Search for top-quark decays t→Hq with 36 fb−1 of pp collision data at s√=13 TeV with the ATLAS detector

Abstract

A search for flavour-changing neutral current decays of a top quark into an up-type quark (q=u,c) and the Standard Model Higgs boson, t→Hq, is presented. The search is based on a dataset of pp collisions at s√=13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponds to an integrated luminosity of 36.1 fb−1. Two complementary analyses are performed that search for top-quark pair events in which one top quark decays into Wb and the other top quark decays into Hq, and target the H→bb¯ and H→τ+τ− decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ-leptons, are exploited in the two analyses respectively. Multivariate techniques are used to separate the signal from the background, which is dominated by top-quark pair production. No significant excess of events above the background expectation is found, and 95% CL upper limits on the t→Hq branching ratios are derived. The combination of these searches with ATLAS searches in diphoton and multilepton final states yields observed (expected) 95% CL upper limits on the t→Hc and t→Hu branching ratios of 1.1×10−3 (8.3×10−4) and 1.2×10−3 (8.3×10−4), respectively. The corresponding combined observed (expected) upper limits on the |λtcH| and |λtuH| couplings are 0.064 (0.055) and 0.066 (0.055), respectively.