Structure of Massive Gauge/Gravity Scattering Amplitudes, Equivalence Theorems, and Extended Double-Copy with Compactified Warped Space

We study the structure of scattering amplitudes of massive Kaluza-Klein (KK) states in the compactified 5-dimensional warped gauge and gravity theories. We present systematic formulations of the gauge theory equivalence theorem (GAET) and the gravitational equivalence theorem (GRET) for warped KK theories in $R_\xi^{}$ gauge, where the GAET connects the scattering amplitudes of longitudinal KK gauge bosons to that of the corresponding KK Goldstone bosons and the GRET connects the scattering amplitudes of KK gravitons of helicity-zero (helicity-one) to that of the corresponding gravitational scalar (vector) KK Goldstone bosons. We analyze the structure of 3-point and 4-point scattering amplitudes of massive KK gauge bosons and of massive KK gravitons as well as their corresponding Goldstone bosons. We first prove the GAET and GRET explicitly for the fundamental 3-point KK gauge/gravity scattering amplitudes. We then demonstrate that the validity of the GAET and GRET for 4-point gauge/gravity scattering amplitudes can be reduced to the validity of GAET and GRET for 3-point gauge/gravity scattering amplitudes at tree level. With these, we study the double-copy construction of KK scattering amplitudes in the warped gauge/gravity theories. We newly realize the double-copy for massive 3-point full gauge/gravity amplitudes at tree level under proper correspondences of color-kinematics and of gauge/gravity couplings, whereas we can construct the double-copy for 4-point KK gauge/gravity amplitudes to the leading order (LO) of high energy expansion. We also conjecture that this LO double-copy construction can be extended to $N$-point scattering amplitudes with $N\!\geqslant 5$.