Lepton flavor violation in photon-induced electron-muon pairs at the HL-LHC

TL;DR

This work investigates charged lepton flavor violation in the process $γγ \to e^{\pm} μ^{\mp}$ at the HL-LHC within a gauge-invariant dimension-7 EFT for photon-photon–lepton interactions. It connects collider sensitivity to the new-physics scale $\Lambda$ while respecting strong bounds from $\mu \to e γ$, and uses a detailed MC analysis with multivariate techniques to optimize signal discrimination against SM backgrounds in ultra-peripheral collisions. Three benchmark points (BP1–BP3) explore a TeV-scale EFT landscape, revealing that a $5\sigma$ discovery is achievable for $\Lambda \sim 1~\mathrm{TeV}$ with $\mathcal{L}_{\rm int} \gtrsim 2700$ fb$^{-1}$ (BP1) and $\lesssim 2$ TeV at comparable luminosities (BP2–BP3). The study demonstrates the HL-LHC’s potential as a powerful probe of cLFV via photon-fusion, and highlights experimental strategies such as forward-proton tagging and Pb-Pb UPCs to enhance sensitivity and motivate dedicated searches.

Abstract

We show the outstanding potential of the High-Luminosity LHC (HL-LHC) to discover charged lepton flavor violation (cLFV) via the ultra-peripheral process $γγ\to e^\pmμ^\mp$. Using a gauge-invariant Effective Field Theory (EFT) framework -consistent with the most stringent bounds from radiative decays- we perform a full Monte Carlo analysis with multivariate techniques. Our results show that a $5σ$ discovery is achievable with an integrated luminosity of $\mathcal{L}_{\rm int} \gtrsim 2700$ fb$^{-1}$ for favorable benchmark scenarios. This study establishes photon-fusion at the HL-LHC as a powerful probe of cLFV, strongly motivating dedicated searches by the experimental collaborations.

Figures (3)

• Figure 1: Parameter space in the $|G_S^{e\mu}| – |G_P^{e\mu}|$. We scan the intervals $|G_{S,\,P}^{e\mu}|, |\tilde{G}_{S,\,P}^{e\mu}|\in[0,10^{-9}]$ GeV$^{-3}$ and $\Lambda\in(100,\,5000)$ GeV. The black points represent those allowed by the constraints in Eqs. \ref{['ul_Gij']} and \ref{['nonchiralcoupling']}.
• Figure 2: Angular separation $\Delta R(e\mu)$. We also include the fit Kernel Density Estimation (KDE).
• Figure 3: Signal significance as a function of integrated luminosity and the cut on the BDT prediction for BP1. Systematic uncertainties ($\kappa =5\%$) are also considered.