A search has been made for lepton-flavor-violating interactions of the type $e^{+}\overrightarrow{p}lX,$ where l denotes a μ or τ with high transverse momentum, at a center-of-mass energy $\sqrt{s}$ of 300 GeV with an integrated luminosity of 47.7 ${\mathrm{pb}}^{\mathrm{-}1}$ using the ZEUS detector at HERA. No evidence was found for lepton-flavor violation and constraints were derived on leptoquarks (LQs) that could mediate such interactions. For LQ masses below $\sqrt{s},$ limits are set on ${\lambda }_{{\mathrm{eq}}_1}$ $\sqrt{{\beta }_{\mathrm{lq}}},$ where ${\lambda }_{{\mathrm{eq}}_1}$ is the coupling of the LQ to an electron and a first-generation quark $q_1$ and ${\beta }_{\mathrm{lq}}$ is the branching ratio of the LQ to l and a quark. For LQ masses exceeding $\sqrt{s},$ limits are set on the four-fermion contact-interaction term ${\lambda }_{{\mathrm{eq}}_{\alpha }}$ ${\lambda }_{{\mathrm{lq}}_{\beta }}{/M}_{\mathrm{LQ}}^2$ for leptoquarks that couple to an electron and a quark $q_{\alpha }$ and also to l and a quark $q_{\beta }.$ Some of the limits are also applicable to lepton-flavor-violating processes mediated by squarks in R-parity-violating supersymmetric models. In some cases involving heavy quarks and especially for $l=\tau ,$ the ZEUS limits are the most stringent published to date.

• Received 8 January 2002

DOI:https://doi.org/10.1103/PhysRevD.65.092004