Search for lepton-flavor violation at HERA

Abstract

A search for lepton-flavor-violating interactions $e p \to μX $ and $e p\to τX $ has been performed with the ZEUS detector using the entire HERA I data sample, corresponding to an integrated luminosity of 130 pb^{-1}. The data were taken at center-of-mass energies, $\sqrt{s}$, of 300 and 318 GeV. No evidence of lepton-flavor violation was found, and constraints were derived on leptoquarks (LQs) that could mediate such interactions. For LQ masses below $\sqrt{s}$, limits were set on $λ_{eq_1} \sqrt{β_{\ell q}}$, where $λ_{eq_1}$ is the coupling of the LQ to an electron and a first-generation quark $q_1$, and $β_{\ell q}$ is the branching ratio of the LQ to the final-state lepton $\ell$ ($μ$ or $τ$) and a quark $q$. For LQ masses much larger than $\sqrt{s}$, limits were set on the four-fermion interaction term $λ_{e q_α} λ_{\ell q_β} / M_{\mathrm{LQ}}^2$ for LQs that couple to an electron and a quark $q_α$ and to a lepton $\ell$ and a quark $q_β$, where $α$ and $β$ are quark generation indices. Some of the limits are also applicable to lepton-flavor-violating processes mediated by squarks in $R$-Parity-violating supersymmetric models. In some cases, especially when a higher-generation quark is involved and for the process $e p\to τX $, the ZEUS limits are the most stringent to date.