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Abstract

This paper presents a search for a new Z 0 vector gauge boson with the ATLAS experiment at the Large Hadron Collider using pp collision data collected at √ s = 13 TeV, corresponding to an integrated luminosity of 139 fb−1 . The new gauge boson Z 0 is predicted by Lµ − Lτ models to address observed phenomena that can not be explained by the Standard Model. The search examines the four-muon (4µ) final state, using a deep learning neural network classifier to separate the Z 0 signal from the Standard Model background events. The di-muon invariant masses in the 4µ events are used to extract the Z 0 resonance signature. No significant excess of events is observed over the predicted background. Upper limits at a 95% confidence level on the Z 0 production cross-section times the decay branching fraction of pp → Z 0µµ → 4µ are set from 0.31 to 4.3 fb for the Z 0 mass ranging from 5 to 81 GeV. The corresponding common coupling strengths, gZ0, of the Z 0 boson to the second and third generation leptons above 0.003 – 0.2 have been excluded.