Search for single vector-like $B$ quark production and decay via $B\rightarrow bH(b\bar{b})$ in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search is presented for single production of a vector-like $B$ quark decaying into a Standard Model $b$-quark and a Standard Model Higgs boson, which decays into a $b\bar{b}$ pair. The search is carried out in 139 fb$^{-1}$ of $\sqrt{s} = 13$ TeV proton-proton collision data collected by the ATLAS detector at the LHC between 2015 and 2018. No significant deviation from the Standard Model background prediction is observed, and mass-dependent exclusion limits at the 95% confidence level are set on the resonance production cross-section in several theoretical scenarios determined by the couplings $c_W$, $c_Z$ and $c_H$ between the $B$ quark and the Standard Model $W$, $Z$ and Higgs bosons, respectively. For a vector-like $B$ occurring as an isospin singlet, the search excludes values of $c_W$ greater than 0.45 for a $B$ resonance mass ($m_B$) between 1.0 and 1.2 TeV. For 1.2 TeV < $m_B$ < 2.0 TeV, $c_W$ values larger than 0.50-0.65 are excluded. If the $B$ occurs as part of a $(B,Y)$ doublet, the smallest excluded $c_Z$ coupling values range between 0.3 and 0.5 across the investigated resonance mass range 1.0 TeV < $m_B$ < 2.0 TeV.