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

We show the outstanding potential of the High-Luminosity LHC (HL-LHC) to discover charged lepton flavor violation (cLFV) via the ultraperipheral process <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>γ</a:mi> <a:mi>γ</a:mi> <a:mo stretchy="false">→</a:mo> <a:msup> <a:mi>e</a:mi> <a:mo>±</a:mo> </a:msup> <a:msup> <a:mi>μ</a:mi> <a:mo>∓</a:mo> </a:msup> </a:math> . Using a gauge-invariant effective field theory 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 <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mn>5</d:mn> <d:mi>σ</d:mi> </d:math> discovery is achievable with an integrated luminosity of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:msub> <f:mi mathvariant="script">L</f:mi> <f:mrow> <f:mi>int</f:mi> </f:mrow> </f:msub> <f:mo>≳</f:mo> <f:mn>2700</f:mn> <f:mtext> </f:mtext> <f:mtext> </f:mtext> <f:msup> <f:mrow> <f:mi>fb</f:mi> </f:mrow> <f:mrow> <f:mo>−</f:mo> <f:mn>1</f:mn> </f:mrow> </f:msup> </f:math> 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.