Search for lepton-flavor violation at HERA

doi:10.1140/epjc/s2005-02399-1

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2024 Impact factor 4.8

Particles and Fields

Eur. Phys. J. C (2005) 44: 463-479
https://doi.org/10.1140/epjc/s2005-02399-1

Experimental Physics

Search for lepton-flavor violation at HERA

The Zeus Collaboration

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 $130 \text{pb}^{-1}$ . The data were taken at center-of-mass energies, $\sqrt{s}$ , of 300 and $318 \text{Ge}\;\text{V\/}$ . 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 $\ell$ (μ 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.