A. de Giorgi arturo.degiorgi@uam.es Departamento de Física Teórica and Instituto de Física Teórica UAM/CSIC, Universidad Autónoma de Madrid, Cantoblanco, 28049, Madrid, Spain Department of Physics & Laboratory for Particle Physics and Cosmology, Harvard University, Cambridge, MA 02138, USA S.Vogl stefan.vogl@physik.uni-freiburg.de Albert-Ludwigs-Universität Freiburg, Physikalisches Institut Hermann-Herder-Str. 3, 79104 Freiburg,Germany
Abstract We prove a set of sum rules needed for KK-graviton pair production from matter in orbifolded extra-dimensional models. The sum rules can be found in full generality by considering the properties of solutions to the Sturm-Liouville problem, which describes the wave functions and the masses of the KK-gravitons in four dimensions.They ensure cancellations in the amplitudes of the processes mentioned above which considerably reduce their growth with s 𝑠 s italic_s in the high-energy limit. This protects extra-dimensional theories from the low-scale unitarity problems that plague other theories with massive spin-2 particles.We argue that such relations are valid for a broader category of models thus generalizing our previous results that were limited to the large μ 𝜇 \mu italic_μ limit of the Randall-Sundrum model.
† † preprint: IFT-UAM/CSIC-23-139Contents I Introduction II Compactification and Sturm-Liouville equation II.1 Compactification II.2 The Sturm-Liouville Equationn II.2.1 General Properties II.2.2 Useful Definitions and basic properties II.3 SL Sum Rules III Physics Applications IV RS Radion Sum Rules beyond the large-μ𝜇\muitalic_μ limit V Summary and Outlook A Explicit Example: Large Extra-Dimensions A.1 The Model A.2 Sum Rules I IntroductionAs is well known the longitudinal mode of the polarization tensor of massive spin-2 particles is proportional to E 2 / m 2 superscript 𝐸 2 superscript 𝑚 2 E^{2}/m^{2} italic_E start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT / italic_m start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT where E 𝐸 E italic_E is the energy of the particle and m 𝑚 m italic_m its mass. This leads to a strong growth of amplitudes with these particles as external states with the Mandelstam s 𝑠 s italic_s in the Fierz-Pauli theory[1 ] . Consequently, perturbative unitarity breaks down at a significantly lower energy scale than the effective theory suggests. For spin-2 scattering the amplitude grows as s 5 superscript 𝑠 5 s^{5} italic_s start_POSTSUPERSCRIPT 5 end_POSTSUPERSCRIPT while the production of pairs of spin-2 particles from matter fields grows as s 3 superscript 𝑠 3 s^{3} italic_s start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT .Due to the clear connection between gravity and spin-2 fields, there is a great interest in curing this behaviour since this might allow the construction of a theory of massive gravity which could help to address various cosmological questions. At present, the best-behaved theory with a single massive spin-2 particle features polynomial interaction terms for the spin-2 particle that reduce the growth in scattering to s 3 superscript 𝑠 3 s^{3} italic_s start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT [2 , 3 , 4 ] .
The situation is different in extra-dimensional theories where massive spin-2 particles arise as degrees of freedom in an effective theory from the Kaluza-Klein(KK) decomposition of higher-dimensional gravity. In this case, the fundamental scale of the theory is given by the higher-dimensional Planck mass. Therefore, a lower breakdown scale, as implied by a growth of the amplitudes faster than s 𝑠 s italic_s , is not expected. It is avoided by a subtle cancellation between the contributions of the full KK tower which restores perturbative unitarity of tree level amplitudes up to the Planck scale [5 , 6 , 7 , 8 , 9 , 10 , 11 ] . To achieve this cancellation the KK-gravitons need to fulfil several intricate consistency conditions that can be expressed as sum rules for masses and the coefficient of the KK-graviton vertices.
In this work, we discuss how the sum rules for graviton pair production can be proven in a general and mathematically exact way by studying the properties of the Sturm-Liouville(SL) equation. The explicit connection between SL equation and scattering amplitudes involving KK gravitons was first carried out in Ref.[6 ] . In such reference and in Refs.[7 , 8 ] the properties of the SL equation are used to show how amplitudes in KK-scattering in large-s 𝑠 s italic_s limit vanish. On the same lines, we apply such a technique to the case of graviton-matter annihilation. This generalizes the results of our earlier work [10 ] were a proof of the sum rules in the large μ 𝜇 \mu italic_μ limit of the RS-model was given 1 1 1 Another work that studies this generalization and also considers bulk matter fields appeared simultaneously to this work in Ref.[12 ] . .
For illustration, we demonstrate how they cancel the leading powers in s 𝑠 s italic_s of an expansion of the amplitudes ins 𝑠 s italic_s
ℳ ( s ) = ℳ ( 3 ) s 3 + … , ℳ 𝑠 superscript ℳ 3 superscript 𝑠 3 … \mathcal{M}(s)=\mathcal{M}^{(3)}s^{3}+\dots\,, caligraphic_M ( italic_s ) = caligraphic_M start_POSTSUPERSCRIPT ( 3 ) end_POSTSUPERSCRIPT italic_s start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT + … , (1.1)
for KK-graviton pair productionexplicitly in RS.These cancellations are of great interest for phenomenological studies. For example, spin-2 particles have been considered as mediators to the dark sector or directly as dark matter particles [13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ] . For these studies it is of crucial importance how the amplitudes that control the cross sections behave at high momentum transfer.
The paper is organized as follows. First, we discuss some general properties of compactifications and the Sturm-Liouville equations that are then used to derive a set of sum rules for the masses and integrals of the wave functions. In Sec.III we apply these to scattering processes in the RS model and demonstrate that they are sufficient to ensure the cancellations of the most offending contributions to the amplitude and bring their growth with energy down. However, they are not sufficient to cancel the spurious growth completely and need to be supplemented by compactification-dependent sum rules for the radion contribution to achieve the full cancellation.Finally, we summarize our results and provide a brief outlook in Sec.V .
II Compactification and Sturm-Liouville equationII.1 CompactificationOur primary focus is on orbifolded 5D theories of gravity with an extra compact dimension of size R 𝑅 R italic_R . More specifically, the 5D space-time is compactified under an S 1 / ℤ 2 superscript 𝑆 1 superscript ℤ 2 S^{1}/\mathbb{Z}^{2} italic_S start_POSTSUPERSCRIPT 1 end_POSTSUPERSCRIPT / roman_ℤ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT orbifold symmetry yielding a 1D bulk bounded by two 4-dimensional (4D) branes located at y = 0 𝑦 0 y=0 italic_y = 0 and y = π R 𝑦 𝜋 𝑅 y=\pi R italic_y = italic_π italic_R , where y 𝑦 y italic_y is the coordinate of the 5th-dimension.We consider Einstein-Hilbert gravity in five dimensions
S bulk = M 5 3 2 ∫ d 4 x ∫ − π R π R d y g ( ℛ − 2 Λ B ) , subscript 𝑆 bulk superscript subscript 𝑀 5 3 2 superscript d 4 𝑥 superscript subscript 𝜋 𝑅 𝜋 𝑅 d 𝑦 𝑔 ℛ 2 subscript Λ 𝐵 \displaystyle S_{\text{bulk}}=\frac{M_{5}^{3}}{2}\int\text{d}^{4}x\int\limits_%{-\pi R}^{\pi R}\text{d}y\sqrt{g}(\mathcal{R}-2\Lambda_{B})\,, italic_S start_POSTSUBSCRIPT bulk end_POSTSUBSCRIPT = divide start_ARG italic_M start_POSTSUBSCRIPT 5 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT end_ARG start_ARG 2 end_ARG ∫ d start_POSTSUPERSCRIPT 4 end_POSTSUPERSCRIPT italic_x ∫ start_POSTSUBSCRIPT - italic_π italic_R end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_π italic_R end_POSTSUPERSCRIPT d italic_y square-root start_ARG italic_g end_ARG ( caligraphic_R - 2 roman_Λ start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT ) , (2.1)
where g 𝑔 g italic_g is the determinant of the 5D metric, ℛ ℛ \mathcal{R} caligraphic_R the Ricci scalar, M 5 subscript 𝑀 5 M_{5} italic_M start_POSTSUBSCRIPT 5 end_POSTSUBSCRIPT the 5D Planck mass, and Λ B subscript Λ 𝐵 \Lambda_{B} roman_Λ start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT denotes the vacuum energy of the bulk.Regarding the matter content, we consider models where matter fields are localized on the branes, i.e. on the boundaries of the 5th dimension.
The gravitational particle content is obtained by expanding the metric around the 5D-Minkowski metric
g M N = η M N + κ h M N , subscript 𝑔 𝑀 𝑁 subscript 𝜂 𝑀 𝑁 𝜅 subscript ℎ 𝑀 𝑁 g_{MN}=\eta_{MN}+\kappa h_{MN}\,, italic_g start_POSTSUBSCRIPT italic_M italic_N end_POSTSUBSCRIPT = italic_η start_POSTSUBSCRIPT italic_M italic_N end_POSTSUBSCRIPT + italic_κ italic_h start_POSTSUBSCRIPT italic_M italic_N end_POSTSUBSCRIPT , (2.2)
where h M N subscript ℎ 𝑀 𝑁 h_{MN} italic_h start_POSTSUBSCRIPT italic_M italic_N end_POSTSUBSCRIPT is the metric perturbation κ ≡ 2 / M 5 3 / 2 𝜅 2 superscript subscript 𝑀 5 3 2 \kappa\equiv 2/M_{5}^{3/2} italic_κ ≡ 2 / italic_M start_POSTSUBSCRIPT 5 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 3 / 2 end_POSTSUPERSCRIPT .In the suitable coordinates, the relevant components of the metric perturbation h M N subscript ℎ 𝑀 𝑁 h_{MN} italic_h start_POSTSUBSCRIPT italic_M italic_N end_POSTSUBSCRIPT are its tensorial, h μ ν subscript ℎ 𝜇 𝜈 h_{\mu\nu} italic_h start_POSTSUBSCRIPT italic_μ italic_ν end_POSTSUBSCRIPT , and scalar, r 𝑟 r italic_r , ones; they are typically dubbed graviton and radion , respectively. Five-dimensional fields can be expanded via the so-called Kaluza-Klein(KK)-decomposition in terms of an orthonormal basis, { ψ n ( y ) } n subscript subscript 𝜓 𝑛 𝑦 𝑛 \{\psi_{n}(y)\}_{n} { italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y ) } start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT , of the compactified dimension. For h μ ν ( x , y ) subscript ℎ 𝜇 𝜈 𝑥 𝑦 h_{\mu\nu}(x,y) italic_h start_POSTSUBSCRIPT italic_μ italic_ν end_POSTSUBSCRIPT ( italic_x , italic_y ) it amounts to
h μ ν ( x , y ) = 1 R ∑ n = 0 ∞ h μ ν ( n ) ( x ) ψ n ( y ) , subscript ℎ 𝜇 𝜈 𝑥 𝑦 1 𝑅 superscript subscript 𝑛 0 subscript superscript ℎ 𝑛 𝜇 𝜈 𝑥 subscript 𝜓 𝑛 𝑦 h_{\mu\nu}(x,y)=\dfrac{1}{\sqrt{R}}\sum\limits_{n=0}^{\infty}h^{(n)}_{\mu\nu}(%x)\psi_{n}(y)\,, italic_h start_POSTSUBSCRIPT italic_μ italic_ν end_POSTSUBSCRIPT ( italic_x , italic_y ) = divide start_ARG 1 end_ARG start_ARG square-root start_ARG italic_R end_ARG end_ARG ∑ start_POSTSUBSCRIPT italic_n = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_h start_POSTSUPERSCRIPT ( italic_n ) end_POSTSUPERSCRIPT start_POSTSUBSCRIPT italic_μ italic_ν end_POSTSUBSCRIPT ( italic_x ) italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y ) , (2.3)
In the effective 4D theory, this procedure generates an infinite tower of 4D gravitons.The expansion of the metric of Eq.(2.2 ) can be conveniently chosen such that the 4D gravitons are already in the mass basis and have canonical kinetic and mass terms.As the specific value of R 𝑅 R italic_R is not relevant to our discussion, we work with y 𝑦 y italic_y in units of R 𝑅 R italic_R , which effectively amounts to setting R = 1 𝑅 1 R=1 italic_R = 1 , and thus y ∈ [ − π , π ] 𝑦 𝜋 𝜋 y\in[-\pi,\pi] italic_y ∈ [ - italic_π , italic_π ] .
The ansatz for the metric can be parametrized as[23 ]
d s 2 = e − 2 k | y | ( η μ ν d x μ d x ν − e 6 l k | y | d y 2 ) , d superscript 𝑠 2 superscript 𝑒 2 𝑘 𝑦 subscript 𝜂 𝜇 𝜈 d superscript 𝑥 𝜇 d superscript 𝑥 𝜈 superscript 𝑒 6 𝑙 𝑘 𝑦 d superscript 𝑦 2 \text{d}s^{2}=e^{-2k|y|}\left(\eta_{\mu\nu}\text{d}x^{\mu}\text{d}x^{\nu}-e^{6%lk|y|}\text{d}y^{2}\right)\,, d italic_s start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = italic_e start_POSTSUPERSCRIPT - 2 italic_k | italic_y | end_POSTSUPERSCRIPT ( italic_η start_POSTSUBSCRIPT italic_μ italic_ν end_POSTSUBSCRIPT d italic_x start_POSTSUPERSCRIPT italic_μ end_POSTSUPERSCRIPT d italic_x start_POSTSUPERSCRIPT italic_ν end_POSTSUPERSCRIPT - italic_e start_POSTSUPERSCRIPT 6 italic_l italic_k | italic_y | end_POSTSUPERSCRIPT d italic_y start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) , (2.4)
where η μ ν = diag ( 1 , − 1 , − 1 , − 1 ) subscript 𝜂 𝜇 𝜈 diag 1 1 1 1 \eta_{\mu\nu}=\text{diag}(1,-1,-1,-1) italic_η start_POSTSUBSCRIPT italic_μ italic_ν end_POSTSUBSCRIPT = diag ( 1 , - 1 , - 1 , - 1 ) is the Minkowski metric and k ∼ − Λ B similar-to 𝑘 subscript Λ 𝐵 k\sim\sqrt{-\Lambda_{B}} italic_k ∼ square-root start_ARG - roman_Λ start_POSTSUBSCRIPT italic_B end_POSTSUBSCRIPT end_ARG .This ansatz interpolates between several popular models, i.e. LED(k = 0 𝑘 0 k=0 italic_k = 0 )[24 ] , RS(l = 1 / 3 𝑙 1 3 l=1/3 italic_l = 1 / 3 )[25 ] and CW(l = 0 𝑙 0 l=0 italic_l = 0 )[23 ] .The corresponding equation of motion for the graviton wave function along the direction of the fifth dimension, ψ n subscript 𝜓 𝑛 \psi_{n} italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT , reads
[ ∂ y 2 − 9 4 ( 1 + l ) 2 k 2 + e 6 l k | y | m n 2 ] e − 3 2 ( 1 + l ) k | y | ψ n ( | y | ) = 0 , , delimited-[] superscript subscript 𝑦 2 9 4 superscript 1 𝑙 2 superscript 𝑘 2 superscript 𝑒 6 𝑙 𝑘 𝑦 superscript subscript 𝑚 𝑛 2 superscript 𝑒 3 2 1 𝑙 𝑘 𝑦 subscript 𝜓 𝑛 𝑦 0 \left[\partial_{y}^{2}-\frac{9}{4}(1+l)^{2}k^{2}+e^{6lk|y|}m_{n}^{2}\right]e^{%-\frac{3}{2}(1+l)k|y|}\psi_{n}(|y|)=0\\,, [ ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT - divide start_ARG 9 end_ARG start_ARG 4 end_ARG ( 1 + italic_l ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_k start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_e start_POSTSUPERSCRIPT 6 italic_l italic_k | italic_y | end_POSTSUPERSCRIPT italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ] italic_e start_POSTSUPERSCRIPT - divide start_ARG 3 end_ARG start_ARG 2 end_ARG ( 1 + italic_l ) italic_k | italic_y | end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( | italic_y | ) = 0 , , (2.5)
where m n subscript 𝑚 𝑛 m_{n} italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT is the mass of the graviton h μ ν ( n ) superscript subscript ℎ 𝜇 𝜈 𝑛 h_{\mu\nu}^{(n)} italic_h start_POSTSUBSCRIPT italic_μ italic_ν end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_n ) end_POSTSUPERSCRIPT .This is a particular case of a Sturm-Liouville (SL) equation as expected given that the Einstein-Hilbert action only contains up to two derivatives and all hom*ogeneous second-order ordinary differential equations can be brought to SL form.
To ease readability of the main text, we briefly review some of them in the next section.
