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Description
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 all-hadronic decay modes $H \rightarrow b\bar{b}$ and $t \rightarrow bW \rightarrow bq\bar{q}'$ are reconstructed as large-radius jets and identified using tagging algorithms.
Improvements in background estimation, signal discrimination, and a larger data sample, contribute to an improvement in sensitivity over previous all-hadronic searches.
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.
This search targets a mass range between 1.0 to 2.3 TeV, and a coupling value between 0.1 to 1.6, expanding the phase space of previous searches.
In the considered mass range, 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 up to 1.6 for $m_{T} = 2.3$ TeV.
