Search for a new $Z'$ gauge boson in $4\mu$ events with the ATLAS experiment

This note presents a search for a new $Z'$ vector gauge boson with the ATLAS experiment at the Large Hadron Collider using $pp$ collision data collected at $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 139 fb$^{-1}$. The new gauge boson $Z'$ is predicted by $L_{\mu}-L_{\tau}$ models %to address the observed anomalies of the muon $g-2$ and rare B decay measurements. to address observed phenomena that can not be explained by the Standard Model. The search examines the four-muon (4$\mu$) final state, using a deep learning neural network classifier to separate the $Z'$ signal from the Standard Model background events. The di-muon invariant masses in the $4\mu$ events are used to extract the $Z'$ resonance signature. No significant excess of events is observed over the predicted background. Upper limits from 0.425 to 2.04 fb, at a 95\% confidence level, are set on the production cross-section times the decay branching fraction of $pp \rightarrow Z'\mu\mu \rightarrow 4\mu$ for the $Z'$ mass ranging from 5 to 75 GeV. Corresponding common coupling strengths, $g_{Z'}$, of the $Z'$ boson to the second and third generation leptons above 0.003 - 0.2 have been excluded.