Search for electroweak production of vector-like leptons in <i>τ</i>-lepton and <i>b</i>-jet final states in <i>pp</i> collisions at √<i>s</i> = 13 TeV with the ATLAS detector

A search for pair-production of vector-like leptons is presented, considering their decays into a third-generation Standard Model (SM) quark and a vector leptoquark (<i>U</i>₁) as predicted by an ultraviolet-complete extension of the SM, referred to as the '4321' model. Given the assumed decay of <i>U</i>₁ into third-generation SM fermions, the final state can contain multiple <i>τ</i>-leptons and <i>b</i>-quarks. This search is based on a dataset of <i>pp</i> collisions at √<i>s</i> = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of up to 140 fb^-1. No significant excess above the SM background prediction is observed, and 95% confidence level limits on the cross-section times branching ratio are derived as a function of the vector-like lepton mass. A lower observed (expected) limit of 910 GeV (970 GeV) is set on the vector-like lepton mass. Additionally, the results are interpreted for a supersymmetric model with an <i>R</i>-parity violating coupling to the third-generation quarks and leptons. Lower observed (expected) limits are obtained on the higgsino mass at 880 GeV (940 GeV) and on the wino mass at 1170 GeV (1170 GeV).