Search for pair-produced vector-like $T$-quarks decaying into $Ht$ final states in the lepton-plus-jets channel in $pp$ collisions at $\sqrt{s}$=13 TeV with the ATLAS detector

A search for pair-produced vector-like $T$-quarks in 139 fb${}^{-1}$ of $pp$ collisions recorded by the ATLAS detector at a centre-of-mass energy $\sqrt{s}$=13 TeV is presented. The search targets signal events where at least one $T$-quark decays into a Higgs boson and a top quark, $T\to Ht$, with subsequent decays of the Higgs boson into a bottom quark-antiquark pair, $H\to b\bar{b}$, and the top quark into a light charged lepton, a neutrino and a bottom quark, $t\to {\mathrm{\ell }}^{+}{\nu }_{\mathrm{\ell }}b$, where $\mathrm{\ell }=e\mathrm{/}\mu$. The decay products of boosted heavy resonances are reconstructed using variable-radius, reclustered jets, and a neural network is employed to discriminate signal from Standard Model background processes. A profile-likelihood fit to the effective mass distribution is performed, and no excess above the Standard Model prediction is observed. Exclusion limits at 95% confidence level are set on the $T$-quark pair-production cross-section, corresponding to lower mass limits of 1.40 TeV in the SU(2) singlet representation, 1.56 TeV in the SU(2) doublet representation, and 1.66 TeV assuming the branching fraction of $T$-quarks to $Ht$ to be 100%.