Searching for charged lepton flavor violation at $ep$ colliders

TL;DR

This paper evaluates the potential of $ep$ colliders, specifically the LHeC, to detect charged lepton flavor violation through effective operators, showing they could surpass current experimental bounds especially for electron-tau conversion.

Contribution

It provides the first detailed sensitivity analysis of $ep$ colliders for cLFV in a model-independent effective theory framework, including realistic background considerations.

Findings

LHeC can probe electron-muon LFV beyond current bounds.

Sensitivity to electron-tau LFV could improve by over an order of magnitude.

Backward emission of the converted lepton aids in signal-background separation.

Abstract

We investigate the sensitivity of electron-proton ($ep$) colliders for charged lepton flavor violation (cLFV) in an effective theory approach, considering a general effective Lagrangian for the conversion of an electron into a muon or a tau via the effective coupling to a neutral gauge boson or a neutral scalar field. For the photon, the $Z$ boson and the Higgs particle of the Standard Model, we present the sensitivities of the LHeC for the coefficients of the effective operators, calculated from an analysis at the reconstructed level. As an example model where such flavor changing neutral current (FCNC) operators are generated at loop level, we consider the extension of the Standard Model by sterile neutrinos. We show that the LHeC could already probe the LFV conversion of an electron into a muon beyond the current experimental bounds, and could reach more than an order of magnitude higher sensitivity than the present limits for LFV conversion of an electron into a tau. We discuss that the high sensitivities are possible because the converted charged lepton is dominantly emitted in the backward direction, enabling an efficient separation of the signal from the background.