II.2 The Sturm-Liouville EquationnII.2.1 General PropertiesThe SL equation is a real second-order linear ordinary differential equation with interesting properties and many applications in physics. In the following, we briefly recapitulate some of the basics that matter for our discussion. For a more systematic presentation of its mathematical properties and the proofs see for example Ref.[26 ] . In its most general form, it reads
− 1 d y [ p ( y ) d ψ d y ] + q ( y ) ψ = λ r ( y ) ψ . 1 d 𝑦 delimited-[] 𝑝 𝑦 d 𝜓 d 𝑦 𝑞 𝑦 𝜓 𝜆 𝑟 𝑦 𝜓 -\frac{1}{\text{d}y}\left[p(y)\frac{\text{d}\psi}{\text{d}y}\right]+q(y)\psi=%\lambda r(y)\psi\,. - divide start_ARG 1 end_ARG start_ARG d italic_y end_ARG [ italic_p ( italic_y ) divide start_ARG d italic_ψ end_ARG start_ARG d italic_y end_ARG ] + italic_q ( italic_y ) italic_ψ = italic_λ italic_r ( italic_y ) italic_ψ . (2.6)
The function r ( y ) 𝑟 𝑦 r(y) italic_r ( italic_y ) is sometimes referred to as the weight and λ 𝜆 \lambda italic_λ as the eigenvalue of the equation.The SL equation together with boundary conditions for the solution constitutes the so-called SL problem . The eigenvalue is generically not specified, and it is part of the SL problem to find suitable eigenvalues for which non-trivial solutions exist.If p ( y ) 𝑝 𝑦 p(y) italic_p ( italic_y ) and r ( y ) > 0 𝑟 𝑦 0 r(y)>0 italic_r ( italic_y ) > 0 while p ( y ) , p ′ ( y ) , q ( y ) , r ( y ) 𝑝 𝑦 superscript 𝑝 ′ 𝑦 𝑞 𝑦 𝑟 𝑦
p(y),\,p^{\prime}(y),\,q(y),\,r(y) italic_p ( italic_y ) , italic_p start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ( italic_y ) , italic_q ( italic_y ) , italic_r ( italic_y ) are continuous in the interval [ a , b ] 𝑎 𝑏 [a,b] [ italic_a , italic_b ] , and the boundary conditions are given in the form
α 1 ψ ( a ) + α 2 ψ ′ ( a ) = 0 , β 1 ψ ( b ) + β 2 ψ ′ ( b ) = 0 , formulae-sequence subscript 𝛼 1 𝜓 𝑎 subscript 𝛼 2 superscript 𝜓 ′ 𝑎 0 subscript 𝛽 1 𝜓 𝑏 subscript 𝛽 2 superscript 𝜓 ′ 𝑏 0 \alpha_{1}\psi(a)+\alpha_{2}\psi^{\prime}(a)=0\,,\qquad\beta_{1}\psi(b)+\beta_%{2}\psi^{\prime}(b)=0\,, italic_α start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT italic_ψ ( italic_a ) + italic_α start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT italic_ψ start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ( italic_a ) = 0 , italic_β start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT italic_ψ ( italic_b ) + italic_β start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT italic_ψ start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ( italic_b ) = 0 , (2.7)
the SL problem is said to be regular . Regular SL problems satisfy the following properties:
1. There exist an infinite countable number of real eigenvalues { λ n } n = 0 ∞ superscript subscript subscript 𝜆 𝑛 𝑛 0 \{\lambda_{n}\}_{n=0}^{\infty} { italic_λ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT } start_POSTSUBSCRIPT italic_n = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT that can be ordered such that
λ 0 < λ 1 < ⋯ < λ n < ⋯ < ∞ . subscript 𝜆 0 subscript 𝜆 1 ⋯ subscript 𝜆 𝑛 ⋯ \lambda_{0}<\lambda_{1}<\dots<\lambda_{n}<\dots<\infty\,. italic_λ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT < italic_λ start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT < ⋯ < italic_λ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT < ⋯ < ∞ . (2.8)
2. For each eigenvalue λ i subscript 𝜆 𝑖 \lambda_{i} italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT there exists a unique eigenfunction ψ i ( y ) subscript 𝜓 𝑖 𝑦 \psi_{i}(y) italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_y ) , up to rescalings.
3. The solutions form an orthonormal basis of the Hilbert space L 2 ( [ a , b ] , r ( y ) d y ) superscript 𝐿 2 𝑎 𝑏 𝑟 𝑦 d 𝑦 L^{2}([a,b],r(y)\text{d}y) italic_L start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( [ italic_a , italic_b ] , italic_r ( italic_y ) d italic_y ) , that is
⟨ ψ i | ψ j ⟩ ≡ ∫ a b d y r ( y ) ψ i ( y ) ψ j ( y ) = δ i j . inner-product subscript 𝜓 𝑖 subscript 𝜓 𝑗 superscript subscript 𝑎 𝑏 d 𝑦 𝑟 𝑦 subscript 𝜓 𝑖 𝑦 subscript 𝜓 𝑗 𝑦 subscript 𝛿 𝑖 𝑗 \innerproduct{\psi_{i}}{\psi_{j}}\equiv\int_{a}^{b}\text{d}y\,r(y)\psi_{i}(y)%\psi_{j}(y)=\delta_{ij}\,. ⟨ start_ARG italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT end_ARG | start_ARG italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT end_ARG ⟩ ≡ ∫ start_POSTSUBSCRIPT italic_a end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_b end_POSTSUPERSCRIPT d italic_y italic_r ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_y ) = italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT . (2.9)
This also implies that any function, f ( y ) 𝑓 𝑦 f(y) italic_f ( italic_y ) can be expanded within the interval ( a , b ) 𝑎 𝑏 (a,b) ( italic_a , italic_b ) as
f ( y ) ≡ | f ⟩ = ∑ n = 0 ∞ | ψ n ⟩ ⟨ ψ n | f ⟩ = ∑ n = 0 ∞ c n ψ n ( y ) , where c n = ∫ a b d y r ( y ) ψ n ( y ) f ( y ) . formulae-sequence 𝑓 𝑦 ket 𝑓 superscript subscript 𝑛 0 ket subscript 𝜓 𝑛 inner-product subscript 𝜓 𝑛 𝑓 superscript subscript 𝑛 0 subscript 𝑐 𝑛 subscript 𝜓 𝑛 𝑦 where subscript 𝑐 𝑛
superscript subscript 𝑎 𝑏 d 𝑦 𝑟 𝑦 subscript 𝜓 𝑛 𝑦 𝑓 𝑦 \begin{split}&f(y)\equiv\ket{f}=\sum\limits_{n=0}^{\infty}\ket{\psi_{n}}%\innerproduct{\psi_{n}}{f}=\sum\limits_{n=0}^{\infty}c_{n}\psi_{n}(y)\,,\\&\text{where}\quad c_{n}=\int_{a}^{b}\text{d}y\,r(y)\psi_{n}(y)f(y)\,.\end{split} start_ROW start_CELL end_CELL start_CELL italic_f ( italic_y ) ≡ | start_ARG italic_f end_ARG ⟩ = ∑ start_POSTSUBSCRIPT italic_n = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT | start_ARG italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_ARG ⟩ ⟨ start_ARG italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_ARG | start_ARG italic_f end_ARG ⟩ = ∑ start_POSTSUBSCRIPT italic_n = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_c start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y ) , end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL where italic_c start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT = ∫ start_POSTSUBSCRIPT italic_a end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_b end_POSTSUPERSCRIPT d italic_y italic_r ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y ) italic_f ( italic_y ) . end_CELL end_ROW (2.10)
Such information can be conveniently encoded in functional representation via the completeness relation
δ ( y − y ′ ) = ∑ k = 0 ∞ r ( y ) ψ k ( y ) ψ k ( y ′ ) . 𝛿 𝑦 superscript 𝑦 ′ superscript subscript 𝑘 0 𝑟 𝑦 subscript 𝜓 𝑘 𝑦 subscript 𝜓 𝑘 superscript 𝑦 ′ \delta(y-y^{\prime})=\sum\limits_{k=0}^{\infty}r(y)\psi_{k}(y)\psi_{k}(y^{%\prime})\,. italic_δ ( italic_y - italic_y start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) = ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_r ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_y start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) . (2.11)
Finally, one can show that, for q = 0 𝑞 0 q=0 italic_q = 0 , if ψ i subscript 𝜓 𝑖 \psi_{i} italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT is a solution of the SL equation, then p ( y ) ∂ y ψ i ( y ) 𝑝 𝑦 subscript 𝑦 subscript 𝜓 𝑖 𝑦 p(y)\partial_{y}\psi_{i}(y) italic_p ( italic_y ) ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_y ) is also a solution with r → 1 / p → 𝑟 1 𝑝 r\to 1/p italic_r → 1 / italic_p and p → 1 / r → 𝑝 1 𝑟 p\to 1/r italic_p → 1 / italic_r . This implies orthogonality relations among the derivatives of ψ i subscript 𝜓 𝑖 \psi_{i} italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT of the form
∫ a b d y p ( y ) ( ∂ y ψ i ) ( ∂ y ψ j ) = λ i δ i j . superscript subscript 𝑎 𝑏 d 𝑦 𝑝 𝑦 subscript 𝑦 subscript 𝜓 𝑖 subscript 𝑦 subscript 𝜓 𝑗 subscript 𝜆 𝑖 subscript 𝛿 𝑖 𝑗 \int_{a}^{b}\text{d}y\,p(y)\left(\partial_{y}\psi_{i}\right)\left(\partial_{y}%\psi_{j}\right)=\lambda_{i}\delta_{ij}\,. ∫ start_POSTSUBSCRIPT italic_a end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_b end_POSTSUPERSCRIPT d italic_y italic_p ( italic_y ) ( ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ) ( ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) = italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT . (2.12)
Given the above-mentioned properties, the SL equation can be recast as an eigenvalue problem (from which the name of λ 𝜆 \lambda italic_λ derives) of a linear operator L 𝐿 L italic_L , such that
L [ ψ i ] ( y ) = λ i ψ i ( y ) , 𝐿 delimited-[] subscript 𝜓 𝑖 𝑦 subscript 𝜆 𝑖 subscript 𝜓 𝑖 𝑦 L[\psi_{i}](y)=\lambda_{i}\psi_{i}(y)\,, italic_L [ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ] ( italic_y ) = italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_y ) , (2.13)
with
L [ f ] ( y ) ≡ − 1 r ( y ) ( d missing d y [ p ( y ) d f d y ] + q ( y ) f ) . 𝐿 delimited-[] 𝑓 𝑦 1 𝑟 𝑦 d missing d 𝑦 delimited-[] 𝑝 𝑦 d 𝑓 d 𝑦 𝑞 𝑦 𝑓 L[f](y)\equiv-\dfrac{1}{r(y)}\left(\dfrac{\text{d}missing}{\text{d}y}\left[p(y%)\dfrac{\text{d}f}{\text{d}y}\right]+q(y)f\right)\,. italic_L [ italic_f ] ( italic_y ) ≡ - divide start_ARG 1 end_ARG start_ARG italic_r ( italic_y ) end_ARG ( divide start_ARG d roman_missing end_ARG start_ARG d italic_y end_ARG [ italic_p ( italic_y ) divide start_ARG d italic_f end_ARG start_ARG d italic_y end_ARG ] + italic_q ( italic_y ) italic_f ) . (2.14)
The SL operator is self-adjoint within the interval [ a , b ] 𝑎 𝑏 [a,b] [ italic_a , italic_b ] . In the following, we will write Ω Ω \Omega roman_Ω and ∂ Ω Ω \partial\Omega ∂ roman_Ω to indicate the domain of integration and its boundary, respectively.
II.2.2 Useful Definitions and basic propertiesThe effective couplings involving the KK-tower of gravitons can be derived by integrating out the 5th dimension. They correspond to n-points integrals containing either no or two derivatives of ψ i subscript 𝜓 𝑖 \psi_{i} italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT . For this reason, it is useful to study the n − limit-from 𝑛 n- italic_n - point integrals
a i 1 , i 2 , … , i n subscript 𝑎 subscript 𝑖 1 subscript 𝑖 2 … subscript 𝑖 𝑛
\displaystyle a_{i_{1},i_{2},\dots,i_{n}} italic_a start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT , … , italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT ≡ ∫ Ω d y r ( y ) ψ i 1 ψ i 2 ψ i 3 … ψ i n , absent subscript Ω d 𝑦 𝑟 𝑦 subscript 𝜓 subscript 𝑖 1 subscript 𝜓 subscript 𝑖 2 subscript 𝜓 subscript 𝑖 3 … subscript 𝜓 subscript 𝑖 𝑛 \displaystyle\equiv\int_{\Omega}\text{d}y\ r(y)\psi_{i_{1}}\psi_{i_{2}}\psi_{i%_{3}}\dots\psi_{i_{n}}\,, ≡ ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_y italic_r ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 3 end_POSTSUBSCRIPT end_POSTSUBSCRIPT … italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT , (2.15) b i 1 , i 2 , … , i n subscript 𝑏 subscript 𝑖 1 subscript 𝑖 2 … subscript 𝑖 𝑛
\displaystyle b_{i_{1},i_{2},\dots,i_{n}} italic_b start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT , … , italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT ≡ ∫ Ω d y p ( y ) ( ∂ y ψ i 1 ) ( ∂ y ψ i 2 ) ψ i 3 … ψ i n , absent subscript Ω d 𝑦 𝑝 𝑦 subscript 𝑦 subscript 𝜓 subscript 𝑖 1 subscript 𝑦 subscript 𝜓 subscript 𝑖 2 subscript 𝜓 subscript 𝑖 3 … subscript 𝜓 subscript 𝑖 𝑛 \displaystyle\equiv\int_{\Omega}\text{d}y\ p(y)\left(\partial_{y}\psi_{i_{1}}%\right)\left(\partial_{y}\psi_{i_{2}}\right)\psi_{i_{3}}\dots\psi_{i_{n}}\,, ≡ ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_y italic_p ( italic_y ) ( ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_POSTSUBSCRIPT ) ( ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_POSTSUBSCRIPT ) italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 3 end_POSTSUBSCRIPT end_POSTSUBSCRIPT … italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT , (2.16)
where the indices i 1 , i 2 , … , i n subscript 𝑖 1 subscript 𝑖 2 … subscript 𝑖 𝑛
i_{1},i_{2},\dots,i_{n} italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT , … , italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT will indicate the number of the n 𝑛 n italic_n gravitons involved in the interaction. Notice that the first two indices of b i 1 , i 2 , … , i n subscript 𝑏 subscript 𝑖 1 subscript 𝑖 2 … subscript 𝑖 𝑛
b_{i_{1},i_{2},\dots,i_{n}} italic_b start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT , … , italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT commute between themselves, but not with the others and vice versa.With Eq.(2.9 ) and (2.12 ) one immediately finds that for n = 2 𝑛 2 n=2 italic_n = 2 the integrals are given by
a i j = δ i j , subscript 𝑎 𝑖 𝑗 subscript 𝛿 𝑖 𝑗 \displaystyle a_{ij}=\delta_{ij}\,, italic_a start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT = italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT , b i j = λ i δ i j . subscript 𝑏 𝑖 𝑗 subscript 𝜆 𝑖 subscript 𝛿 𝑖 𝑗 \displaystyle b_{ij}=\lambda_{i}\delta_{ij}\,. italic_b start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT = italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT . (2.17)
The a i 1 , … , i n subscript 𝑎 subscript 𝑖 1 … subscript 𝑖 𝑛
a_{i_{1},\dots,i_{n}} italic_a start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , … , italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT and b i 1 , … , i n subscript 𝑏 subscript 𝑖 1 … subscript 𝑖 𝑛
b_{i_{1},\dots,i_{n}} italic_b start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , … , italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT coefficients are not fully independent one from another. They can be related by integration by parts via Eq.(2.6 ). In fact, for any function f ( y ) 𝑓 𝑦 f(y) italic_f ( italic_y ) we have that
∫ Ω d y p ( y ) ∂ y f ∂ y ψ i = p ( y ) f ∂ y ψ i | ∂ Ω + λ i ∫ d y r ( y ) f ψ i + ∫ d y q ( y ) f ψ i . subscript Ω d 𝑦 𝑝 𝑦 subscript 𝑦 𝑓 subscript 𝑦 subscript 𝜓 𝑖 evaluated-at 𝑝 𝑦 𝑓 subscript 𝑦 subscript 𝜓 𝑖 Ω subscript 𝜆 𝑖 d 𝑦 𝑟 𝑦 𝑓 subscript 𝜓 𝑖 d 𝑦 𝑞 𝑦 𝑓 subscript 𝜓 𝑖 \begin{split}\int_{\Omega}\text{d}y\ p(y)\partial_{y}f\partial_{y}\psi_{i}=&%\left.p(y)f\partial_{y}\psi_{i}\right|_{\partial\Omega}+\lambda_{i}\int\text{d%}y\ r(y)f\psi_{i}\\&+\int\text{d}y\ q(y)f\psi_{i}\,.\end{split} start_ROW start_CELL ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_y italic_p ( italic_y ) ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_f ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT = end_CELL start_CELL italic_p ( italic_y ) italic_f ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT | start_POSTSUBSCRIPT ∂ roman_Ω end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∫ d italic_y italic_r ( italic_y ) italic_f italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL + ∫ d italic_y italic_q ( italic_y ) italic_f italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT . end_CELL end_ROW (2.18)
The above relation can be further simplified in the case q = 0 𝑞 0 q=0 italic_q = 0 and ∂ y ψ i | ∂ Ω = 0 evaluated-at subscript 𝑦 subscript 𝜓 𝑖 Ω 0 \partial_{y}\psi_{i}|_{\partial\Omega}=0 ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT | start_POSTSUBSCRIPT ∂ roman_Ω end_POSTSUBSCRIPT = 0 . By choosing ψ i = ψ i 1 subscript 𝜓 𝑖 subscript 𝜓 subscript 𝑖 1 \psi_{i}=\psi_{i_{1}} italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_POSTSUBSCRIPT and f ( y ) = ψ i 2 … ψ i n 𝑓 𝑦 subscript 𝜓 subscript 𝑖 2 … subscript 𝜓 subscript 𝑖 𝑛 f(y)=\psi_{i_{2}}\dots\psi_{i_{n}} italic_f ( italic_y ) = italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_POSTSUBSCRIPT … italic_ψ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT , one obtains
λ i 1 a i 1 , i 2 , … , i n = ∑ j ∈ i 2 , … , i n b i 1 , j , X j , subscript 𝜆 subscript 𝑖 1 subscript 𝑎 subscript 𝑖 1 subscript 𝑖 2 … subscript 𝑖 𝑛
subscript 𝑗 subscript 𝑖 2 … subscript 𝑖 𝑛
subscript 𝑏 subscript 𝑖 1 𝑗 subscript 𝑋 𝑗
\lambda_{i_{1}}a_{i_{1},i_{2},\dots,i_{n}}=\sum\limits_{j\in{i_{2},\dots,i_{n}%}}b_{i_{1},j,X_{j}}\,, italic_λ start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_POSTSUBSCRIPT italic_a start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT , … , italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT = ∑ start_POSTSUBSCRIPT italic_j ∈ italic_i start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT , … , italic_i start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_POSTSUBSCRIPT italic_b start_POSTSUBSCRIPT italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT , italic_j , italic_X start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT end_POSTSUBSCRIPT , (2.19)
where X j subscript 𝑋 𝑗 X_{j} italic_X start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT includes all the indices except i 1 subscript 𝑖 1 i_{1} italic_i start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT and j 𝑗 j italic_j .For n = 2 𝑛 2 n=2 italic_n = 2 this recovers the orthogonality relations. The first non-trivial case is for n = 3 𝑛 3 n=3 italic_n = 3 . It reads
λ i a i j k = b i j k + b i k j , subscript 𝜆 𝑖 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝑏 𝑖 𝑗 𝑘 subscript 𝑏 𝑖 𝑘 𝑗 \lambda_{i}a_{ijk}=b_{ijk}+b_{ikj}\ , italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT + italic_b start_POSTSUBSCRIPT italic_i italic_k italic_j end_POSTSUBSCRIPT , (2.20)
which implies
b i j k = 1 2 a i j k ( λ i + λ j − λ k ) . subscript 𝑏 𝑖 𝑗 𝑘 1 2 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜆 𝑖 subscript 𝜆 𝑗 subscript 𝜆 𝑘 b_{ijk}=\frac{1}{2}a_{ijk}\left(\lambda_{i}+\lambda_{j}-\lambda_{k}\right)\ . italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = divide start_ARG 1 end_ARG start_ARG 2 end_ARG italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ) . (2.21)
The above equation shows that for n = 3 𝑛 3 n=3 italic_n = 3 all b 𝑏 b italic_b -coefficients can be unambiguously traded for a 𝑎 a italic_a -coefficients. This special case allows the translation of many of the results for the sum rules of one type of coefficient to the other and vice versa.
