中国物理学会高能物理分会第十三届全国粒子物理学术会议（2021）

A search for flavor-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 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 1.1X10−3 (8.3X10−4) and 1.2X10−3 (8.3X10−4), 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