Search for Lepton Flavour Violating Signals at the Future Electron-Proton Colliders

Abstract: The search for lepton flavor violation (LFV) is a powerful probe to look for new physics beyond the Standard Model. We explored the possibility of searches for LFV $Z$ boson couplings to electron and muon pairs at the upcoming electron-proton colliders, namely the Large Hadron Electron Collider (LHeC) and the Future Circular lepton-hadron Collider (FCC-eh). We employed the study via a single muon plus an associated jet channel to search for the LFV signal. We used a multivariate technique to obtain an improved signal-background analysis. By using the condition on nonobservation of any significant deviation of the signal over the expected background, we provide an upper limit on the LFV $Z$ boson coupling and corresponding branching ratio. We find that an upper limit of $2.0\times 10^{-7}$ and $9.7 \times 10^{-8}$ can be set on BR($Z\to e \mu$) at 95% C.L. with one year run of LHeC and FCC-eh, respectively, if the LFV coupling is governed by vector or axial-vector coupling. For tensor or axial-tensor coupling, these limits can be improved to $6.2\times 10^{-8}$ and $2.2\times 10^{-8}$ for LHeC and FCC-eh machines, respectively. The projected numbers improve significantly over the existing limit of $2.62\times 10^{-7}$ set by ATLAS.