In the next section, we will make use of such relations and definitions to prove sum rules involving a 𝑎 a italic_a - and b 𝑏 b italic_b -coefficients that will be relevant for the physics involving the scattering of gravitons and gravitons pair production.
II.3 SL Sum RulesFor convenience, the most relevant sum rules and relations needed for the amplitudes are collected in Table1 .
Sum Rule ℳ ℳ \mathcal{M} caligraphic_M ∑ k = 0 ∞ ψ k ( π ) a i j k = ψ i ( π ) ψ j ( π ) superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 \sum\limits_{k=0}^{\infty}\psi_{k}(\pi)a_{ijk}=\psi_{i}(\pi)\psi_{j}(\pi) ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) ℳ ( 3 ) superscript ℳ 3 \mathcal{M}^{(3)} caligraphic_M start_POSTSUPERSCRIPT ( 3 ) end_POSTSUPERSCRIPT ∑ k = 0 ∞ ψ k ( π ) λ k a i j k = ψ i ( π ) ψ j ( π ) ( λ i + λ j ) superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 subscript 𝜆 𝑖 subscript 𝜆 𝑗 \sum\limits_{k=0}^{\infty}\psi_{k}(\pi)\lambda_{k}a_{ijk}=\psi_{i}(\pi)\psi_{j%}(\pi)(\lambda_{i}+\lambda_{j}) ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) ℳ ( 2 ) superscript ℳ 2 \mathcal{M}^{(2)} caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ∑ k = 0 ∞ ψ k ( π ) b k i j = λ i ψ i ( π ) ψ j ( π ) superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝑏 𝑘 𝑖 𝑗 subscript 𝜆 𝑖 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 \sum\limits_{k=0}^{\infty}\psi_{k}(\pi)b_{kij}=\lambda_{i}\psi_{i}(\pi)\psi_{j%}(\pi) ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_b start_POSTSUBSCRIPT italic_k italic_i italic_j end_POSTSUBSCRIPT = italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) ℳ ( 2 ) superscript ℳ 2 \mathcal{M}^{(2)} caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ∑ k = 0 ∞ ψ k ( π ) b i j k = 0 superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝑏 𝑖 𝑗 𝑘 0 \sum\limits_{k=0}^{\infty}\psi_{k}(\pi)b_{ijk}=0 ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = 0 ℳ ( 2 ) superscript ℳ 2 \mathcal{M}^{(2)} caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ∑ k = 1 ∞ ψ k ( π ) λ k a i j k ( λ i − λ j ) 2 = ( λ i + λ j ) [ ψ i ( π ) ψ j ( π ) + ψ 0 2 δ i j ] − 6 ψ r b i j r superscript subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 superscript subscript 𝜆 𝑖 subscript 𝜆 𝑗 2 subscript 𝜆 𝑖 subscript 𝜆 𝑗 delimited-[] subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 superscript subscript 𝜓 0 2 subscript 𝛿 𝑖 𝑗 6 subscript 𝜓 𝑟 subscript 𝑏 𝑖 𝑗 𝑟 \sum\limits_{k=1}^{\infty}\dfrac{\psi_{k}(\pi)}{\lambda_{k}}a_{ijk}(\lambda_{i%}-\lambda_{j})^{2}=(\lambda_{i}+\lambda_{j})\left[\psi_{i}(\pi)\psi_{j}(\pi)+%\psi_{0}^{2}\delta_{ij}\right]-6\psi_{r}b_{ijr} ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) [ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) + italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ] - 6 italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_b start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT ℳ ( 2 ) superscript ℳ 2 \mathcal{M}^{(2)} caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ∑ k = 1 ψ k ( π ) b k i r λ k = A π − 2 ψ r ψ i ( π ) − ψ r a i r r subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝑏 𝑘 𝑖 𝑟 subscript 𝜆 𝑘 superscript subscript 𝐴 𝜋 2 subscript 𝜓 𝑟 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑟 subscript 𝑎 𝑖 𝑟 𝑟 \sum\limits_{k=1}\psi_{k}(\pi)\dfrac{b_{kir}}{\lambda_{k}}=A_{\pi}^{-2}\psi_{r%}\psi_{i}(\pi)-\psi_{r}a_{irr} ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) divide start_ARG italic_b start_POSTSUBSCRIPT italic_k italic_i italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG = italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) - italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT ℳ ( 2 ) superscript ℳ 2 \mathcal{M}^{(2)} caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT b i j k = 1 2 a i j k ( λ i + λ j − λ k ) subscript 𝑏 𝑖 𝑗 𝑘 1 2 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜆 𝑖 subscript 𝜆 𝑗 subscript 𝜆 𝑘 b_{ijk}=\frac{1}{2}a_{ijk}\left(\lambda_{i}+\lambda_{j}-\lambda_{k}\right) italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = divide start_ARG 1 end_ARG start_ARG 2 end_ARG italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ) -
The diagrams that involve such sums are shown in Fig.1 .
As can be seen, the graviton wave functions enter in the total amplitude either localized on the brane or at most linearly via a i j k subscript 𝑎 𝑖 𝑗 𝑘 a_{ijk} italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT when integrated over the bulk. Since we have already in mind the physical application, we will consider ∂ Ω = { 0 , π } Ω 0 𝜋 \partial\Omega=\{0,\pi\} ∂ roman_Ω = { 0 , italic_π } , but the results only require the constant parts to be evaluated on the boundary, i.e. ψ i ( ∂ Ω ) subscript 𝜓 𝑖 Ω \psi_{i}(\partial\Omega) italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( ∂ roman_Ω ) , ( ∂ y ψ i ) ( ∂ Ω ) subscript 𝑦 subscript 𝜓 𝑖 Ω (\partial_{y}\psi_{i})(\partial\Omega) ( ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ) ( ∂ roman_Ω ) .
The main task is to compute sum rules involving ∑ k ψ k ( π ) a i j k subscript 𝑘 subscript 𝜓 𝑘 𝜋 subscript 𝑎 𝑖 𝑗 𝑘 \sum_{k}\psi_{k}(\pi)a_{ijk} ∑ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT 2 2 2 Notice that all the sum rules we derive for y = π 𝑦 𝜋 y=\pi italic_y = italic_π equally apply to y = 0 𝑦 0 y=0 italic_y = 0 . and related.Applying the Liouville operator, one finds the general relation ( α ≥ 0 ) 𝛼 0 (\alpha\geq 0) ( italic_α ≥ 0 )
∑ n = 0 ∞ ψ n ( π ) λ n α ( ∫ Ω d y r ( y ) ψ n ( y ) f ( y ) ) = L α [ f ( y ) ] | y = π . superscript subscript 𝑛 0 subscript 𝜓 𝑛 𝜋 superscript subscript 𝜆 𝑛 𝛼 subscript Ω d 𝑦 𝑟 𝑦 subscript 𝜓 𝑛 𝑦 𝑓 𝑦 evaluated-at superscript 𝐿 𝛼 delimited-[] 𝑓 𝑦 𝑦 𝜋 \sum\limits_{n=0}^{\infty}\psi_{n}(\pi)\lambda_{n}^{\alpha}\left(\int_{\Omega}%\text{d}y\,r(y)\psi_{n}(y)f(y)\right)=\left.L^{\alpha}[f(y)]\right|_{y=\pi}\,. ∑ start_POSTSUBSCRIPT italic_n = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_π ) italic_λ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_α end_POSTSUPERSCRIPT ( ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_y italic_r ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y ) italic_f ( italic_y ) ) = italic_L start_POSTSUPERSCRIPT italic_α end_POSTSUPERSCRIPT [ italic_f ( italic_y ) ] | start_POSTSUBSCRIPT italic_y = italic_π end_POSTSUBSCRIPT . (2.22)
Taking f ( y ) = ψ i ψ j 𝑓 𝑦 subscript 𝜓 𝑖 subscript 𝜓 𝑗 f(y)=\psi_{i}\psi_{j} italic_f ( italic_y ) = italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT and α = 0 𝛼 0 \alpha=0 italic_α = 0 , leads to
∑ k = 0 ∞ ψ k ( π ) a i j k = ψ i ( π ) ψ j ( π ) . superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 \displaystyle\sum\limits_{k=0}^{\infty}\psi_{k}(\pi)a_{ijk}=\psi_{i}(\pi)\psi_%{j}(\pi)\,. ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) . (2.23)
This result is completely general, independently of q ( y ) 𝑞 𝑦 q(y) italic_q ( italic_y ) or on the boundary conditions. As for the quadratic sum rules, by dimensional analysis, Eq.(2.23 ) is relevant to cancel the leading order, ℳ ( 3 ) superscript ℳ 3 \mathcal{M}^{(3)} caligraphic_M start_POSTSUPERSCRIPT ( 3 ) end_POSTSUPERSCRIPT . Instead, sum rules which involve one power of λ k subscript 𝜆 𝑘 \lambda_{k} italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT or b i j k subscript 𝑏 𝑖 𝑗 𝑘 b_{ijk} italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT contribute to partially cancel parts of the amplitudes that scale slower than s 3 superscript 𝑠 3 s^{3} italic_s start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT in ℳ ℳ \mathcal{M} caligraphic_M .In the following, we will employ q ( y ) = 0 𝑞 𝑦 0 q(y)=0 italic_q ( italic_y ) = 0 . In this case, the Liouville operator can be written as
L [ f ] ( y ) ≡ − 1 r ( y ) d missing d y [ p ( y ) d f d y ] . 𝐿 delimited-[] 𝑓 𝑦 1 𝑟 𝑦 d missing d 𝑦 delimited-[] 𝑝 𝑦 d 𝑓 d 𝑦 L[f](y)\equiv-\dfrac{1}{r(y)}\dfrac{\text{d}missing}{\text{d}y}\left[p(y)%\dfrac{\text{d}f}{\text{d}y}\right]\,. italic_L [ italic_f ] ( italic_y ) ≡ - divide start_ARG 1 end_ARG start_ARG italic_r ( italic_y ) end_ARG divide start_ARG d roman_missing end_ARG start_ARG d italic_y end_ARG [ italic_p ( italic_y ) divide start_ARG d italic_f end_ARG start_ARG d italic_y end_ARG ] . (2.24)
The linearity of the Liouville operator allows us to write the relation
L [ f g ] = g L [ f ] + f L [ g ] − 2 p r ( ∂ f ) ( ∂ g ) . 𝐿 delimited-[] 𝑓 𝑔 𝑔 𝐿 delimited-[] 𝑓 𝑓 𝐿 delimited-[] 𝑔 2 𝑝 𝑟 𝑓 𝑔 L[fg]=gL[f]+fL[g]-2\dfrac{p}{r}(\partial f)(\partial g)\,. italic_L [ italic_f italic_g ] = italic_g italic_L [ italic_f ] + italic_f italic_L [ italic_g ] - 2 divide start_ARG italic_p end_ARG start_ARG italic_r end_ARG ( ∂ italic_f ) ( ∂ italic_g ) . (2.25)
For ∂ y ψ i | y = π = 0 evaluated-at subscript 𝑦 subscript 𝜓 𝑖 𝑦 𝜋 0 \partial_{y}\psi_{i}|_{y=\pi}=0 ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT | start_POSTSUBSCRIPT italic_y = italic_π end_POSTSUBSCRIPT = 0 and by means of Eq.(2.22 )-(2.24 ), one can show
∑ k = 0 ∞ ψ k ( π ) λ k a i j k = ψ i ( π ) ψ j ( π ) ( λ i + λ j ) , superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 subscript 𝜆 𝑖 subscript 𝜆 𝑗 \displaystyle\sum\limits_{k=0}^{\infty}\psi_{k}(\pi)\lambda_{k}a_{ijk}=\psi_{i%}(\pi)\psi_{j}(\pi)(\lambda_{i}+\lambda_{j})\,, ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) , (2.26) ∑ k = 0 ∞ ψ k ( π ) λ k 2 a i j k = ψ i ( π ) ψ j ( π ) ( λ i 2 + 6 λ i λ j + λ j 2 ) , superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 superscript subscript 𝜆 𝑘 2 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 superscript subscript 𝜆 𝑖 2 6 subscript 𝜆 𝑖 subscript 𝜆 𝑗 superscript subscript 𝜆 𝑗 2 \displaystyle\sum\limits_{k=0}^{\infty}\psi_{k}(\pi)\lambda_{k}^{2}a_{ijk}=%\psi_{i}(\pi)\psi_{j}(\pi)(\lambda_{i}^{2}+6\lambda_{i}\lambda_{j}+\lambda_{j}%^{2})\,, ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + 6 italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) , (2.27)
These sum rules generalize the ones in Ref.[10 , 18 ] .
Similar sum rules can be obtained for the b 𝑏 b italic_b -integrals. In principle, such relations are not strictly speaking needed, as one can use Eq.(2.21 ) and trade b 𝑏 b italic_b - for a 𝑎 a italic_a - coefficients. However, we report them here for completeness, as they save some work avoiding the conversion. For example,
∑ k = 0 ∞ ψ k ( π ) b i j k = ∑ k = 0 ∞ ψ k ( π ) ∫ Ω d y p ( y ) ∂ y ψ i ∂ y ψ j ψ k = ∫ Ω d y p ( y ) r ( y ) ∂ y ψ i ∂ y ψ j ∑ k = 0 ∞ r ( y ) ψ k ψ k ( π ) = ∫ Ω d y p ( y ) r ( y ) ∂ y ψ i ∂ y ψ j δ ( y − π ) = p ( y ) r ( y ) ∂ y ψ i ∂ y ψ j | y = π = 0 , superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝑏 𝑖 𝑗 𝑘 superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript Ω d 𝑦 𝑝 𝑦 subscript 𝑦 subscript 𝜓 𝑖 subscript 𝑦 subscript 𝜓 𝑗 subscript 𝜓 𝑘 subscript Ω d 𝑦 𝑝 𝑦 𝑟 𝑦 subscript 𝑦 subscript 𝜓 𝑖 subscript 𝑦 subscript 𝜓 𝑗 superscript subscript 𝑘 0 𝑟 𝑦 subscript 𝜓 𝑘 subscript 𝜓 𝑘 𝜋 subscript Ω d 𝑦 𝑝 𝑦 𝑟 𝑦 subscript 𝑦 subscript 𝜓 𝑖 subscript 𝑦 subscript 𝜓 𝑗 𝛿 𝑦 𝜋 evaluated-at 𝑝 𝑦 𝑟 𝑦 subscript 𝑦 subscript 𝜓 𝑖 subscript 𝑦 subscript 𝜓 𝑗 𝑦 𝜋 0 \begin{split}\sum\limits_{k=0}^{\infty}\psi_{k}(\pi)b_{ijk}&=\sum\limits_{k=0}%^{\infty}\psi_{k}(\pi)\int_{\Omega}\text{d}y\ p(y)\partial_{y}\psi_{i}\partial%_{y}\psi_{j}\psi_{k}\\&=\int_{\Omega}\text{d}y\frac{p(y)}{r(y)}\partial_{y}\psi_{i}\partial_{y}\psi_%{j}\sum\limits_{k=0}^{\infty}\ r(y)\psi_{k}\psi_{k}(\pi)\\&=\int_{\Omega}\text{d}y\frac{p(y)}{r(y)}\partial_{y}\psi_{i}\partial_{y}\psi_%{j}\delta(y-\pi)\\&=\left.\frac{p(y)}{r(y)}\partial_{y}\psi_{i}\partial_{y}\psi_{j}\right|_{y=%\pi}=0\,,\end{split} start_ROW start_CELL ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT end_CELL start_CELL = ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_y italic_p ( italic_y ) ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_y divide start_ARG italic_p ( italic_y ) end_ARG start_ARG italic_r ( italic_y ) end_ARG ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_r ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_y divide start_ARG italic_p ( italic_y ) end_ARG start_ARG italic_r ( italic_y ) end_ARG ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT italic_δ ( italic_y - italic_π ) end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = divide start_ARG italic_p ( italic_y ) end_ARG start_ARG italic_r ( italic_y ) end_ARG ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT | start_POSTSUBSCRIPT italic_y = italic_π end_POSTSUBSCRIPT = 0 , end_CELL end_ROW (2.28)
where we have used the fact that r ( y ) ≠ 0 𝑟 𝑦 0 r(y)\neq 0 italic_r ( italic_y ) ≠ 0 and the boundary condition ∂ y ψ i ( y ) | y = π = 0 evaluated-at subscript 𝑦 subscript 𝜓 𝑖 𝑦 𝑦 𝜋 0 \partial_{y}\psi_{i}(y)|_{y=\pi}=0 ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_y ) | start_POSTSUBSCRIPT italic_y = italic_π end_POSTSUBSCRIPT = 0 .Finally, through Eq.(2.20 ), multiplying both sides by appropriate ψ ( π ) 𝜓 𝜋 \psi(\pi) italic_ψ ( italic_π ) s and summing it over the appropriate indices, it can be proved that
∑ k = 0 ∞ ψ k ( π ) b k i j = λ i ψ i ( π ) ψ j ( π ) . superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝑏 𝑘 𝑖 𝑗 subscript 𝜆 𝑖 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 \displaystyle\sum\limits_{k=0}^{\infty}\psi_{k}(\pi)b_{kij}=\lambda_{i}\psi_{i%}(\pi)\psi_{j}(\pi)\,. ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_b start_POSTSUBSCRIPT italic_k italic_i italic_j end_POSTSUBSCRIPT = italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) . (2.29)
Explicit verification of these sum rules can be found in App.A for LED and in Ref.[10 ] for the large μ 𝜇 \mu italic_μ limit of RS.
