Search for Third-Generation Vectorlike Leptons in pp Collisions at √𝒔 = 13 TeV with the ATLAS detector

A search for vectorlike leptons coupling to the third generation Standard Model leptons in the multilepton (two, three, and four or more light leptons) final state with zero or more hadronic tau lepton decays is presented. The search was performed using a dataset of proton-proton collisions corresponding to an integrated luminosity of 139 fb at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the LHC. To maximize the separation of signal from background events, a machine learning classifier was used. No excess of events was observed beyond the standard model expectation. Using a doublet vectorlike lepton model, vectorlike leptons coupling to third generation Standard Model leptons are excluded in the mass range 130 GeV to 900 GeV at the 95% confidence level, with an expected upper mass limit of 970 GeV.