Search for pair production of third-generation leptoquarks decaying into a bottom quark and a $τ$-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-protoncollisions at a centre-of-mass energy of $\sqrt{s} =13$ TeV. Events in which atleast one $\tau$-lepton decays hadronically are considered, and multivariatediscriminants are used to extract the signals. No significant deviations fromthe Standard Model expectation are observed and 95% confidence-level upperlimits on the production cross-section are derived as a function of leptoquarkmass and branching ratio $B$ into a $\tau$-lepton and $b$-quark. For scalarleptoquarks, masses below 1460 GeV are excluded assuming $B=100$%, while forvector leptoquarks the corresponding limit is 1650 GeV (1910 GeV) in theminimal-coupling (Yang-Mills) scenario.