III Physics ApplicationsThe equation of motion for the y 𝑦 y italic_y -component of the gravitons shown in Eq.(2.5 ) can be matched to the SL equation with
p ( y ) = e − 3 ( l + 1 ) k | y | , 𝑝 𝑦 superscript 𝑒 3 𝑙 1 𝑘 𝑦 \displaystyle p(y)=e^{-3(l+1)k|y|}\,, italic_p ( italic_y ) = italic_e start_POSTSUPERSCRIPT - 3 ( italic_l + 1 ) italic_k | italic_y | end_POSTSUPERSCRIPT , q ( y ) = 0 , 𝑞 𝑦 0 \displaystyle q(y)=0\,, italic_q ( italic_y ) = 0 , r ( y ) = e 3 ( l − 1 ) k | y | , 𝑟 𝑦 superscript 𝑒 3 𝑙 1 𝑘 𝑦 \displaystyle r(y)=e^{3(l-1)k|y|}\,, italic_r ( italic_y ) = italic_e start_POSTSUPERSCRIPT 3 ( italic_l - 1 ) italic_k | italic_y | end_POSTSUPERSCRIPT , λ n = m n 2 . subscript 𝜆 𝑛 superscript subscript 𝑚 𝑛 2 \displaystyle\lambda_{n}=m_{n}^{2}\,. italic_λ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT = italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT . (3.1)
Matching to the boundary conditions of the orbifold symmetry, the SL problem is defined by vanishing derivatives on the boundary, i.e.
∂ ψ i ( 0 ) = ∂ ψ i ( π ) = 0 , subscript 𝜓 𝑖 0 subscript 𝜓 𝑖 𝜋 0 \partial\psi_{i}(0)=\partial\psi_{i}(\pi)=0\,, ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( 0 ) = ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) = 0 , (3.2)
which makes it regular according to the definition of Eq.(2.7 ).
The a 𝑎 a italic_a - and b 𝑏 b italic_b -coefficients enter the effective Lagrangian of the massive gravitons stemming from EH action, which can be decomposed as
ℒ ⊃ ∑ i j ( a i j O i j ( h 2 ) + b i j O i j ′ ( h 2 ) ) + ∑ i j k ( a i j k O i j k ( h 3 ) + b i j k O i j k ′ ( h 3 ) ) + … , subscript 𝑖 𝑗 subscript 𝑎 𝑖 𝑗 subscript 𝑂 𝑖 𝑗 superscript ℎ 2 subscript 𝑏 𝑖 𝑗 subscript superscript 𝑂 ′ 𝑖 𝑗 superscript ℎ 2 subscript 𝑖 𝑗 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝑂 𝑖 𝑗 𝑘 superscript ℎ 3 subscript 𝑏 𝑖 𝑗 𝑘 subscript superscript 𝑂 ′ 𝑖 𝑗 𝑘 superscript ℎ 3 … ℒ \begin{split}\mathcal{L}\supset&\,\sum\limits_{ij}\left(a_{ij}O_{ij}(h^{2})+b_%{ij}O^{\prime}_{ij}(h^{2})\right)\\&+\sum\limits_{ijk}\left(a_{ijk}O_{ijk}(h^{3})+b_{ijk}O^{\prime}_{ijk}(h^{3})%\right)+\dots\,,\end{split} start_ROW start_CELL caligraphic_L ⊃ end_CELL start_CELL ∑ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ( italic_a start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT italic_O start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ( italic_h start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) + italic_b start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT italic_O start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ( italic_h start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) ) end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL + ∑ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT italic_O start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_h start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ) + italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT italic_O start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_h start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ) ) + … , end_CELL end_ROW (3.3)
where O i j … ( h n ) subscript 𝑂 𝑖 𝑗 … superscript ℎ 𝑛 O_{ij\dots}(h^{n}) italic_O start_POSTSUBSCRIPT italic_i italic_j … end_POSTSUBSCRIPT ( italic_h start_POSTSUPERSCRIPT italic_n end_POSTSUPERSCRIPT ) is a an operator which involves n 𝑛 n italic_n -powers of the field h ℎ h italic_h with indices ( i , j , … ) 𝑖 𝑗 … (i,j,\dots) ( italic_i , italic_j , … ) . The superscript′ indicates that the operator originally contained two y 𝑦 y italic_y -derivatives that have been absorbed in the definition of the corresponding b 𝑏 b italic_b -coefficient (cfr.Eq.(2.16 )).The normalization of the 5D wave functions discussed in the previous section is such that the O ( ′ ) ( h 2 ) O^{(^{\prime})}(h^{2}) italic_O start_POSTSUPERSCRIPT ( start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ( italic_h start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) part of the Lagrangian reproduces the Fierz-Pauli Lagrangian for a tower of massive gravitons[1 ] .The explicit expansion up to 𝒪 ( h 4 ) 𝒪 superscript ℎ 4 \mathcal{O}(h^{4}) caligraphic_O ( italic_h start_POSTSUPERSCRIPT 4 end_POSTSUPERSCRIPT ) can be found e.g. in Ref.s[8 , 10 ] .Regarding the matter content, we can consider a toy model with a scalar field such that
S ⊃ ∫ y = π d 4 x − g ( 1 2 g μ ν ∂ μ ϕ ∂ ν ϕ − 1 2 m ϕ 2 ϕ 2 ) . subscript 𝑦 𝜋 superscript d 4 𝑥 𝑔 1 2 superscript 𝑔 𝜇 𝜈 subscript 𝜇 italic-ϕ subscript 𝜈 italic-ϕ 1 2 superscript subscript 𝑚 italic-ϕ 2 superscript italic-ϕ 2 𝑆 S\supset\int_{y=\pi}\text{d}^{4}x\sqrt{-g}\left(\dfrac{1}{2}g^{\mu\nu}\partial%_{\mu}\phi\partial_{\nu}\phi-\dfrac{1}{2}m_{\phi}^{2}\phi^{2}\right)\,. italic_S ⊃ ∫ start_POSTSUBSCRIPT italic_y = italic_π end_POSTSUBSCRIPT d start_POSTSUPERSCRIPT 4 end_POSTSUPERSCRIPT italic_x square-root start_ARG - italic_g end_ARG ( divide start_ARG 1 end_ARG start_ARG 2 end_ARG italic_g start_POSTSUPERSCRIPT italic_μ italic_ν end_POSTSUPERSCRIPT ∂ start_POSTSUBSCRIPT italic_μ end_POSTSUBSCRIPT italic_ϕ ∂ start_POSTSUBSCRIPT italic_ν end_POSTSUBSCRIPT italic_ϕ - divide start_ARG 1 end_ARG start_ARG 2 end_ARG italic_m start_POSTSUBSCRIPT italic_ϕ end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_ϕ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) . (3.4)
The conclusions do not depend on the matter content as long as it is localized on the brane.
Before moving forward, it is important to notice that LED, RS, and Clockwork share important features.To make it more manifest, we move to conformal coordinates, z 𝑧 z italic_z , defined such that the metric of Eq.(2.4 ) becomes conformally flat
d y = A 3 l d z , d s 2 = A ( z ) 2 ( η μ ν d x μ d x ν − d z 2 ) , formulae-sequence d 𝑦 superscript 𝐴 3 𝑙 d 𝑧 d superscript 𝑠 2 𝐴 superscript 𝑧 2 subscript 𝜂 𝜇 𝜈 d superscript 𝑥 𝜇 d superscript 𝑥 𝜈 d superscript 𝑧 2 \text{d}y=A^{3l}\text{d}z\,,\qquad\text{d}s^{2}=A(z)^{2}\left(\eta_{\mu\nu}%\text{d}x^{\mu}\text{d}x^{\nu}-\text{d}z^{2}\right)\,, d italic_y = italic_A start_POSTSUPERSCRIPT 3 italic_l end_POSTSUPERSCRIPT d italic_z , d italic_s start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = italic_A ( italic_z ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( italic_η start_POSTSUBSCRIPT italic_μ italic_ν end_POSTSUBSCRIPT d italic_x start_POSTSUPERSCRIPT italic_μ end_POSTSUPERSCRIPT d italic_x start_POSTSUPERSCRIPT italic_ν end_POSTSUPERSCRIPT - d italic_z start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) , (3.5)
whereA ( y ) ≡ e − k | y | 𝐴 𝑦 superscript 𝑒 𝑘 𝑦 A(y)\equiv e^{-k|y|} italic_A ( italic_y ) ≡ italic_e start_POSTSUPERSCRIPT - italic_k | italic_y | end_POSTSUPERSCRIPT .In such coordinates, the SL equation is determined by the functions
r ( z ) = p ( z ) = A 3 , q ( z ) = 0 . r(z)=p(z)=A^{3}\quad,\quad q(z)=0\,. italic_r ( italic_z ) = italic_p ( italic_z ) = italic_A start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT , italic_q ( italic_z ) = 0 . (3.6)
As formally there is no l 𝑙 l italic_l -dependent term in these coordinates, it shows that all these models are conformally flat and equivalent, and hence characterized by the same structure of gravitons interactions in the 4D EFT. Furthermore, as the couplings stemming from integrating out the 5th dimension formally follow the same equations, they obey the same sum rules.
We now exemplify the applications in the RS model by demonstrating that the higher s 𝑠 s italic_s powers of the amplitude vanish after the sum rules are applied. Here we draw on the results of Ref.[10 ] .It turns out that the leading term in the amplitude is ℳ ( 3 ) superscript ℳ 3 \mathcal{M}^{(3)} caligraphic_M start_POSTSUPERSCRIPT ( 3 ) end_POSTSUPERSCRIPT and depends solely on graviton interactions.Such amplitude is given by
ℳ ( 3 ) = − i sin 2 ( θ ) 24 M 5 3 λ i λ j ( ∑ k = 0 ∞ ψ k ( π ) a i j k − ψ i ( π ) ψ j ( π ) ) . superscript ℳ 3 𝑖 superscript 2 𝜃 24 superscript subscript 𝑀 5 3 subscript 𝜆 𝑖 subscript 𝜆 𝑗 superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 \mathcal{M}^{(3)}=-\frac{i\sin^{2}(\theta)}{24M_{5}^{3}\lambda_{i}\lambda_{j}}%\left(\sum\limits_{k=0}^{\infty}\psi_{k}(\pi)a_{ijk}-\psi_{i}(\pi)\psi_{j}(\pi%)\right)\,. caligraphic_M start_POSTSUPERSCRIPT ( 3 ) end_POSTSUPERSCRIPT = - divide start_ARG italic_i roman_sin start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( italic_θ ) end_ARG start_ARG 24 italic_M start_POSTSUBSCRIPT 5 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT end_ARG ( ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT - italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) ) . (3.7)
As can be seen, by means of Eq.(2.23 ) this contribution vanishes. Cancellation of the contributions to the amplitudes that scale slower than s 3 superscript 𝑠 3 s^{3} italic_s start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT involves at least one power of λ k , b i j k subscript 𝜆 𝑘 subscript 𝑏 𝑖 𝑗 𝑘
\lambda_{k},b_{ijk} italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT , italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT and/or the scalar mode of the metric, the radion.The former sum rules are relevant to cancelling manyterms of the amplitude both at ℳ ( 2 ) superscript ℳ 2 \mathcal{M}^{(2)} caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT and ℳ ( 3 / 2 ) superscript ℳ 3 2 \mathcal{M}^{(3/2)} caligraphic_M start_POSTSUPERSCRIPT ( 3 / 2 ) end_POSTSUPERSCRIPT which are proportional to
ℳ ( 2 ) , ( 3 / 2 ) ∝ ( ∑ k = 0 ∞ ψ k ( π ) λ k a i j k − ψ i ( π ) ψ j ( π ) ( λ i + λ j ) ) . proportional-to superscript ℳ 2 3 2
superscript subscript 𝑘 0 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 subscript 𝜆 𝑖 subscript 𝜆 𝑗 \mathcal{M}^{(2),(3/2)}\propto\left(\sum\limits_{k=0}^{\infty}\psi_{k}(\pi)%\lambda_{k}a_{ijk}-\psi_{i}(\pi)\psi_{j}(\pi)(\lambda_{i}+\lambda_{j})\right)\,. caligraphic_M start_POSTSUPERSCRIPT ( 2 ) , ( 3 / 2 ) end_POSTSUPERSCRIPT ∝ ( ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT - italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) ) . (3.8)
They vanish due to Eq.(2.26 ).Instead, the latter ones, which include the radion must be treated differently as the radion wave function and its normalization are not generated directly by the same SL equation of the gravitons.As the treatment of the radion cannot be done in full generality as we did for the graviton, we show the amplitudes and the sum rules that cure them in the following section for the RS model.
IV RS Radion Sum Rules beyond the large-μ 𝜇 \mu italic_μ limitWe supplement the linear sum rules presented the previous section with two additional sum rules that involve the radion in the RS model. These are needed to cancel all contributions to the amplitude that grow faster than s 𝑠 s italic_s , as shown for the scattering of KK-gravitons both in unstabilized and stabilized models[27 , 11 ] . The missing contribution corresponds to the diagram in Fig.2(a) . We showed this previously in Ref.[10 ] for the limiting case μ ≡ k R ≫ 1 𝜇 𝑘 𝑅 much-greater-than 1 \mu\equiv kR\gg 1 italic_μ ≡ italic_k italic_R ≫ 1 . The proof was built on the explicit solutions of the SL equation in this limit. The following discussion generalizes this to any value of μ 𝜇 \mu italic_μ . Analogous results were published simultaneously to this paper in Ref.[12 ] .
We work in the so-called unitary gauge , i.e. in coordinates in which only the tensorial and scalar excitations of the metric are physical.Similar techniques can be employed also for different theories.
The couplings involving the radion are special as they do not depend on the SL functions, ψ i subscript 𝜓 𝑖 \psi_{i} italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT . Their impact in the 4D effective couplings shows up by adding extra powers of e − k | y | superscript 𝑒 𝑘 𝑦 e^{-k|y|} italic_e start_POSTSUPERSCRIPT - italic_k | italic_y | end_POSTSUPERSCRIPT in the n-points integrals of the gravitons. Even though this seems to eliminate the precious insights given to us by the SL theory, a geometric underlying connection inherited by the original 5D theory is still present.We will work in conformal coordinates, z ( y ) 𝑧 𝑦 z(y) italic_z ( italic_y ) , as defined in Eq.(3.5 ).In such coordinates the difference in the definitions between a a 𝑎 a italic_a and b 𝑏 b italic_b coefficients is merely given by the presence of the two derivatives ∂ z subscript 𝑧 \partial_{z} ∂ start_POSTSUBSCRIPT italic_z end_POSTSUBSCRIPT in the b 𝑏 b italic_b -one, while they have the same powers of A ( z ) 𝐴 𝑧 A(z) italic_A ( italic_z ) .
In conformal coordinates, for every radion in the expansion, a factor of A − 2 superscript 𝐴 2 A^{-2} italic_A start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT comes along. Therefore we define the couplings with the radion as
ψ r 2 ∫ Ω d z A ( z ) − 1 ≡ 1 , superscript subscript 𝜓 𝑟 2 subscript Ω d 𝑧 𝐴 superscript 𝑧 1 1 \displaystyle\psi_{r}^{2}\int_{\Omega}\text{d}z\,\,A(z)^{-1}\equiv 1\,, italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT ≡ 1 , (4.1) b i j r ≡ ψ r ∫ Ω d z A ( z ) ∂ z ψ i ∂ z ψ j , subscript 𝑏 𝑖 𝑗 𝑟 subscript 𝜓 𝑟 subscript Ω d 𝑧 𝐴 𝑧 subscript 𝑧 subscript 𝜓 𝑖 subscript 𝑧 subscript 𝜓 𝑗 \displaystyle b_{ijr}\equiv\psi_{r}\int_{\Omega}\text{d}z\,\,A(z)\partial_{z}%\psi_{i}\partial_{z}\psi_{j}\ \,, italic_b start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT ≡ italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) ∂ start_POSTSUBSCRIPT italic_z end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ start_POSTSUBSCRIPT italic_z end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT , (4.2) a i r r ≡ ψ r 2 ∫ Ω d z A ( z ) − 1 ψ i . subscript 𝑎 𝑖 𝑟 𝑟 superscript subscript 𝜓 𝑟 2 subscript Ω d 𝑧 𝐴 superscript 𝑧 1 subscript 𝜓 𝑖 \displaystyle a_{irr}\equiv\psi_{r}^{2}\int_{\Omega}\text{d}z\,\,A(z)^{-1}\psi%_{i}\,. italic_a start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT ≡ italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT . (4.3)
With a slight abuse of notation, we write ψ n ( π ) subscript 𝜓 𝑛 𝜋 \psi_{n}(\pi) italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_π ) or z = π 𝑧 𝜋 z=\pi italic_z = italic_π in the integrals, which have to be understood as z = π ∼ z ( y = π ) 𝑧 𝜋 similar-to 𝑧 𝑦 𝜋 z=\pi\sim z(y=\pi) italic_z = italic_π ∼ italic_z ( italic_y = italic_π ) .
