Abstract
A search for pair-production of vector-like leptons is presented, considering their decays into a thirdgeneration Standard Model (SM) quark and a vector leptoquark (U-1) as predicted by an ultraviolet-complete extension of the SM, referred to as the '4321' model. Given the assumed decay of U-1 into third-generation SMfermions, the final state can contain multiple t -leptons and b-quarks. This search is based on a dataset of pp collisions at root s = 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 vectorlike lepton mass. Additionally, the results are interpreted for a supersymmetric model with an R-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).