  The presence of a new decay mode relaxes the current mass exclusion limits on
vectorlike quarks considerably. We consider the case of a weak-singlet
vectorlike $B$ quark that can decay to a singlet scalar or pseudoscalar $\Phi$.
In an earlier paper [A. Bhardwaj et al., Roadmap to explore vectorlike quarks
decaying to a new scalar or pseudoscalar, Phys. Rev. D, 106 (2022) 095014;
arXiv:2203.13753], we mapped the possibilities to explore such setups at the
LHC. We showed that it is possible for a $B$ quark to decay into $\Phi$ and the
$\Phi$ to dominantly decay to a pair of gluons or $b$ quark(s) without
fine-tuning the parameters. In this paper, we present a collider search
strategy to look for the pair production of singlet $B$ quarks. If both $B$
quarks decay into $b\Phi$ pairs, the final state is fully hadronic:
$B{B}\to(b\Phi)({b}\Phi)\to (bgg)({b}gg)/(bb{b})({b}b{b})$, which is very
challenging to probe. Therefore, we consider a simpler mixed decay specific to
the singlet $B$ case, $BB\to(b\Phi)(tW)$ with a lepton in the final state, to
achieve the best sensitivity at the high-luminosity LHC. We use a deep neural
network with a weighted cross-entropy loss to separate the signal from the huge
SM background. Our analysis shows that large areas of the $M_{B}-M_{\Phi}$
parameter space are discoverable through this signature. We show how the
discovery and exclusion regions scale with the branching ratio in the new decay
mode. We also estimate the reach in the inclusive monolepton channel with the
same network model.
