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

Altres ajuts: Hadron-Hadron scattering (experiments); Rare decay; Top physics 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 corresponding to an integrated luminosity of 36.1 fb. Two complementary analyses are performed to 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 → ττ decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ-leptons, is 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 (8.3 × 10) and 1.2 × 10 (8.3 × 10), respectively. The corresponding combined observed (expected) upper limits on the couplings are 0.064 (0.055) and 0.066 (0.055), respectively.