Search for pair production of heavy vector-like quarks decaying to high-$p_T$ $W$ bosons and b quarks in the lepton-plus-jets final state in pp collisions at $\sqrt{s}$=13 TeV with the ATLAS detector

A search is presented for the pair production of a heavy vector-like quark T, together with its antiparticle, targetting a T quark decay to a W boson and a b-quark. The search is based on 14.7±0.4 fb−1 of pp collisions at $\sqrt{s}$ = 13 TeV recorded in 2015 and early 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton plus jets final state, characterised by a high-transverse-momentum isolated electron or muon, large missing transverse momentum, multiple jets, of which at least one is b-tagged, and a single large-radius jet or system of two small-radius jets identified as originating from the hadronic decay of a W boson. No significant deviation from the Standard Model expectation is observed in the reconstructed mass distribution of the leptonically-decaying vector-like T quark candidate. Under two different assumptions, that of a branching ratio BR(T→Wb) = 1 or an SU(2) singlet, an observed (expected) 95% CL lower limit on the vector-like quark mass of 1090 GeV (980 GeV) and 810 GeV (870 GeV) is obtained, respectively. For vector-like T quarks, under the assumption that only the T→Wb, T→Zt and T→Ht decay modes contribute, 95% CL upper limits are derived for various masses in the two-dimensional plane of BR(T→Ht) versus BR(T→Wb).