Search for lepton-flavor violation at HERA

Abstract

A search for lepton-flavor-violating interactions $ep\to \mu X$ and $ep\to \tau X$ 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 ${\lambda }_{e{q}_1}\sqrt{{\beta }_{\ell q}}$, where ${\lambda }_{e{q}_1}$ is the coupling of the LQ to an electron and a first-generation quark $q_1$, and ${\beta }_{\ell q}$ is the branching ratio of the LQ to the final-state lepton $\ell$ ($\mu$ or $\tau$) and a quark $q$. For LQ masses much larger than $\sqrt{s}$, limits were set on the four-fermion interaction term ${\lambda }_{e{q}_{\alpha }}{\lambda }_{\ell q_{\beta }}/M_{\mathrm{LQ}}^2$ for LQs that couple to an electron and a quark $q_{\alpha }$ and to a lepton $\ell$ and a quark $q_{\beta }$, where $\alpha$ and $\beta$ 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 $ep\to \tau X$, the ZEUS limits are the most stringent to date.

37 pages, 10 figures, Accepted by EPJC. References and 1 figure (Fig. 6) added