(eng)
A search for lepton-flavor-violating interactions $e p \to \mu X $ and $e
p\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 $\lambda_{eq_1} \sqrt{\beta_{\ell q}}$, where
$\lambda_{eq_1}$ 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 $e p\to \tau X $, the ZEUS limits are the most
stringent to date.
Comment: 37 pages, 10 figures, Accepted by EPJC. References and 1 figure (Fig.
6) added
