Prospects for discovering strongly decaying doubly heavy $T_{bc}$ tetraquark states at LHCb

Abstract

We investigate the discovery potential of the $T_{bc}$ state with $J^P = 0^+$ in proton-proton ($pp$) collisions at LHCb at a center-of-mass energy of $\sqrt{s} = 13~\mathrm{TeV}$. The study focuses on the decay channel $T_{bc} \to B^- D^+$. A phenomenological approach is employed to construct the background model based on the associated production of $B$ and $D$ mesons, incorporating previously published LHCb results. Background processes are simulated using $\texttt{MadGraph5\_aMC@NLO}$ and $\texttt{Pythia8.3}$. We explore the parameter space of the $T_{bc}$ mass, width, production cross section, and the effective double-parton scattering cross section ($\sigma_{\mathrm{eff}}$) relevant for the $B D$ meson background. The integrated luminosity required for a $5\sigma$ discovery at LHCb is evaluated under various assumptions. In particular, we consider three representative $T_{bc}$ production cross section scenarios: an optimistic estimate of $103~\mathrm{nb}$, an intermediate value of $18~\mathrm{nb}$ obtained by scaling from the $T_{cc}^+$ production cross section, and a conservative lower bound of $0.3~\mathrm{nb}$. We find that a $5\sigma$ observation is achievable for a production cross section of $103~\mathrm{nb}$, which is expected to be within reach during Run~4. In contrast, the more realistic cross section estimate of $18~\mathrm{nb}$ requires the full Run~5 dataset ($300~\mathrm{fb}^{-1}$) under the most favorable parameter choices. For the conservative scenario, no significant signal would be observable even with $300~\mathrm{fb}^{-1}$. In addition, we estimate the minimum observable $\sigma(T_{bc}) \times BR(T_{bc} \to B^- D^+)$ for a $5\sigma$ discovery under different luminosity scenarios, providing guidance for future experimental searches at LHCb.