Light-by-light scattering with intact protons at the LHC: from Standard Model to New Physics

TL;DR

This work assesses the discovery potential for light-by-light scattering at the 14 TeV LHC using intact protons detected in forward detectors, to probe new charged particles and neutral resonances in a model-independent way. It moves beyond EFT by implementing full one-loop amplitudes for spins 1/2 and 1 in the Forward Physics Monte Carlo, enabling reliable sensitivity across all masses, parameterized by $S$, $m$, and $Q_{ m eff}$. The analysis shows that a vector with $Q_{ m eff}=4$ is discoverable up to $m\approx 640$ GeV (and a fermion up to $m\approx 300$ GeV) at 300 fb$^{-1}$, with modest HL-LHC gains, and finds multi-TeV reach for neutral states like Kaluza-Klein gravitons and dilatons. Forward proton tagging thus provides a powerful, complementary route to direct searches, enabling precise kinematic reconstruction and strong background suppression for new physics in the diphoton channel.

Abstract

We discuss the discovery potential of light-by-light scattering at the Large Hadron Collider (LHC), induced by the Standard Model (SM) and by new exotic charged particles. Our simulation relies on intact proton detection in the planned forward detectors of CMS and ATLAS. The full four-photon amplitudes generated by any electrically charged particles of spins $1/2$ and $1$, including the SM processes involving loops of leptons, quarks and $W$ bosons are implemented in the Forward Physics Monte Carlo generator. Our method provides model-independent bounds on massive charged particles, only parametrized by the spin, mass and "effective charge" $Q_{\rm eff}$ of the new particle. We find that a new charged vector (fermion) with $Q_{\rm eff}=4$ can be discovered up to $m=640~\rm GeV$ ($m=300~\rm GeV$) with an integrated luminosity of $300~\rm fb^{-1}$ at the LHC. We also discuss the sensitivities to neutral particles such as a strongly-interacting heavy dilaton and warped Kaluza-Klein gravitons, whose effects could be discovered for masses in the multi-TeV range.

Figures (11)

• Figure 1: Typical diagrams of electrically charged particles contributing to light-by-light scattering.
• Figure 2: Feynman diagrams predicted by the Standard Model leading to the exclusive production of two photons and two intact protons in the final state at the lowest order of perturbation theory.
• Figure 3: Integrated cross sections of the different exclusive di-photon processes with intact protons at the $13$ TeV LHC, plotted against the required minimum di-photon mass. Both photons are required to have a transverse momentum above 10 GeV.
• Figure 4: Scheme of the AFP detector. Roman pot hosting Si and timing detectors will be installed on both sides of ATLAS at 206 and 214 m from the ATLAS nominal interaction point. The CMS-TOTEM collaborations will have similar detectors.
• Figure 5: Di-photon mass acceptance for the AFP detectors for the LHC nominal running at $\beta^*=$ 0.6 m.