After imposing the graviton sum rules already derived, the non-vanishing amplitude at order ℳ ( 2 ) superscript ℳ 2 \mathcal{M}^{(2)} caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT reads
ℳ ( 2 ) = − i s 2 36 M 5 3 λ i λ j [ ∑ k = 1 ∞ ψ k ( π ) λ k a i j k − ( λ i − λ j ) 2 ( λ i + λ j ) [ ψ i ( π ) ψ j ( π ) + ψ 0 2 δ i j ] + 6 ψ r b i j r ] . superscript ℳ 2 𝑖 superscript 𝑠 2 36 superscript subscript 𝑀 5 3 subscript 𝜆 𝑖 subscript 𝜆 𝑗 delimited-[] superscript subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 superscript subscript 𝜆 𝑖 subscript 𝜆 𝑗 2 subscript 𝜆 𝑖 subscript 𝜆 𝑗 delimited-[] subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 superscript subscript 𝜓 0 2 subscript 𝛿 𝑖 𝑗 6 subscript 𝜓 𝑟 subscript 𝑏 𝑖 𝑗 𝑟 \begin{split}\mathcal{M}^{(2)}=-\frac{is^{2}}{36M_{5}^{3}\lambda_{i}\lambda_{j%}}&\left[\sum_{k=1}^{\infty}\dfrac{\psi_{k}(\pi)}{\lambda_{k}}a_{ijk}-\left(%\lambda_{i}-\lambda_{j}\right)^{2}(\lambda_{i}+\lambda_{j})\left[\psi_{i}(\pi)%\psi_{j}(\pi)+\psi_{0}^{2}\delta_{ij}\right]+6\psi_{r}b_{ijr}\right]\,.\end{split} start_ROW start_CELL caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT = - divide start_ARG italic_i italic_s start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG start_ARG 36 italic_M start_POSTSUBSCRIPT 5 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT end_ARG end_CELL start_CELL [ ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT - ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) [ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) + italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ] + 6 italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_b start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT ] . end_CELL end_ROW (4.4)
At order ℳ ( 3 / 2 ) superscript ℳ 3 2 \mathcal{M}^{(3/2)} caligraphic_M start_POSTSUPERSCRIPT ( 3 / 2 ) end_POSTSUPERSCRIPT the amplitudes vanish employing the graviton sum rules, leaving the final amplitude of order ℳ ( 1 ) superscript ℳ 1 \mathcal{M}^{(1)} caligraphic_M start_POSTSUPERSCRIPT ( 1 ) end_POSTSUPERSCRIPT .Finally, we consider matter annihilation into a massive graviton and a radion, ϕ ϕ → h i r → italic-ϕ italic-ϕ subscript ℎ 𝑖 𝑟 \phi\phi\to h_{i}r italic_ϕ italic_ϕ → italic_h start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_r . The corresponding diagrams are shown in Fig.2(b) . The leading amplitude is of order ℳ ( 2 ) superscript ℳ 2 \mathcal{M}^{(2)} caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT and reads
ℳ ( 2 ) = i s 2 24 M 5 3 λ i ( ∑ k = 1 ∞ ψ k ( π ) b i k r λ k + ψ r a i r r − A π − 2 ψ r ψ i ( π ) ) , superscript ℳ 2 𝑖 superscript 𝑠 2 24 superscript subscript 𝑀 5 3 subscript 𝜆 𝑖 superscript subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝑏 𝑖 𝑘 𝑟 subscript 𝜆 𝑘 subscript 𝜓 𝑟 subscript 𝑎 𝑖 𝑟 𝑟 superscript subscript 𝐴 𝜋 2 subscript 𝜓 𝑟 subscript 𝜓 𝑖 𝜋 \mathcal{M}^{(2)}=\dfrac{is^{2}}{24M_{5}^{3}\lambda_{i}}\left(\sum\limits_{k=1%}^{\infty}\psi_{k}(\pi)\frac{b_{ikr}}{\lambda_{k}}+\psi_{r}a_{irr}-A_{\pi}^{-2%}\psi_{r}\psi_{i}(\pi)\right)\,, caligraphic_M start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT = divide start_ARG italic_i italic_s start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG start_ARG 24 italic_M start_POSTSUBSCRIPT 5 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT end_ARG ( ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) divide start_ARG italic_b start_POSTSUBSCRIPT italic_i italic_k italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG + italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT - italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) ) , (4.5)
To cancel these contributions two additional sum rules are required.
The starting point for their proof is the identity
A ( π ) α − 3 f ( π ) = ∑ n = 0 ∞ ψ n ( π ) ( ∫ Ω d z A ( z ) α ψ n ( z ) f ( z ) ) . 𝐴 superscript 𝜋 𝛼 3 𝑓 𝜋 superscript subscript 𝑛 0 subscript 𝜓 𝑛 𝜋 subscript Ω d 𝑧 𝐴 superscript 𝑧 𝛼 subscript 𝜓 𝑛 𝑧 𝑓 𝑧 A(\pi)^{\alpha-3}f(\pi)=\sum\limits_{n=0}^{\infty}\psi_{n}(\pi)\left(\int_{%\Omega}\text{d}z\,A(z)^{\alpha}\psi_{n}(z)f(z)\right)\,. italic_A ( italic_π ) start_POSTSUPERSCRIPT italic_α - 3 end_POSTSUPERSCRIPT italic_f ( italic_π ) = ∑ start_POSTSUBSCRIPT italic_n = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_π ) ( ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) start_POSTSUPERSCRIPT italic_α end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_z ) italic_f ( italic_z ) ) . (4.6)
Through the SL equation and integrating by parts, we can write the above expression as
A ( π ) α − 3 f ( π ) = ψ 0 2 ( ∫ Ω d z A ( z ) α f ( z ) ) + ∑ n = 1 ∞ ψ n ( π ) λ n ( ∫ Ω d z A ( z ) 3 ∂ ψ n ( z ) ∂ ( A α − 3 f ( z ) ) ) . 𝐴 superscript 𝜋 𝛼 3 𝑓 𝜋 superscript subscript 𝜓 0 2 subscript Ω d 𝑧 𝐴 superscript 𝑧 𝛼 𝑓 𝑧 superscript subscript 𝑛 1 subscript 𝜓 𝑛 𝜋 subscript 𝜆 𝑛 subscript Ω d 𝑧 𝐴 superscript 𝑧 3 subscript 𝜓 𝑛 𝑧 superscript 𝐴 𝛼 3 𝑓 𝑧 \begin{split}&A(\pi)^{\alpha-3}f(\pi)=\psi_{0}^{2}\left(\int_{\Omega}\text{d}z%\,A(z)^{\alpha}f(z)\right)\\&+\sum\limits_{n=1}^{\infty}\dfrac{\psi_{n}(\pi)}{\lambda_{n}}\left(\int_{%\Omega}\text{d}z\,A(z)^{3}\partial\psi_{n}(z)\partial\left(A^{\alpha-3}f(z)%\right)\right)\,.\end{split} start_ROW start_CELL end_CELL start_CELL italic_A ( italic_π ) start_POSTSUPERSCRIPT italic_α - 3 end_POSTSUPERSCRIPT italic_f ( italic_π ) = italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) start_POSTSUPERSCRIPT italic_α end_POSTSUPERSCRIPT italic_f ( italic_z ) ) end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL + ∑ start_POSTSUBSCRIPT italic_n = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_ARG ( ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_z ) ∂ ( italic_A start_POSTSUPERSCRIPT italic_α - 3 end_POSTSUPERSCRIPT italic_f ( italic_z ) ) ) . end_CELL end_ROW (4.7)
Now we choose
f ( z ) = A ( z ) 3 − α ∫ π z d z ′ A β − 3 g ( z ′ ) , 𝑓 𝑧 𝐴 superscript 𝑧 3 𝛼 superscript subscript 𝜋 𝑧 d superscript 𝑧 ′ superscript 𝐴 𝛽 3 𝑔 superscript 𝑧 ′ f(z)=A(z)^{3-\alpha}\int\limits_{\pi}^{z}\text{d}z^{\prime}A^{\beta-3}g(z^{%\prime})\,, italic_f ( italic_z ) = italic_A ( italic_z ) start_POSTSUPERSCRIPT 3 - italic_α end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT d italic_z start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT italic_β - 3 end_POSTSUPERSCRIPT italic_g ( italic_z start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) , (4.8)
such that we obtain the result of interest:
∑ n = 1 ∞ ψ n ( π ) λ n ( ∫ Ω d z A ( z ) β ∂ ψ n ( z ) g ( z ) ) = − ψ 0 2 ( ∫ Ω d z A ( z ) 3 ∫ π z d z ′ A β − 3 g ( z ′ ) ) . superscript subscript 𝑛 1 subscript 𝜓 𝑛 𝜋 subscript 𝜆 𝑛 subscript Ω d 𝑧 𝐴 superscript 𝑧 𝛽 subscript 𝜓 𝑛 𝑧 𝑔 𝑧 superscript subscript 𝜓 0 2 subscript Ω d 𝑧 𝐴 superscript 𝑧 3 superscript subscript 𝜋 𝑧 d superscript 𝑧 ′ superscript 𝐴 𝛽 3 𝑔 superscript 𝑧 ′ \begin{split}&\sum\limits_{n=1}^{\infty}\dfrac{\psi_{n}(\pi)}{\lambda_{n}}%\left(\int_{\Omega}\text{d}z\,A(z)^{\beta}\partial\psi_{n}(z)g(z)\right)\\&=-\psi_{0}^{2}\left(\int_{\Omega}\text{d}z\,A(z)^{3}\int\limits_{\pi}^{z}%\text{d}z^{\prime}A^{\beta-3}g(z^{\prime})\right)\,.\end{split} start_ROW start_CELL end_CELL start_CELL ∑ start_POSTSUBSCRIPT italic_n = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT end_ARG ( ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) start_POSTSUPERSCRIPT italic_β end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_z ) italic_g ( italic_z ) ) end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = - italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT d italic_z start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT italic_β - 3 end_POSTSUPERSCRIPT italic_g ( italic_z start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) ) . end_CELL end_ROW (4.9)
This will be a key ingredient to proving the two sum rules. Its convenience relies upon the possibility of trading the infinite sum with the eigenvalues in the denominator into a double integral.
Sum Rule 1. We are interested in computing
∑ k = 1 ∞ ψ k ( π ) λ k a i j k ( λ i − λ j ) 2 . superscript subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 superscript subscript 𝜆 𝑖 subscript 𝜆 𝑗 2 \sum\limits_{k=1}^{\infty}\dfrac{\psi_{k}(\pi)}{\lambda_{k}}a_{ijk}(\lambda_{i%}-\lambda_{j})^{2}\,. ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT . (4.10)
By means of Eq.(4.9 ), it is convenient to define the following quantity
S i j ≡ ∑ k = 1 ∞ ψ k ( π ) λ k b k i j = − ψ 0 2 ∫ Ω d z A 3 ∫ π z ∂ ψ i ψ j . subscript 𝑆 𝑖 𝑗 superscript subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑏 𝑘 𝑖 𝑗 superscript subscript 𝜓 0 2 subscript Ω d 𝑧 superscript 𝐴 3 superscript subscript 𝜋 𝑧 subscript 𝜓 𝑖 subscript 𝜓 𝑗 S_{ij}\equiv\sum\limits_{k=1}^{\infty}\dfrac{\psi_{k}(\pi)}{\lambda_{k}}b_{kij%}=-\psi_{0}^{2}\int_{\Omega}\text{d}z\,A^{3}\int\limits_{\pi}^{z}\partial\psi_%{i}\psi_{j}\,. italic_S start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ≡ ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG italic_b start_POSTSUBSCRIPT italic_k italic_i italic_j end_POSTSUBSCRIPT = - italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT . (4.11)
Via Eq.(2.21 ), one proves that the problem is then reduced to
∑ k = 1 ∞ ψ k ( π ) λ k a i j k ( λ i − λ j ) 2 = ( S i j − S j i ) ( λ i − λ j ) . superscript subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 superscript subscript 𝜆 𝑖 subscript 𝜆 𝑗 2 subscript 𝑆 𝑖 𝑗 subscript 𝑆 𝑗 𝑖 subscript 𝜆 𝑖 subscript 𝜆 𝑗 \sum\limits_{k=1}^{\infty}\dfrac{\psi_{k}(\pi)}{\lambda_{k}}a_{ijk}(\lambda_{i%}-\lambda_{j})^{2}=(S_{ij}-S_{ji})(\lambda_{i}-\lambda_{j})\,. ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = ( italic_S start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT - italic_S start_POSTSUBSCRIPT italic_j italic_i end_POSTSUBSCRIPT ) ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) . (4.12)
The strategy is to prove two independent relations for S i j subscript 𝑆 𝑖 𝑗 S_{ij} italic_S start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT and to solve for the latter.The first relation can be obtained pretty straightforwardly
S i j + S j i = ψ i ( π ) ψ j ( π ) − ψ 0 2 δ i j . subscript 𝑆 𝑖 𝑗 subscript 𝑆 𝑗 𝑖 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 superscript subscript 𝜓 0 2 subscript 𝛿 𝑖 𝑗 S_{ij}+S_{ji}=\psi_{i}(\pi)\psi_{j}(\pi)-\psi_{0}^{2}\delta_{ij}\,. italic_S start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT + italic_S start_POSTSUBSCRIPT italic_j italic_i end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) - italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT . (4.13)
The second one requires some work. Similarly to the approach used in Ref.[10 ] , we consider
λ j S i j + λ i S j i = ψ 0 2 [ λ i δ i j + 6 ∫ Ω d z A 3 ∫ π z d z ′ A − 1 ∂ ( A ) ∂ ψ i ∂ ψ j ] subscript 𝜆 𝑗 subscript 𝑆 𝑖 𝑗 subscript 𝜆 𝑖 subscript 𝑆 𝑗 𝑖 superscript subscript 𝜓 0 2 delimited-[] subscript 𝜆 𝑖 subscript 𝛿 𝑖 𝑗 6 subscript Ω d 𝑧 superscript 𝐴 3 superscript subscript 𝜋 𝑧 d superscript 𝑧 ′ superscript 𝐴 1 𝐴 subscript 𝜓 𝑖 subscript 𝜓 𝑗 \begin{split}&\lambda_{j}S_{ij}+\lambda_{i}S_{ji}\\&=\psi_{0}^{2}\left[\lambda_{i}\delta_{ij}+6\int_{\Omega}\text{d}z\,A^{3}\int%\limits_{\pi}^{z}\text{d}z^{\prime}A^{-1}\partial(A)\partial\psi_{i}\partial%\psi_{j}\right]\end{split} start_ROW start_CELL end_CELL start_CELL italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT italic_S start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_S start_POSTSUBSCRIPT italic_j italic_i end_POSTSUBSCRIPT end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT [ italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT + 6 ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT d italic_z start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT ∂ ( italic_A ) ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ] end_CELL end_ROW (4.14)
Given that d y ≡ A d z d 𝑦 𝐴 d 𝑧 \text{d}y\equiv A\text{d}z d italic_y ≡ italic_A d italic_z , it follows that3 3 3 This is the point where the value of l 𝑙 l italic_l would have entered the discussion if we kept it unspecified.
