Search for pair-produced scalar and vector leptoquarks decaying into third-generation quarks and first- or second-generation leptons in $pp$ collisions with the ATLAS detector

A search for pair-produced scalar and vector leptoquarks decaying into quarks and leptons of different generations is presented using the full LHC Run~2 (2015-2018) dataset of $139\,\mbox{fb}^{-1}$ collected with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13\,\mbox{TeV}$. Scalar leptoquarks with charge -1/3$e$ as well as scalar and vector leptoquarks with charge +2/3$e$ are considered. All possible decays of the pair-produced leptoquarks into quarks of the third ($t, b$) generation and charged or neutral leptons of the first or second ($e, \mu, \nu$) generation with exactly one electron or muon in the final state are investigated. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section are provided for eight models as a function of the leptoquark mass and the branching ratio of the leptoquark into the charged or neutral lepton. In addition, lower limits on the leptoquark masses are derived for all models across a range of branching ratios. Two of these models are directly aiming at an explanation for the recent $B$-anomalies. Here a $U_1$ vector leptoquark decays into charged and neutral leptons of the second generation with a similar branching fraction. Lower limits on the mass of 1980 GeV and 1710 GeV have been found for two different coupling scenarios.