Searches for extra-dimensional excitations in light-by-light scattering

Abstract

We present a comprehensive phenomenological analysis of the Radion in the Randall-Sundrum model, focusing on its production via light-by-light scattering in ultra-peripheral proton-proton collisions at the LHC. We provide a consistent derivation of the effective couplings to Standard Model fields, clarifying the normalization of the trace anomaly-induced coupling to photons and the role of kinetic mixing with the Higgs boson. We demonstrate that while the pure gravitational coupling is loop-suppressed relative to Axion-Like Particles (ALPs), making the unmixed Radion elusive, the non-minimal mixing with the Higgs sector can induce constructive interference that enhances the signal by orders of magnitude. Using forward proton tagging to select exclusive high-mass events, we reinterpret recent experimental limits on ALPs to derive the first exclusion contours for the Radion in the $(\Lambda_r, \xi)$ plane, showing that mixing scenarios are beginning to be constrained by current LHC data.