∂ z ( A ) = A ∂ y ( e − μ y ) = − μ A 2 . subscript 𝑧 𝐴 𝐴 subscript 𝑦 superscript 𝑒 𝜇 𝑦 𝜇 superscript 𝐴 2 \partial_{z}(A)=A\partial_{y}(e^{-\mu y})=-\mu A^{2}\,. ∂ start_POSTSUBSCRIPT italic_z end_POSTSUBSCRIPT ( italic_A ) = italic_A ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT ( italic_e start_POSTSUPERSCRIPT - italic_μ italic_y end_POSTSUPERSCRIPT ) = - italic_μ italic_A start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT . (4.15)
Let us denote for simplicity A π ≡ A ( π ) subscript 𝐴 𝜋 𝐴 𝜋 A_{\pi}\equiv A(\pi) italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT ≡ italic_A ( italic_π ) and consider
∫ Ω d z A β ∂ ψ i ∂ ψ j = ∫ Ω d z A α + β ∂ ∫ π z A − α ∂ ψ i ∂ ψ j = A π α + β ∫ Ω d z A − α ∂ ψ i ∂ ψ j − ( α + β ) ∫ Ω d z A α + β − 1 ∂ ( A ) ∫ π z A − α ∂ ψ i ∂ ψ j = A π α + β ∫ Ω d z A − α ∂ ψ i ∂ ψ j + μ ( α + β ) ∫ Ω d z A α + β + 1 ∫ π z A − α ∂ ψ i ∂ ψ j , subscript Ω d 𝑧 superscript 𝐴 𝛽 subscript 𝜓 𝑖 subscript 𝜓 𝑗 subscript Ω d 𝑧 superscript 𝐴 𝛼 𝛽 superscript subscript 𝜋 𝑧 superscript 𝐴 𝛼 subscript 𝜓 𝑖 subscript 𝜓 𝑗 superscript subscript 𝐴 𝜋 𝛼 𝛽 subscript Ω d 𝑧 superscript 𝐴 𝛼 subscript 𝜓 𝑖 subscript 𝜓 𝑗 𝛼 𝛽 subscript Ω d 𝑧 superscript 𝐴 𝛼 𝛽 1 𝐴 superscript subscript 𝜋 𝑧 superscript 𝐴 𝛼 subscript 𝜓 𝑖 subscript 𝜓 𝑗 superscript subscript 𝐴 𝜋 𝛼 𝛽 subscript Ω d 𝑧 superscript 𝐴 𝛼 subscript 𝜓 𝑖 subscript 𝜓 𝑗 𝜇 𝛼 𝛽 subscript Ω d 𝑧 superscript 𝐴 𝛼 𝛽 1 superscript subscript 𝜋 𝑧 superscript 𝐴 𝛼 subscript 𝜓 𝑖 subscript 𝜓 𝑗 \begin{split}\int_{\Omega}\text{d}z\,A^{\beta}\partial\psi_{i}\partial\psi_{j}%&=\int_{\Omega}\text{d}z\,A^{\alpha+\beta}\partial\int\limits_{\pi}^{z}A^{-%\alpha}\partial\psi_{i}\partial\psi_{j}=A_{\pi}^{\alpha+\beta}\int_{\Omega}%\text{d}z\,A^{-\alpha}\partial\psi_{i}\partial\psi_{j}-(\alpha+\beta)\int_{%\Omega}\text{d}z\,A^{\alpha+\beta-1}\partial(A)\int\limits_{\pi}^{z}A^{-\alpha%}\partial\psi_{i}\partial\psi_{j}\\&=A_{\pi}^{\alpha+\beta}\int_{\Omega}\text{d}z\,A^{-\alpha}\partial\psi_{i}%\partial\psi_{j}+\mu(\alpha+\beta)\int_{\Omega}\text{d}z\,A^{\alpha+\beta+1}%\int\limits_{\pi}^{z}A^{-\alpha}\partial\psi_{i}\partial\psi_{j}\,,\end{split} start_ROW start_CELL ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT italic_β end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT end_CELL start_CELL = ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT italic_α + italic_β end_POSTSUPERSCRIPT ∂ ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT - italic_α end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT = italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_α + italic_β end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT - italic_α end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT - ( italic_α + italic_β ) ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT italic_α + italic_β - 1 end_POSTSUPERSCRIPT ∂ ( italic_A ) ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT - italic_α end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_α + italic_β end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT - italic_α end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT + italic_μ ( italic_α + italic_β ) ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT italic_α + italic_β + 1 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT - italic_α end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT , end_CELL end_ROW (4.16)
where we have also used that A ( − π ) = A ( π ) . 𝐴 𝜋 𝐴 𝜋 A(-\pi)=A(\pi). italic_A ( - italic_π ) = italic_A ( italic_π ) . We choose α = − 1 𝛼 1 \alpha=-1 italic_α = - 1 and β = 3 𝛽 3 \beta=3 italic_β = 3 to find
λ i δ i j = A π 2 ∫ Ω d z A ∂ ψ i ∂ ψ j + 2 μ ∫ Ω d z A 3 ∫ π z A ∂ ψ i ∂ ψ j , = A π 2 ∫ Ω d z A ∂ ψ i ∂ ψ j − 2 ∫ Ω d z A 3 ∫ π z A − 1 ∂ ( A ) ∂ ψ i ∂ ψ j . \begin{split}&\lambda_{i}\delta_{ij}=A_{\pi}^{2}\int_{\Omega}\text{d}z\,A%\partial\psi_{i}\partial\psi_{j}+2\mu\int_{\Omega}\text{d}z\,A^{3}\int\limits_%{\pi}^{z}A\partial\psi_{i}\partial\psi_{j}\,,\\&\,=A_{\pi}^{2}\int_{\Omega}\text{d}z\,A\partial\psi_{i}\partial\psi_{j}-2\int%_{\Omega}\text{d}z\,A^{3}\int\limits_{\pi}^{z}A^{-1}\partial(A)\partial\psi_{i%}\partial\psi_{j}\,.\end{split} start_ROW start_CELL end_CELL start_CELL italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT = italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT + 2 italic_μ ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_A ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT , end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT - 2 ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT ∂ ( italic_A ) ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT . end_CELL end_ROW (4.17)
We shuffle the terms to write the main result of interest
2 ∫ Ω d z A 3 ∫ π z A − 1 ∂ ( A ) ∂ ψ i ∂ ψ j = − λ i δ i j + A π 2 ∫ Ω d z A ∂ ψ i ∂ ψ j . 2 subscript Ω d 𝑧 superscript 𝐴 3 superscript subscript 𝜋 𝑧 superscript 𝐴 1 𝐴 subscript 𝜓 𝑖 subscript 𝜓 𝑗 subscript 𝜆 𝑖 subscript 𝛿 𝑖 𝑗 superscript subscript 𝐴 𝜋 2 subscript Ω d 𝑧 𝐴 subscript 𝜓 𝑖 subscript 𝜓 𝑗 \begin{split}&2\int_{\Omega}\text{d}z\,A^{3}\int\limits_{\pi}^{z}A^{-1}%\partial(A)\partial\psi_{i}\partial\psi_{j}\\&=-\lambda_{i}\delta_{ij}+A_{\pi}^{2}\int_{\Omega}\text{d}z\,A\partial\psi_{i}%\partial\psi_{j}\,.\end{split} start_ROW start_CELL end_CELL start_CELL 2 ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT ∂ ( italic_A ) ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = - italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT + italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT . end_CELL end_ROW (4.18)
We recognize the last integral term to be ψ r − 1 b i j r superscript subscript 𝜓 𝑟 1 subscript 𝑏 𝑖 𝑗 𝑟 \psi_{r}^{-1}b_{ijr} italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT italic_b start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT .This implies that
λ j S i j + λ i S j i = ψ 0 2 { 3 A π 2 ψ r − 1 b i j r − 2 λ i δ i j } . subscript 𝜆 𝑗 subscript 𝑆 𝑖 𝑗 subscript 𝜆 𝑖 subscript 𝑆 𝑗 𝑖 superscript subscript 𝜓 0 2 3 superscript subscript 𝐴 𝜋 2 superscript subscript 𝜓 𝑟 1 subscript 𝑏 𝑖 𝑗 𝑟 2 subscript 𝜆 𝑖 subscript 𝛿 𝑖 𝑗 \lambda_{j}S_{ij}+\lambda_{i}S_{ji}=\psi_{0}^{2}\left\{3A_{\pi}^{2}\psi_{r}^{-%1}b_{ijr}-2\lambda_{i}\delta_{ij}\right\}\,. italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT italic_S start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_S start_POSTSUBSCRIPT italic_j italic_i end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT { 3 italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT italic_b start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT - 2 italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT } . (4.19)
By combining Eq.s(4.13 )-(4.19 ), we get the desired result
S i j = λ i [ ψ i ( π ) ψ j ( π ) + ψ 0 2 δ i j ] − 3 A π 2 ψ r − 1 ψ 0 2 b i j r λ i − λ j , subscript 𝑆 𝑖 𝑗 subscript 𝜆 𝑖 delimited-[] subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 superscript subscript 𝜓 0 2 subscript 𝛿 𝑖 𝑗 3 superscript subscript 𝐴 𝜋 2 superscript subscript 𝜓 𝑟 1 superscript subscript 𝜓 0 2 subscript 𝑏 𝑖 𝑗 𝑟 subscript 𝜆 𝑖 subscript 𝜆 𝑗 \begin{split}&S_{ij}=\dfrac{\lambda_{i}\left[\psi_{i}(\pi)\psi_{j}(\pi)+\psi_{%0}^{2}\delta_{ij}\right]-3A_{\pi}^{2}\psi_{r}^{-1}\psi_{0}^{2}b_{ijr}}{\lambda%_{i}-\lambda_{j}}\,,\end{split} start_ROW start_CELL end_CELL start_CELL italic_S start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT = divide start_ARG italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT [ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) + italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ] - 3 italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_b start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT end_ARG , end_CELL end_ROW (4.20)
which finally leads to
∑ k = 1 ∞ ψ k ( π ) λ k a i j k ( λ i − λ j ) 2 = ( S i j − S j i ) ( λ i − λ j ) , = ( λ i + λ j ) [ ψ i ( π ) ψ j ( π ) + ψ 0 2 δ i j ] − 6 A π 2 ψ r − 1 ψ 0 2 b i j r . \begin{split}&\sum\limits_{k=1}^{\infty}\dfrac{\psi_{k}(\pi)}{\lambda_{k}}a_{%ijk}(\lambda_{i}-\lambda_{j})^{2}=(S_{ij}-S_{ji})(\lambda_{i}-\lambda_{j})\,,%\\&\,=(\lambda_{i}+\lambda_{j})\left[\psi_{i}(\pi)\psi_{j}(\pi)+\psi_{0}^{2}%\delta_{ij}\right]-6A_{\pi}^{2}\psi_{r}^{-1}\psi_{0}^{2}b_{ijr}\,.\end{split} start_ROW start_CELL end_CELL start_CELL ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = ( italic_S start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT - italic_S start_POSTSUBSCRIPT italic_j italic_i end_POSTSUBSCRIPT ) ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) , end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) [ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) + italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ] - 6 italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_b start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT . end_CELL end_ROW (4.21)
The presence of ψ 0 2 superscript subscript 𝜓 0 2 \psi_{0}^{2} italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT is at first glance somewhat surprising as the massless graviton does not necessarily play a role in the amplitude, e.g. in the limit μ ≫ 1 much-greater-than 𝜇 1 \mu\gg 1 italic_μ ≫ 1 . We can replace it by noticing that
ψ 0 2 = μ 1 − e − 2 μ π , ψ r 2 = μ e 2 π μ − 1 , \psi_{0}^{2}=\frac{\mu}{1-e^{-2\mu\pi}}\quad,\quad\psi_{r}^{2}=\frac{\mu}{e^{2%\pi\mu}-1}\,, italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = divide start_ARG italic_μ end_ARG start_ARG 1 - italic_e start_POSTSUPERSCRIPT - 2 italic_μ italic_π end_POSTSUPERSCRIPT end_ARG , italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = divide start_ARG italic_μ end_ARG start_ARG italic_e start_POSTSUPERSCRIPT 2 italic_π italic_μ end_POSTSUPERSCRIPT - 1 end_ARG , (4.22)
so that
ψ 0 2 A π 2 = ψ r 2 . superscript subscript 𝜓 0 2 superscript subscript 𝐴 𝜋 2 superscript subscript 𝜓 𝑟 2 \psi_{0}^{2}A_{\pi}^{2}=\psi_{r}^{2}\,. italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT . (4.23)
Thus we have
∑ k = 1 ∞ ψ k ( π ) λ k a i j k ( λ i − λ j ) 2 = ( λ i + λ j ) [ ψ i ( π ) ψ j ( π ) + ψ 0 2 δ i j ] − 6 ψ r b i j r . superscript subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝜆 𝑘 subscript 𝑎 𝑖 𝑗 𝑘 superscript subscript 𝜆 𝑖 subscript 𝜆 𝑗 2 subscript 𝜆 𝑖 subscript 𝜆 𝑗 delimited-[] subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑗 𝜋 superscript subscript 𝜓 0 2 subscript 𝛿 𝑖 𝑗 6 subscript 𝜓 𝑟 subscript 𝑏 𝑖 𝑗 𝑟 \begin{split}&\sum\limits_{k=1}^{\infty}\dfrac{\psi_{k}(\pi)}{\lambda_{k}}a_{%ijk}(\lambda_{i}-\lambda_{j})^{2}\\&=(\lambda_{i}+\lambda_{j})\left[\psi_{i}(\pi)\psi_{j}(\pi)+\psi_{0}^{2}\delta%_{ij}\right]-6\psi_{r}b_{ijr}\,.\end{split} start_ROW start_CELL end_CELL start_CELL ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = ( italic_λ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT + italic_λ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ) [ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_π ) + italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ] - 6 italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_b start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT . end_CELL end_ROW (4.24)
This proves the sum rule.
Sum Rule 2. We use Eq.(4.9 ) to find
∑ k = 1 ∞ ψ k ( π ) b k i r λ k = − ψ 0 2 ψ r ∫ Ω d z A ( z ) 3 ∫ π z A − 2 ∂ ψ i , = − ψ 0 2 ψ r [ c i − ψ 0 − 2 A π − 2 ψ i ( π ) − 2 μ ∫ Ω d z A 3 ∫ π z ψ i A − 1 ] , \begin{split}&\sum\limits_{k=1}^{\infty}\psi_{k}(\pi)\dfrac{b_{kir}}{\lambda_{%k}}=-\psi_{0}^{2}\psi_{r}\int_{\Omega}\text{d}z\,A(z)^{3}\int\limits_{\pi}^{z}%A^{-2}\partial\psi_{i}\,,\\&\,=-\psi_{0}^{2}\psi_{r}\left[c_{i}-\psi_{0}^{-2}A_{\pi}^{-2}\psi_{i}(\pi)-2%\mu\int_{\Omega}\text{d}z\,A^{3}\int\limits_{\pi}^{z}\psi_{i}A^{-1}\right]\,,%\end{split} start_ROW start_CELL end_CELL start_CELL ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) divide start_ARG italic_b start_POSTSUBSCRIPT italic_k italic_i italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG = - italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A ( italic_z ) start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT ∂ italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT , end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = - italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT [ italic_c start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) - 2 italic_μ ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_A start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT ] , end_CELL end_ROW (4.25)
where c i ≡ ∫ Ω d z A ψ i subscript 𝑐 𝑖 subscript Ω d 𝑧 𝐴 subscript 𝜓 𝑖 c_{i}\equiv\int_{\Omega}\text{d}z\,A\psi_{i} italic_c start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ≡ ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT .
Using the same strategy used in Eq.(4.16 ), one can prove that
μ ( α + β ) ∫ Ω d z A α + β + 1 ∫ π z A − α ψ i = ∫ Ω d z A β ψ i − A π α + β ∫ Ω d z A − α ψ i . 𝜇 𝛼 𝛽 subscript Ω d 𝑧 superscript 𝐴 𝛼 𝛽 1 superscript subscript 𝜋 𝑧 superscript 𝐴 𝛼 subscript 𝜓 𝑖 subscript Ω d 𝑧 superscript 𝐴 𝛽 subscript 𝜓 𝑖 superscript subscript 𝐴 𝜋 𝛼 𝛽 subscript Ω d 𝑧 superscript 𝐴 𝛼 subscript 𝜓 𝑖 \begin{split}&\mu(\alpha+\beta)\int_{\Omega}\text{d}z\,A^{\alpha+\beta+1}\int%\limits_{\pi}^{z}A^{-\alpha}\psi_{i}\\&=\int_{\Omega}\text{d}z\,A^{\beta}\psi_{i}-A_{\pi}^{\alpha+\beta}\int_{\Omega%}\text{d}z\,A^{-\alpha}\psi_{i}\,.\end{split} start_ROW start_CELL end_CELL start_CELL italic_μ ( italic_α + italic_β ) ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT italic_α + italic_β + 1 end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_z end_POSTSUPERSCRIPT italic_A start_POSTSUPERSCRIPT - italic_α end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT italic_β end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT - italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_α + italic_β end_POSTSUPERSCRIPT ∫ start_POSTSUBSCRIPT roman_Ω end_POSTSUBSCRIPT d italic_z italic_A start_POSTSUPERSCRIPT - italic_α end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT . end_CELL end_ROW (4.26)
We can now match the previous result by choosing β = 1 𝛽 1 \beta=1 italic_β = 1 and α = 1 𝛼 1 \alpha=1 italic_α = 1 such that α + β = 2 𝛼 𝛽 2 \alpha+\beta=2 italic_α + italic_β = 2 . Remarkably, by merging the two contributions, the c i subscript 𝑐 𝑖 c_{i} italic_c start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT part cancels and leads to the final result
∑ k = 1 ∞ ψ k ( π ) b k i r λ k = A π − 2 ψ r ψ i ( π ) − ψ 0 2 ψ r − 1 A π 2 a i r r = A π − 2 ψ r ψ i ( π ) − ψ r a i r r . superscript subscript 𝑘 1 subscript 𝜓 𝑘 𝜋 subscript 𝑏 𝑘 𝑖 𝑟 subscript 𝜆 𝑘 superscript subscript 𝐴 𝜋 2 subscript 𝜓 𝑟 subscript 𝜓 𝑖 𝜋 superscript subscript 𝜓 0 2 superscript subscript 𝜓 𝑟 1 superscript subscript 𝐴 𝜋 2 subscript 𝑎 𝑖 𝑟 𝑟 superscript subscript 𝐴 𝜋 2 subscript 𝜓 𝑟 subscript 𝜓 𝑖 𝜋 subscript 𝜓 𝑟 subscript 𝑎 𝑖 𝑟 𝑟 \begin{split}\sum\limits_{k=1}^{\infty}\psi_{k}(\pi)\dfrac{b_{kir}}{\lambda_{k%}}&=A_{\pi}^{-2}\psi_{r}\psi_{i}(\pi)-\psi_{0}^{2}\psi_{r}^{-1}A_{\pi}^{2}a_{%irr}\\&=A_{\pi}^{-2}\psi_{r}\psi_{i}(\pi)-\psi_{r}a_{irr}\,.\end{split} start_ROW start_CELL ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT ( italic_π ) divide start_ARG italic_b start_POSTSUBSCRIPT italic_k italic_i italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_λ start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT end_ARG end_CELL start_CELL = italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) - italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 1 end_POSTSUPERSCRIPT italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = italic_A start_POSTSUBSCRIPT italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_π ) - italic_ψ start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT italic_a start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT . end_CELL end_ROW (4.27)
This concludes the proof.
V Summary and OutlookIn this work, we have proven in full generality a set of sum rules for solutions to the Sturm-Lioville(SL) problem. This generalizes our previous results put forward in [10 ] where similar conclusions were reached for the large μ 𝜇 \mu italic_μ limit of the RS model. We have then applied them and shown their power in the framework of orbifolded extra-dimensional gravity, with particular emphasis on LED, RS, and CW models. We exploit the fact that these models are conformally equivalent and thus formally possess the same KK-graviton interactions.In graviton pair-productions from matter, the sum rules have been shown to cancel the unphysical high-energy growth of the amplitudes, reducing them from 𝒪 ( s 3 ) → 𝒪 ( s 2 ) → 𝒪 superscript 𝑠 3 𝒪 superscript 𝑠 2 \mathcal{O}(s^{3})\to\mathcal{O}(s^{2}) caligraphic_O ( italic_s start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT ) → caligraphic_O ( italic_s start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) .The full reduction of the amplitudes to 𝒪 ( s ) 𝒪 𝑠 \mathcal{O}(s) caligraphic_O ( italic_s ) requires the contribution of the radion.These depend on the compactification and we have not found a general form. However, we give an example of how they can be found in the RS-model without making simplifying assumption regarding the wave-functions.
Regarding the amplitudes that do not involve the radion, we would like to comment on the applicability of our results to other models in the following. The 5D theory and its 4D limit need to fulfil a number of conditions for our approach to work. We need the 4D theory to consist of spin-2 fields with a Fierz-Pauli action. The 5D theory has to possess a covariant action with at most two derivatives and matter is localized on a brane. Under these conditions:
1. Regardless of cancellations in the amplitude, the sum rules still apply. If the action contains at most two derivatives, then the 5D part of each graviton will satisfy a second-order linear differential equation. All second-order linear differential equations can be recast as SL equations by multiplying them by an appropriate integration factor. The diagrams that can contribute to the scattering processes considered do not change, so the sum rules that can enter are only those derived in the previous section.
2. In such theories, the action fixes
q ( y ) = 0 . 𝑞 𝑦 0 q(y)=0\,. italic_q ( italic_y ) = 0 . (5.1)
This is an important requirement for the derivation of some of the sum rules, which should be otherwise slightly modified. It can be understood by considering the derivative structure of the action. As it can contain at most two 5D derivatives of the fields, in the basis in which the 5D graviton does not mix with any other component of the metric, only terms with two purely ∂ μ subscript 𝜇 \partial_{\mu} ∂ start_POSTSUBSCRIPT italic_μ end_POSTSUBSCRIPT or ∂ 5 subscript 5 \partial_{5} ∂ start_POSTSUBSCRIPT 5 end_POSTSUBSCRIPT can be present. Therefore, a term of the q ( y ) 𝑞 𝑦 q(y) italic_q ( italic_y ) -type would necessarily include two ∂ μ subscript 𝜇 \partial_{\mu} ∂ start_POSTSUBSCRIPT italic_μ end_POSTSUBSCRIPT , which belong to the kinetic term part of the Lagrangian, not to the mass part. Furthermore, from a physical point of view, this condition guarantees the presence of a massless KK-graviton (cfr. Eq.(2.6 )), since it ensures that a constant wave function with a zero eigenvalue is a solution.
3. Using the previous argument, the corresponding SL equations are also expected to have q ( y ) = 0 𝑞 𝑦 0 q(y)=0 italic_q ( italic_y ) = 0 . The possible relevant differences with respect to what is derived in this work depend on the type of compactification and they are hidden in the boundary conditions. In the case of orbifold compactification, we used
d ψ i d y | ∂ Ω = 0 . evaluated-at d subscript 𝜓 𝑖 d 𝑦 Ω 0 \left.\dfrac{\text{d}\psi_{i}}{\text{d}y}\right|_{\partial\Omega}=0\,. divide start_ARG d italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT end_ARG start_ARG d italic_y end_ARG | start_POSTSUBSCRIPT ∂ roman_Ω end_POSTSUBSCRIPT = 0 . (5.2)
The absence of such a condition might result in a modification of some results, which has then to be taken into account during the derivation of the sum rules.
