Search for the charged-lepton-flavor-violating decay $Z\rightarrow e\mu$ in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

TL;DR

This paper reports a search for the rare decay $Z\rightarrow e\mu$ in proton-proton collisions at 13 TeV using ATLAS data, setting the most stringent upper limit to date on its branching fraction, with no evidence of such decay.

Contribution

First search for $Z\rightarrow e\mu$ decay at 13 TeV with ATLAS, establishing the most stringent upper limit on its branching fraction to date.

Findings

No excess observed near the $Z$ boson mass.

Upper limit on $B(Z\rightarrow e\mu)$ is $2.62 \times 10^{-7}$ at 95% CL.

Provides the most restrictive constraint on charged-lepton-flavor violation in $Z$ decays.

Abstract

A search for the charged-lepton-flavor-violating process $Z\rightarrow e\mu$ is presented, using 139 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data collected by the ATLAS experiment at the LHC. An excess in the $e\mu$ invariant mass spectrum near the $Z$ boson mass would be a striking signature of new physics. No excess is observed, and an upper limit $B(Z\rightarrow e\mu) < 2.62 \times 10^{-7}$ is placed on the branching fraction at 95% confidence level, which is the most stringent limit to date.