Search for Single Production of a Vector-like $T$ Quark Decaying into a Higgs Boson and Top Quark with Fully Hadronic Final States Using the ATLAS Detector

A search is made for a vector-like $T$ quark decaying into a Higgs boson and a top quark in 13 TeV proton-proton collisions using the ATLAS detector at the Large Hadron Collider with a data sample corresponding to an integrated luminosity of 139 fb$^{−1}$. The Higgs-boson and top-quark candidates are identified in the all-hadronic decay mode, where $H \rightarrow b\bar{b}$ and $t \rightarrow bW \rightarrow q\bar{q}'$ are reconstructed as large-radius jets. The candidate Higgs boson, top quark, and associated B-hadrons are identified using tagging algorithms. No significant excess is observed above the background, so limits are set on the production cross-section of a singlet $T$ quark at 95% confidence level, depending on the mass, $m_{T}$, and coupling, $\kappa_{T}$, of the vector-like $T$ quark to Standard Model particles. In the considered mass range between 1.0 and 2.3 TeV, the upper limit on the allowed coupling values increases with $m_{T}$ from a minimum value of 0.35 for $1.07 < m_{T} < 1.4$ TeV to 1.6 for $m_{T} =2.3$ TeV.