Search for top-quark decays $t\to Hq$ with 36 fb${}^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ 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, $t\to Hq$, is presented. The search is based on a dataset of $pp$ collisions at $\sqrt{s}=13$ 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 $H\to b\bar{b}$ and $H\to {\tau }^{+}{\tau }^{-}$ decay modes, respectively. The high multiplicity of $b$-quark jets, or the presence of hadronically decaying $\tau$-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 $t\to Hq$ 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 $t\to Hc$ and $t\to Hu$ branching ratios of $1.1\times 10^{-3}$ ($8.3\times 10^{-4}$) and $1.2\times 10^{-3}$ ($8.3\times 10^{-4}$), respectively. The corresponding combined observed (expected) upper limits on the $|{\lambda }_{tcH}|$ and $|{\lambda }_{tuH}|$ couplings are 0.064 (0.055) and 0.066 (0.055), respectively.

68 pages in total, author list starting page 52, 15 figures, 9 tables, published in JHEP, JHEP 05 (2019) 123. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2017-07