Search for pair production of third-generation leptoquarks decaying into a bottom quark and a $\tau $ -lepton with the ATLAS detector

A search for pair-produced scalar or vector leptoquarks decaying into a b-quark and a $\tau $ -lepton is presented using the full LHC Run 2 (2015–2018) data sample of 139 fb ${}^{-1}$ collected with the ATLAS detector in proton–proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Events in which at least one $\tau $ -lepton decays hadronically are considered, and multivariate discriminants are used to extract the signals. No significant deviations from the Standard Model expectation are observed and 95% confidence-level upper limits on the production cross-section are derived as a function of leptoquark mass and branching ratio $B$ into a $\tau $ -lepton and b-quark. For scalar leptoquarks, masses below 1460 GeV are excluded assuming $B=100$ %, while for vector leptoquarks the corresponding limit is 1650 GeV (1910 GeV) in the minimal-coupling (Yang–Mills) scenario.