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

TL;DR

This paper analyzes the potential to detect Radion excitations from extra dimensions via light-by-light scattering at the LHC, highlighting how Higgs mixing can significantly enhance the signal and current data can constrain models.

Contribution

It provides the first detailed derivation of Radion couplings, explores the impact of Higgs mixing on detectability, and reinterprets LHC ALP limits to constrain Radion parameters.

Findings

Higgs-Radion mixing can boost the signal by orders of magnitude.

Pure gravitational Radion coupling is loop-suppressed, making detection challenging.

Current LHC data already constrains some Radion parameter space.

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.