Probing Heavy Neutral Higgs Bosons via Single Vector-Like Bottom Quark Production at the HL-LHC

We investigate the discovery prospects of a singly produced vector-like bottom quark in the Type-II Two-Higgs-Doublet Model extended by an $SU(2{)}_L$ vector-like $(T,B)$ doublet. We focus on the non-standard decay chain $B\to \phi b$, followed by $\phi \to t\bar{t}$, where $\phi =H$ or $A$, leading to a final state with one charged lepton, missing transverse energy, and multiple $b$-jets. We perform a full simulation of both signal and Standard Model backgrounds at $\sqrt{s}=14$ TeV. We show that the exotic channels $B\to \phi b$ can dominate over the conventional decay modes, reaching branching ratios of order $50\mathrm{\%}$ for both neutral scalars in the alignment limit. A conventional cut-based analysis provides a $5\sigma$ discovery significance only at sufficiently high integrated luminosity. By contrast, an XGBoost-based multivariate analysis substantially improves the signal-background discrimination and extends the discovery reach up to $m_B\simeq 1.3$ TeV with $600{\mathrm{f}\mathrm{b}}^{-1}$ and up to $m_B\simeq 1.6$ TeV with $3{\mathrm{a}\mathrm{b}}^{-1}$, even in the presence of systematic uncertainties as large as $15\mathrm{\%}$.