Search for pair-production of vector-like quarks in lepton+jets final states containing at least one $b$-tagged jet using the Run 2 data from the ATLAS experiment

Abstract: A search is presented for the pair-production of heavy vector-like quarks in the lepton+jets final state using 140 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}= 13$ TeV collected with the ATLAS detector. The search is optimised for vector-like top-quarks ($T$) that decay into a $W$ boson and a $b$-quark, with one $W$ boson decaying leptonically and the other hadronically. Other vector-like quark flavours and decay modes are also considered. Events are selected with one high transverse-momentum electron or muon, large missing transverse momentum, a large-radius jet identified as a $W$ boson, and multiple small-radius jets, at least one of which is $b$-tagged. Vector-like $T$-quarks with 100% branching ratio to $Wb$ are excluded at 95% CL for masses below 1700 GeV. These limits are also applied to vector-like $Y$-quarks, which decay exclusively into a $W$ boson and a $b$-quark. Isospin singlets with $ {\cal B}(T \to Wb:Ht:Zt)={1/2}:{1/4}:{1/4}$ are excluded for masses below 1360 GeV.