The cancellations of ℳ ( n ) superscript ℳ 𝑛 \mathcal{M}^{(n)} caligraphic_M start_POSTSUPERSCRIPT ( italic_n ) end_POSTSUPERSCRIPT for n > 1 𝑛 1 n>1 italic_n > 1 that are encoded in the sum rules have a very notable impact in phenomenological studies involving KK-graviton production, e.g. in studies of the early universe where high-momentum transfer is common. Such cancellations are possible thanks to the underlying geometric structure of the original 5D theory.
Acknowledgments A.d.G. thanks Alejandro Pérez Rodríguez for useful discussions and especially thanks Carlos A. Argüelles and the group of Palfrey House for their hospitality and the stimulating working environment during which a core part of this work was realised.The work of A.d.G. is supported by the European Union’s Horizon 2020 Marie Skłodowska-Curie grant agreement No 860881-HIDDeN.
Note added The initial version of this work contained an extensive discussion of the sum rules for graviton-scattering. After the first version appeared on the arXiv, it was brought to our attention that the proof of the sum rules reported in Ref.[8 ] is also based on the properties of solutions of the Sturm-Liouville problem and not limited to the Randall-Sundrum model. Therefore, this proof is already general and we decided to remove this part from the updated version of this work.
Appendix A Explicit Example: Large Extra-DimensionsA.1 The ModelThe LED[24 ] can be considered as the RS in the limit k → 0 → 𝑘 0 k\to 0 italic_k → 0 (thus μ → 0 → 𝜇 0 \mu\to 0 italic_μ → 0 ) while R 𝑅 R italic_R is kept fixed. As in the main text, we will work in units of R 𝑅 R italic_R , or, equivalently, we will set R = 1 𝑅 1 R=1 italic_R = 1 .The numerical coefficients that enter the sum rules in Table1 can be computed exactly.The EOMs of ψ n subscript 𝜓 𝑛 \psi_{n} italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT are given by
( ∂ y 2 + m n 2 ) ψ n = 0 , superscript subscript 𝑦 2 superscript subscript 𝑚 𝑛 2 subscript 𝜓 𝑛 0 \left(\partial_{y}^{2}+m_{n}^{2}\right)\psi_{n}=0\,, ( ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT = 0 , (A.1)
with solutions
ψ 0 ( y ) = N 0 [ 1 + α 0 | y | ] , subscript 𝜓 0 𝑦 subscript 𝑁 0 delimited-[] 1 subscript 𝛼 0 𝑦 \displaystyle\psi_{0}(y)=N_{0}\left[1+\alpha_{0}|y|\right]\,, italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT ( italic_y ) = italic_N start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT [ 1 + italic_α start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT | italic_y | ] , (A.2) ψ n ( y ) = N n [ cos ( ( m n | y | ) ) + α n sin ( ( m n | y | ) ) ] , subscript 𝜓 𝑛 𝑦 subscript 𝑁 𝑛 delimited-[] subscript 𝑚 𝑛 𝑦 subscript 𝛼 𝑛 subscript 𝑚 𝑛 𝑦 \displaystyle\psi_{n}(y)=N_{n}\left[\cos{\left(m_{n}|y|\right)}+\alpha_{n}\sin%{\left(m_{n}|y|\right)}\right]\,, italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y ) = italic_N start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT [ roman_cos ( start_ARG ( italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT | italic_y | ) end_ARG ) + italic_α start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT roman_sin ( start_ARG ( italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT | italic_y | ) end_ARG ) ] , (A.3)
where we have set m 0 = 0 subscript 𝑚 0 0 m_{0}=0 italic_m start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT = 0 .Imposing the boundary conditions ∂ y ψ n = 0 subscript 𝑦 subscript 𝜓 𝑛 0 \partial_{y}\psi_{n}=0 ∂ start_POSTSUBSCRIPT italic_y end_POSTSUBSCRIPT italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT = 0 , we need to set α n = α 0 = 0 subscript 𝛼 𝑛 subscript 𝛼 0 0 \alpha_{n}=\alpha_{0}=0 italic_α start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT = italic_α start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT = 0 , and hence
ψ 0 = N 0 , ψ n ( y ) = N n cos ( ( m n y ) ) . \psi_{0}=N_{0}\quad,\quad\psi_{n}(y)=N_{n}\cos{\left(m_{n}y\right)}\,. italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT = italic_N start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT , italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y ) = italic_N start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT roman_cos ( start_ARG ( italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT italic_y ) end_ARG ) . (A.4)
By periodicity of the solutions, we can also fix the masses
cos ( ( m n y ) ) = cos ( ( m n ( y + 2 π ) ) ) ⇒ m n = n . formulae-sequence subscript 𝑚 𝑛 𝑦 subscript 𝑚 𝑛 𝑦 2 𝜋 ⇒
subscript 𝑚 𝑛 𝑛 \cos{(m_{n}y)}=\cos{(m_{n}(y+2\pi))}\quad\Rightarrow\quad m_{n}=n\,. roman_cos ( start_ARG ( italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT italic_y ) end_ARG ) = roman_cos ( start_ARG ( italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y + 2 italic_π ) ) end_ARG ) ⇒ italic_m start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT = italic_n . (A.5)
With such definitions the wave functions are orthogonal and normalized4 4 4 We have chosen here to incorporate in the scalar product an overall factor of 1 / π 1 𝜋 1/\pi 1 / italic_π .
1 π ∫ − π π d y ψ i ( y ) ψ j ( y ) = δ i j , 1 𝜋 superscript subscript 𝜋 𝜋 d 𝑦 subscript 𝜓 𝑖 𝑦 subscript 𝜓 𝑗 𝑦 subscript 𝛿 𝑖 𝑗 \frac{1}{\pi}\int\limits_{-\pi}^{\pi}\text{d}y\ \psi_{i}(y)\psi_{j}(y)=\delta_%{ij}\,, divide start_ARG 1 end_ARG start_ARG italic_π end_ARG ∫ start_POSTSUBSCRIPT - italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_π end_POSTSUPERSCRIPT d italic_y italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT ( italic_y ) italic_ψ start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT ( italic_y ) = italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT , (A.6)
fixing
N 0 = 1 2 , N n = 1 . N_{0}=\frac{1}{\sqrt{2}}\quad,\quad N_{n}=1\,. italic_N start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT = divide start_ARG 1 end_ARG start_ARG square-root start_ARG 2 end_ARG end_ARG , italic_N start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT = 1 . (A.7)
To simplify the discussion and avoid redundant ± plus-or-minus \pm ± signs, we consider the matter content to be localized on the brane at y = 0 𝑦 0 y=0 italic_y = 0 . This does not affect any of the results expected from the discussion in the main text.Thus, the fifth-dimensional wave functions are given by
ψ 0 ( y ) = N 0 = 1 2 ψ n ( y ) = N n cos ( ( n y ) ) = cos ( ( n y ) ) . subscript 𝜓 0 𝑦 subscript 𝑁 0 1 2 subscript 𝜓 𝑛 𝑦 subscript 𝑁 𝑛 𝑛 𝑦 𝑛 𝑦 \begin{split}&\psi_{0}(y)=N_{0}=\frac{1}{\sqrt{2}}\\&\psi_{n}(y)=N_{n}\cos{(ny)}=\cos{(ny)}\,.\end{split} start_ROW start_CELL end_CELL start_CELL italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT ( italic_y ) = italic_N start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT = divide start_ARG 1 end_ARG start_ARG square-root start_ARG 2 end_ARG end_ARG end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL italic_ψ start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT ( italic_y ) = italic_N start_POSTSUBSCRIPT italic_n end_POSTSUBSCRIPT roman_cos ( start_ARG ( italic_n italic_y ) end_ARG ) = roman_cos ( start_ARG ( italic_n italic_y ) end_ARG ) . end_CELL end_ROW (A.8)
A.2 Sum RulesWe turn now to the sum rules.According to the definitions of Eq.s(2.15 ) and (2.16 ), we define
a i j k ≡ N i N j N k χ i j k , subscript 𝑎 𝑖 𝑗 𝑘 subscript 𝑁 𝑖 subscript 𝑁 𝑗 subscript 𝑁 𝑘 subscript 𝜒 𝑖 𝑗 𝑘 \displaystyle a_{ijk}\equiv N_{i}N_{j}N_{k}\,\chi_{ijk}\,, italic_a start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ≡ italic_N start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT , (A.9) b i j k ≡ N i N j N k χ ~ i j k , subscript 𝑏 𝑖 𝑗 𝑘 subscript 𝑁 𝑖 subscript 𝑁 𝑗 subscript 𝑁 𝑘 subscript ~ 𝜒 𝑖 𝑗 𝑘 \displaystyle b_{ijk}\equiv N_{i}N_{j}N_{k}\,\tilde{\chi}_{ijk}\,, italic_b start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ≡ italic_N start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT , (A.10)
so that
χ i j k ≡ 1 π ∫ − π π d y cos ( i y ) cos ( j y ) cos ( k y ) , subscript 𝜒 𝑖 𝑗 𝑘 1 𝜋 superscript subscript 𝜋 𝜋 d 𝑦 𝑖 𝑦 𝑗 𝑦 𝑘 𝑦 \displaystyle\chi_{ijk}\equiv\frac{1}{\pi}\int\limits_{-\pi}^{\pi}\text{d}y\ %\cos(iy)\cos(jy)\cos(ky)\,, italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ≡ divide start_ARG 1 end_ARG start_ARG italic_π end_ARG ∫ start_POSTSUBSCRIPT - italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_π end_POSTSUPERSCRIPT d italic_y roman_cos ( start_ARG italic_i italic_y end_ARG ) roman_cos ( start_ARG italic_j italic_y end_ARG ) roman_cos ( start_ARG italic_k italic_y end_ARG ) , (A.11) χ ~ i j k ≡ i j π ∫ − π π d y sin ( i y ) sin ( j y ) cos ( k y ) . subscript ~ 𝜒 𝑖 𝑗 𝑘 𝑖 𝑗 𝜋 superscript subscript 𝜋 𝜋 d 𝑦 𝑖 𝑦 𝑗 𝑦 𝑘 𝑦 \displaystyle\tilde{\chi}_{ijk}\equiv\frac{ij}{\pi}\int\limits_{-\pi}^{\pi}%\text{d}y\ \sin(iy)\sin(jy)\cos(ky)\,. over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ≡ divide start_ARG italic_i italic_j end_ARG start_ARG italic_π end_ARG ∫ start_POSTSUBSCRIPT - italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_π end_POSTSUPERSCRIPT d italic_y roman_sin ( start_ARG italic_i italic_y end_ARG ) roman_sin ( start_ARG italic_j italic_y end_ARG ) roman_cos ( start_ARG italic_k italic_y end_ARG ) . (A.12)
One can calculate explicitly the integrals which gives the rather simple expressions
χ i j k = 1 2 [ δ ( i − j − k ) , 0 + δ ( i + j − k ) , 0 + δ ( i − j + k ) , 0 + δ ( i + j + k ) , 0 ] , subscript 𝜒 𝑖 𝑗 𝑘 1 2 delimited-[] subscript 𝛿 𝑖 𝑗 𝑘 0
subscript 𝛿 𝑖 𝑗 𝑘 0
subscript 𝛿 𝑖 𝑗 𝑘 0
subscript 𝛿 𝑖 𝑗 𝑘 0
\displaystyle\chi_{ijk}=\frac{1}{2}\left[\delta_{(i-j-k),0}+\delta_{(i+j-k),0}%+\delta_{(i-j+k),0}+\delta_{(i+j+k),0}\right]\,, italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = divide start_ARG 1 end_ARG start_ARG 2 end_ARG [ italic_δ start_POSTSUBSCRIPT ( italic_i - italic_j - italic_k ) , 0 end_POSTSUBSCRIPT + italic_δ start_POSTSUBSCRIPT ( italic_i + italic_j - italic_k ) , 0 end_POSTSUBSCRIPT + italic_δ start_POSTSUBSCRIPT ( italic_i - italic_j + italic_k ) , 0 end_POSTSUBSCRIPT + italic_δ start_POSTSUBSCRIPT ( italic_i + italic_j + italic_k ) , 0 end_POSTSUBSCRIPT ] , (A.13) χ ~ i j k = i j 2 [ δ ( i − j − k ) , 0 − δ ( i + j − k ) , 0 + δ ( i − j + k ) , 0 − δ ( i + j + k ) , 0 ] , subscript ~ 𝜒 𝑖 𝑗 𝑘 𝑖 𝑗 2 delimited-[] subscript 𝛿 𝑖 𝑗 𝑘 0
subscript 𝛿 𝑖 𝑗 𝑘 0
subscript 𝛿 𝑖 𝑗 𝑘 0
subscript 𝛿 𝑖 𝑗 𝑘 0
\displaystyle\tilde{\chi}_{ijk}=\frac{ij}{2}\left[\delta_{(i-j-k),0}-\delta_{(%i+j-k),0}+\delta_{(i-j+k),0}-\delta_{(i+j+k),0}\right]\,, over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = divide start_ARG italic_i italic_j end_ARG start_ARG 2 end_ARG [ italic_δ start_POSTSUBSCRIPT ( italic_i - italic_j - italic_k ) , 0 end_POSTSUBSCRIPT - italic_δ start_POSTSUBSCRIPT ( italic_i + italic_j - italic_k ) , 0 end_POSTSUBSCRIPT + italic_δ start_POSTSUBSCRIPT ( italic_i - italic_j + italic_k ) , 0 end_POSTSUBSCRIPT - italic_δ start_POSTSUBSCRIPT ( italic_i + italic_j + italic_k ) , 0 end_POSTSUBSCRIPT ] , (A.14)
where δ i , j subscript 𝛿 𝑖 𝑗
\delta_{i,j} italic_δ start_POSTSUBSCRIPT italic_i , italic_j end_POSTSUBSCRIPT is the Kronecker delta.The above quantities satisfy Eq.(2.20 ),
i 2 χ i j k = χ ~ i j k + χ ~ i k j . superscript 𝑖 2 subscript 𝜒 𝑖 𝑗 𝑘 subscript ~ 𝜒 𝑖 𝑗 𝑘 subscript ~ 𝜒 𝑖 𝑘 𝑗 i^{2}\chi_{ijk}=\tilde{\chi}_{ijk}+\tilde{\chi}_{ikj}\,. italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT + over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_k italic_j end_POSTSUBSCRIPT . (A.15)
Similarly, we define the coefficients associated with the radion as
b ~ i j r → N i N j N r χ ~ i j r , → subscript ~ 𝑏 𝑖 𝑗 𝑟 subscript 𝑁 𝑖 subscript 𝑁 𝑗 subscript 𝑁 𝑟 subscript ~ 𝜒 𝑖 𝑗 𝑟 \displaystyle\tilde{b}_{ijr}\to N_{i}N_{j}N_{r}\,\tilde{\chi}_{ijr}\,, over~ start_ARG italic_b end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT → italic_N start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT , (A.16) a i r r → N i N r 2 χ i r r , → subscript 𝑎 𝑖 𝑟 𝑟 subscript 𝑁 𝑖 superscript subscript 𝑁 𝑟 2 subscript 𝜒 𝑖 𝑟 𝑟 \displaystyle a_{irr}\to N_{i}N_{r}^{2}\,\chi_{irr}\,, italic_a start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT → italic_N start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT , (A.17)
with
χ ~ i j r ≡ i j π ∫ − π π d y sin ( i y ) sin ( j y ) = i 2 δ i , j , subscript ~ 𝜒 𝑖 𝑗 𝑟 𝑖 𝑗 𝜋 superscript subscript 𝜋 𝜋 d 𝑦 𝑖 𝑦 𝑗 𝑦 superscript 𝑖 2 subscript 𝛿 𝑖 𝑗
\displaystyle\tilde{\chi}_{ijr}\equiv\frac{ij}{\pi}\int\limits_{-\pi}^{\pi}%\text{d}y\sin(iy)\sin(jy)=i^{2}\delta_{i,j}\,, over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT ≡ divide start_ARG italic_i italic_j end_ARG start_ARG italic_π end_ARG ∫ start_POSTSUBSCRIPT - italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_π end_POSTSUPERSCRIPT d italic_y roman_sin ( start_ARG italic_i italic_y end_ARG ) roman_sin ( start_ARG italic_j italic_y end_ARG ) = italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i , italic_j end_POSTSUBSCRIPT , (A.18) χ i r r ≡ 1 π ∫ − π π d y cos ( i y ) = 2 δ i , 0 . subscript 𝜒 𝑖 𝑟 𝑟 1 𝜋 superscript subscript 𝜋 𝜋 d 𝑦 𝑖 𝑦 2 subscript 𝛿 𝑖 0
\displaystyle\chi_{irr}\equiv\frac{1}{\pi}\int\limits_{-\pi}^{\pi}\text{d}y%\cos(iy)=2\delta_{i,0}\,. italic_χ start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT ≡ divide start_ARG 1 end_ARG start_ARG italic_π end_ARG ∫ start_POSTSUBSCRIPT - italic_π end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_π end_POSTSUPERSCRIPT d italic_y roman_cos ( start_ARG italic_i italic_y end_ARG ) = 2 italic_δ start_POSTSUBSCRIPT italic_i , 0 end_POSTSUBSCRIPT . (A.19)
The sum rules needed for the cancellation of the gravitons pair-production from matter amplitudes, counterparts of those in Table1 , are given in Table2 .
