Search for lepton-flavor violation at HERA

A search for lepton-flavor-violating interactions ep -> mu X and ep -> 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, root 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 root s, limits were set on lambda(eq1) root beta(lq), where lambda(eq1) is the coupling of the LQ to an electron and a first-generation quark q(1), and beta(eq) is the branching ratio of the LQ to the final-state lepton l (mu or tau) and a quark q. For LQ masses much larger than root s, limits were set on the four-fermion interaction term lambda(eq alpha)lambda(eq beta)/M(L)(2)Q for LQs that couple to an electron and a quark q(alpha) and to a lepton l 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 -> tau X, the ZEUS limits are the most stringent to date.