Rare two-body decays of the top quark into a bottom meson plus an up or charm quark

Rare two-body decays of the top quark into a neutral bottom-quark meson plus an up- or charm-quark: $t\to {\bar{B}}^0+u,c$; $t\to {\overline B}^0_{s}+ c,u$;and$t \to \Upsilon(nS)+ c,u$, are studied for the first time. The corresponding partials widths are computed at leading order in the non-relativistic QCD framework. The sums of all two-body branching ratios amount to $$\mathcal{B}$(t \to {\overline B}^0+ {\rm jet}) \approx $\mathcal{B}$(t \to {\overline B}^0_{s}+ {\rm jet}) \approx 4.2\cdot 10^{-5}$and$$\mathcal{B}$(t \to \Upsilon(nS)+ {\rm jet}) \approx 2\cdot 10^{-9}$, respectively. The feasibility to observe the $t\to {\overline B}^0_{(s)}+{\rm jet}$ decay is estimated in top-pair events produced in proton-proton collisions at $\sqrt{s}=14,100$ TeV at the LHC and FCC, respectively. Combining many exclusive hadronic ${\bar{B}}_{(s)}^0$ decays, with $J\mathrm{/}\psi$ or $D^{0,\pm }$ final states, about 50 (16000) events are expected in 3 (20) ab${}^{-1}$ of integrated luminosity at the LHC (FCC), after typical selection criteria, acceptance, and efficiency losses. An observation of the two-body top-quark decay can also be achieved in the interesting $t\to b(\rm{jet})+c(\rm{jet})$dijetfinalstate,wherethe${\overline B}^0_{(s)}$ decay products are reconstructed as a jet, with 5300 and 1.4 million signal events above backgrounds expected after selection criteria at the LHC and FCC, respectively. Such unique final states provide a new direct method to precisely measure the top-quark mass via simple 2-body invariant mass analyses.