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

A search is presented for single production of a vector-like $B$ quark decaying to a Standard Model $b$-quark and the Standard Model Higgs boson, which decays into $b\bar{b}$. The search is carried out on 139\,fb$^{-1}$ of proton--proton collision data at $\sqrt{s} = 13\text{ TeV}$ collected by the ATLAS detector at the LHC between 2015 and 2018. No significant deviation from the Standard Model background prediction is observed. Mass-dependent exclusion limits at the $95\%$ confidence level are drawn on the single production cross-section of a vector-like $B$ quark occurring as part of a ($B$, $Y$) weak isospin doublet and various hypotheses on the coupling strength $\kappa$. Within such hypotheses the search excludes the presence of a vector-like $B$ quark resonance with $1.0\text{ TeV} < m_B < 1.28\text{ TeV}$ and $1.46\text{ TeV} < m_B < 2.0\text{ TeV}$ for $\kappa = 0.25$, and the $1.0\text{ TeV} < m_B < 2.0\text{ TeV}$ resonance mass range for $\kappa = 0.3$. These intervals exclude resonances of relative width larger than 5$\%$ for masses $1.0\text{ TeV} < m_B < 1.76\text{ TeV}$ .