Search for lepton-flavor violation at HERA

TL;DR

This study searched for lepton-flavor-violating interactions at HERA using ZEUS data, found no evidence, and set new constraints on leptoquark models and related processes involving electrons, muons, and taus.

Contribution

First comprehensive search for lepton-flavor violation at HERA, establishing new limits on leptoquark couplings and masses, including scenarios with high-generation quarks and supersymmetric models.

Findings

No evidence of lepton-flavor violation was observed.

Set the most stringent limits to date on certain leptoquark couplings.

Constraints apply to models involving squarks in R-Parity-violating supersymmetry.

Abstract

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.