SR 1: ∑ k = 0 ∞ N k 2 χ i j k = 1 superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 subscript 𝜒 𝑖 𝑗 𝑘 1 \sum\limits_{k=0}^{\infty}N_{k}^{2}\chi_{ijk}=1 ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = 1 SR 2: ∑ k = 0 ∞ N k 2 k 2 χ i j k = i 2 + j 2 superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 superscript 𝑘 2 subscript 𝜒 𝑖 𝑗 𝑘 superscript 𝑖 2 superscript 𝑗 2 \sum\limits_{k=0}^{\infty}N_{k}^{2}k^{2}\chi_{ijk}=i^{2}+j^{2} ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_k start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT SR 3: ∑ k = 0 ∞ N k 2 χ ~ k i j = i 2 superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 subscript ~ 𝜒 𝑘 𝑖 𝑗 superscript 𝑖 2 \sum\limits_{k=0}^{\infty}N_{k}^{2}\tilde{\chi}_{kij}=i^{2} ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_k italic_i italic_j end_POSTSUBSCRIPT = italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT SR 4: ∑ k = 0 ∞ N k 2 χ ~ i j k = 0 superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 subscript ~ 𝜒 𝑖 𝑗 𝑘 0 \sum\limits_{k=0}^{\infty}N_{k}^{2}\tilde{\chi}_{ijk}=0 ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = 0 SR 5: ∑ k = 1 ∞ N k 2 N i N j χ i j k k 2 ( i 2 − j 2 ) 2 = ( i 2 + j 2 ) ( N i N j + N 0 2 δ i j ) superscript subscript 𝑘 1 superscript subscript 𝑁 𝑘 2 subscript 𝑁 𝑖 subscript 𝑁 𝑗 subscript 𝜒 𝑖 𝑗 𝑘 superscript 𝑘 2 superscript superscript 𝑖 2 superscript 𝑗 2 2 superscript 𝑖 2 superscript 𝑗 2 subscript 𝑁 𝑖 subscript 𝑁 𝑗 superscript subscript 𝑁 0 2 subscript 𝛿 𝑖 𝑗 \sum\limits_{k=1}^{\infty}N_{k}^{2}N_{i}N_{j}\frac{\chi_{ijk}}{k^{2}}(i^{2}-j^%{2})^{2}=\left(i^{2}+j^{2}\right)\left(N_{i}N_{j}+N_{0}^{2}\delta_{ij}\right) ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT divide start_ARG italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT end_ARG start_ARG italic_k start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT - italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) ( italic_N start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT + italic_N start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ) − 6 N r 2 χ ~ i j r 6 superscript subscript 𝑁 𝑟 2 subscript ~ 𝜒 𝑖 𝑗 𝑟 -6N_{r}^{2}\tilde{\chi}_{ijr} - 6 italic_N start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT SR 6: ∑ j = 1 ∞ N j 2 χ ~ i j r j 2 = 1 − N r 2 χ i r r superscript subscript 𝑗 1 superscript subscript 𝑁 𝑗 2 subscript ~ 𝜒 𝑖 𝑗 𝑟 superscript 𝑗 2 1 superscript subscript 𝑁 𝑟 2 subscript 𝜒 𝑖 𝑟 𝑟 \sum\limits_{j=1}^{\infty}N_{j}^{2}\frac{\tilde{\chi}_{ijr}}{j^{2}}=1-N_{r}^{2%}\chi_{irr} ∑ start_POSTSUBSCRIPT italic_j = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT divide start_ARG over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG = 1 - italic_N start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT
Sum Rule 1 Let us focus first on χ i j k subscript 𝜒 𝑖 𝑗 𝑘 \chi_{ijk} italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT proving that
∑ k = 0 ∞ N k 2 χ i j k = 1 . superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 subscript 𝜒 𝑖 𝑗 𝑘 1 \sum\limits_{k=0}^{\infty}N_{k}^{2}\chi_{ijk}=1\ . ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = 1 . (A.20)
It is clear from the definition of Eq.(A.13 ) that χ 00 k = 2 δ k , 0 subscript 𝜒 00 𝑘 2 subscript 𝛿 𝑘 0
\chi_{00k}=2\delta_{k,0} italic_χ start_POSTSUBSCRIPT 00 italic_k end_POSTSUBSCRIPT = 2 italic_δ start_POSTSUBSCRIPT italic_k , 0 end_POSTSUBSCRIPT thus the sum rule is satisfied for i = j = 0 𝑖 𝑗 0 i=j=0 italic_i = italic_j = 0 as N 0 2 = 1 / 2 superscript subscript 𝑁 0 2 1 2 N_{0}^{2}=1/2 italic_N start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = 1 / 2 . If i ≠ j = 0 𝑖 𝑗 0 i\neq j=0 italic_i ≠ italic_j = 0 , then χ i 0 k = δ k i subscript 𝜒 𝑖 0 𝑘 subscript 𝛿 𝑘 𝑖 \chi_{i0k}=\delta_{ki} italic_χ start_POSTSUBSCRIPT italic_i 0 italic_k end_POSTSUBSCRIPT = italic_δ start_POSTSUBSCRIPT italic_k italic_i end_POSTSUBSCRIPT which sets the sum to 1 1 1 1 , as N i ≠ 0 = 1 subscript 𝑁 𝑖 0 1 N_{i\neq 0}=1 italic_N start_POSTSUBSCRIPT italic_i ≠ 0 end_POSTSUBSCRIPT = 1 . Finally, in case i , j ≠ 0 𝑖 𝑗
0 i,j\neq 0 italic_i , italic_j ≠ 0 , one can easily check that just two terms contribute with 1 / 2 1 2 1/2 1 / 2 , thus giving 1 / 2 + 1 / 2 = 1 1 2 1 2 1 1/2+1/2=1 1 / 2 + 1 / 2 = 1 , proving the last case of the sum rule.
Sum Rules 2,3,4 Let us now focus on the sum rules involving k 2 χ i j k superscript 𝑘 2 subscript 𝜒 𝑖 𝑗 𝑘 k^{2}\chi_{ijk} italic_k start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT . First, we look at ∑ k = 0 ∞ N k 2 χ ~ i j k superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 subscript ~ 𝜒 𝑖 𝑗 𝑘 \sum\limits_{k=0}^{\infty}N_{k}^{2}\tilde{\chi}_{ijk} ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT . It is clear from Eq.(A.14 ) that χ ~ 0 j k = χ ~ i 0 k = χ ~ 00 k = 0 subscript ~ 𝜒 0 𝑗 𝑘 subscript ~ 𝜒 𝑖 0 𝑘 subscript ~ 𝜒 00 𝑘 0 \tilde{\chi}_{0jk}=\tilde{\chi}_{i0k}=\tilde{\chi}_{00k}=0 over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT 0 italic_j italic_k end_POSTSUBSCRIPT = over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i 0 italic_k end_POSTSUBSCRIPT = over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT 00 italic_k end_POSTSUBSCRIPT = 0 , thus the only non trivial case is for i , j ≠ 0 𝑖 𝑗
0 i,j\neq 0 italic_i , italic_j ≠ 0 . However, as k 𝑘 k italic_k runs from 0 0 to ∞ \infty ∞ , it is clear that only two terms contribute and they are opposite in sign, i.e. ∑ k = 0 ∞ N k 2 χ ~ i j k ∝ ( 1 − 1 ) = 0 proportional-to superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 subscript ~ 𝜒 𝑖 𝑗 𝑘 1 1 0 \sum\limits_{k=0}^{\infty}N_{k}^{2}\tilde{\chi}_{ijk}\propto(1-1)=0 ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT ∝ ( 1 - 1 ) = 0 . In general, then we have that
∑ k = 0 ∞ N k 2 χ ~ i j k = 0 . superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 subscript ~ 𝜒 𝑖 𝑗 𝑘 0 \sum\limits_{k=0}^{\infty}N_{k}^{2}\tilde{\chi}_{ijk}=0\ . ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = 0 . (A.21)
The other rules can be obtained using the sum rules we have derived up to now with Eq.(A.15 ), which implies
N j 2 i 2 χ i j k = N j 2 ( χ ~ i j k + χ ~ i k j ) . superscript subscript 𝑁 𝑗 2 superscript 𝑖 2 subscript 𝜒 𝑖 𝑗 𝑘 superscript subscript 𝑁 𝑗 2 subscript ~ 𝜒 𝑖 𝑗 𝑘 subscript ~ 𝜒 𝑖 𝑘 𝑗 N_{j}^{2}i^{2}\chi_{ijk}=N_{j}^{2}(\tilde{\chi}_{ijk}+\tilde{\chi}_{ikj})\ . italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ( over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT + over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_k italic_j end_POSTSUBSCRIPT ) . (A.22)
Summing over the index-j 𝑗 j italic_j and using both Eq.s(A.20 ) and (A.21 ) we get
∑ j = 0 ∞ N j 2 χ ~ i j k = i 2 . superscript subscript 𝑗 0 superscript subscript 𝑁 𝑗 2 subscript ~ 𝜒 𝑖 𝑗 𝑘 superscript 𝑖 2 \sum\limits_{j=0}^{\infty}N_{j}^{2}\tilde{\chi}_{ijk}=i^{2}\ . ∑ start_POSTSUBSCRIPT italic_j = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT . (A.23)
Finally, we can use Eq.(A.15 ) and the above relations again to find the second sum rule of interest
∑ k = 0 ∞ N k 2 k 2 χ i j k = i 2 + j 2 . superscript subscript 𝑘 0 superscript subscript 𝑁 𝑘 2 superscript 𝑘 2 subscript 𝜒 𝑖 𝑗 𝑘 superscript 𝑖 2 superscript 𝑗 2 \sum\limits_{k=0}^{\infty}N_{k}^{2}k^{2}\chi_{ijk}=i^{2}+j^{2}\ . ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_k start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT = italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT .
It is instructive to make an explicit check of such relation since we have not verified all of them explicitly; let us assume without loss of generality that j > k ≠ 0 𝑗 𝑘 0 j>k\neq 0 italic_j > italic_k ≠ 0 , then
∑ k = 0 ∞ k 2 χ k i j = 1 2 [ ( i + j ) 2 + ( i − j ) 2 ] = 1 2 [ 2 i 2 + 2 j 2 ] = i 2 + j 2 , superscript subscript 𝑘 0 superscript 𝑘 2 subscript 𝜒 𝑘 𝑖 𝑗 1 2 delimited-[] superscript 𝑖 𝑗 2 superscript 𝑖 𝑗 2 1 2 delimited-[] 2 superscript 𝑖 2 2 superscript 𝑗 2 superscript 𝑖 2 superscript 𝑗 2 \begin{split}\sum\limits_{k=0}^{\infty}k^{2}\chi_{kij}&=\frac{1}{2}\left[(i+j)%^{2}+(i-j)^{2}\right]\\&=\frac{1}{2}\left[2i^{2}+2j^{2}\right]=i^{2}+j^{2}\,,\end{split} start_ROW start_CELL ∑ start_POSTSUBSCRIPT italic_k = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_k start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_k italic_i italic_j end_POSTSUBSCRIPT end_CELL start_CELL = divide start_ARG 1 end_ARG start_ARG 2 end_ARG [ ( italic_i + italic_j ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + ( italic_i - italic_j ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ] end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = divide start_ARG 1 end_ARG start_ARG 2 end_ARG [ 2 italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + 2 italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ] = italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT , end_CELL end_ROW (A.24)
which is what we expected.
Sum Rule 5 We need to prove that ∑ k = 1 ∞ N k 2 χ i j k k 2 ( i 2 − j 2 ) 2 = ( i 2 + j 2 ) ( 1 + N 0 2 χ i j 0 ) − 6 N r 2 χ ~ i j r superscript subscript 𝑘 1 superscript subscript 𝑁 𝑘 2 subscript 𝜒 𝑖 𝑗 𝑘 superscript 𝑘 2 superscript superscript 𝑖 2 superscript 𝑗 2 2 superscript 𝑖 2 superscript 𝑗 2 1 superscript subscript 𝑁 0 2 subscript 𝜒 𝑖 𝑗 0 6 superscript subscript 𝑁 𝑟 2 subscript ~ 𝜒 𝑖 𝑗 𝑟 \sum\limits_{k=1}^{\infty}N_{k}^{2}\frac{\chi_{ijk}}{k^{2}}(i^{2}-j^{2})^{2}=%\left(i^{2}+j^{2}\right)\left(1+N_{0}^{2}\chi_{ij0}\right)-6N_{r}^{2}\tilde{%\chi}_{ijr} ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT divide start_ARG italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT end_ARG start_ARG italic_k start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT - italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) ( 1 + italic_N start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_j 0 end_POSTSUBSCRIPT ) - 6 italic_N start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT . The fastest method is to verify it explicitly. Looking at the definitions of the coefficients of Eq.s(A.11 ) and (A.18 ), it is clear that if i = j = 0 𝑖 𝑗 0 i=j=0 italic_i = italic_j = 0 the sum rule is trivially satisfied. The right-hand-side of the sum rule can be written as ( i 2 + j 2 ) ( 1 + 1 / 2 δ i j ) − 3 i 2 δ i , j superscript 𝑖 2 superscript 𝑗 2 1 1 2 subscript 𝛿 𝑖 𝑗 3 superscript 𝑖 2 subscript 𝛿 𝑖 𝑗
\left(i^{2}+j^{2}\right)\left(1+1/2\delta_{ij}\right)-3i^{2}\delta_{i,j} ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) ( 1 + 1 / 2 italic_δ start_POSTSUBSCRIPT italic_i italic_j end_POSTSUBSCRIPT ) - 3 italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_δ start_POSTSUBSCRIPT italic_i , italic_j end_POSTSUBSCRIPT , so let us consider the case i = j ≠ 0 𝑖 𝑗 0 i=j\neq 0 italic_i = italic_j ≠ 0 and i ≠ j 𝑖 𝑗 i\neq j italic_i ≠ italic_j separately. In the first case, the left-hand-side is 0 0 , while the right-hand-side becomes 2 i 2 ⋅ 3 / 2 − 3 i 2 = 0 ⋅ 2 superscript 𝑖 2 3 2 3 superscript 𝑖 2 0 2i^{2}\cdot 3/2-3i^{2}=0 2 italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ⋅ 3 / 2 - 3 italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = 0 , and thus it is satisfied. The case in which i ≠ j = 0 𝑖 𝑗 0 i\neq j=0 italic_i ≠ italic_j = 0 can be easily verified too as most of the terms disappear. Finally if i ≠ j ≠ 0 𝑖 𝑗 0 i\neq j\neq 0 italic_i ≠ italic_j ≠ 0
∑ k = 1 ∞ N k 2 χ i j k k 2 ( i 2 − j 2 ) 2 = ( i 2 − j 2 ) 2 2 [ 1 ( i + j ) 2 + 1 ( i − j ) 2 ] , = ( i 2 − j 2 ) 2 1 2 2 ( i 2 + j 2 ) ( i 2 − j 2 ) 2 = i 2 + j 2 , \begin{split}\sum\limits_{k=1}^{\infty}N_{k}^{2}\frac{\chi_{ijk}}{k^{2}}(i^{2}%-j^{2})^{2}&=\frac{(i^{2}-j^{2})^{2}}{2}\left[\frac{1}{(i+j)^{2}}+\frac{1}{(i-%j)^{2}}\right]\,,\\&=(i^{2}-j^{2})^{2}\frac{1}{2}\frac{2(i^{2}+j^{2})}{(i^{2}-j^{2})^{2}}=i^{2}+j%^{2}\ ,\end{split} start_ROW start_CELL ∑ start_POSTSUBSCRIPT italic_k = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_k end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT divide start_ARG italic_χ start_POSTSUBSCRIPT italic_i italic_j italic_k end_POSTSUBSCRIPT end_ARG start_ARG italic_k start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT - italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_CELL start_CELL = divide start_ARG ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT - italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG start_ARG 2 end_ARG [ divide start_ARG 1 end_ARG start_ARG ( italic_i + italic_j ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG + divide start_ARG 1 end_ARG start_ARG ( italic_i - italic_j ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG ] , end_CELL end_ROW start_ROW start_CELL end_CELL start_CELL = ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT - italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT divide start_ARG 1 end_ARG start_ARG 2 end_ARG divide start_ARG 2 ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) end_ARG start_ARG ( italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT - italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG = italic_i start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT , end_CELL end_ROW (A.25)
which proves the last scenario and thus the sum rule.
Sum Rule 6 The sum rule we are trying to prove is ∑ j = 1 ∞ N j 2 χ ~ i j r j 2 = 1 − N r 2 χ i r r = 1 − δ i , 0 superscript subscript 𝑗 1 superscript subscript 𝑁 𝑗 2 subscript ~ 𝜒 𝑖 𝑗 𝑟 superscript 𝑗 2 1 superscript subscript 𝑁 𝑟 2 subscript 𝜒 𝑖 𝑟 𝑟 1 subscript 𝛿 𝑖 0
\sum\limits_{j=1}^{\infty}N_{j}^{2}\frac{\tilde{\chi}_{ijr}}{j^{2}}=1-N_{r}^{2%}\chi_{irr}=1-\delta_{i,0} ∑ start_POSTSUBSCRIPT italic_j = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT divide start_ARG over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG = 1 - italic_N start_POSTSUBSCRIPT italic_r end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT italic_χ start_POSTSUBSCRIPT italic_i italic_r italic_r end_POSTSUBSCRIPT = 1 - italic_δ start_POSTSUBSCRIPT italic_i , 0 end_POSTSUBSCRIPT . In the case i = 0 𝑖 0 i=0 italic_i = 0 the proof is trivial as χ ~ 0 j r = 0 subscript ~ 𝜒 0 𝑗 𝑟 0 \tilde{\chi}_{0jr}=0 over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT 0 italic_j italic_r end_POSTSUBSCRIPT = 0 and hence we get 0 = 0 0 0 0=0 0 = 0 . If i ≠ 0 𝑖 0 i\neq 0 italic_i ≠ 0 we need to check that ∑ j = 1 ∞ N j 2 χ ~ i j r j 2 = 1 superscript subscript 𝑗 1 superscript subscript 𝑁 𝑗 2 subscript ~ 𝜒 𝑖 𝑗 𝑟 superscript 𝑗 2 1 \sum\limits_{j=1}^{\infty}N_{j}^{2}\frac{\tilde{\chi}_{ijr}}{j^{2}}=1 ∑ start_POSTSUBSCRIPT italic_j = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT divide start_ARG over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG = 1 . This holds as ∑ j = 1 ∞ N j 2 χ ~ i j r j 2 = ∑ j = 1 ∞ i j δ i , j = 1 superscript subscript 𝑗 1 superscript subscript 𝑁 𝑗 2 subscript ~ 𝜒 𝑖 𝑗 𝑟 superscript 𝑗 2 superscript subscript 𝑗 1 𝑖 𝑗 subscript 𝛿 𝑖 𝑗
1 \sum\limits_{j=1}^{\infty}N_{j}^{2}\frac{\tilde{\chi}_{ijr}}{j^{2}}=\sum%\limits_{j=1}^{\infty}\frac{i}{j}\delta_{i,j}=1 ∑ start_POSTSUBSCRIPT italic_j = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT italic_N start_POSTSUBSCRIPT italic_j end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT divide start_ARG over~ start_ARG italic_χ end_ARG start_POSTSUBSCRIPT italic_i italic_j italic_r end_POSTSUBSCRIPT end_ARG start_ARG italic_j start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT end_ARG = ∑ start_POSTSUBSCRIPT italic_j = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ∞ end_POSTSUPERSCRIPT divide start_ARG italic_i end_ARG start_ARG italic_j end_ARG italic_δ start_POSTSUBSCRIPT italic_i , italic_j end_POSTSUBSCRIPT = 1 , concluding the proof.
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