Testing the W-exchange mechanism with two-body baryonic B decays

The role of $W$-exchange diagrams in baryonic $B$ decays is poorly understood, and often taken as insignificant and neglected. We show that charmful two-body baryonic $B\to {\bf B}_c \bar {\bf B}'$ decays provide a good test-bed for the study of the $W$-exchange topology, whose contribution is found to be non-negligible; here ${\bf B}_c$ is an anti-triplet or a sextet charmed baryon, and ${\bf \bar B}'$ an octet charmless (anti-)baryon. In particular, we calculate that ${\cal B}(\bar B^0\to\Sigma_c^{+}\bar p)=(2.9^{+0.8}_{-0.9})\times 10^{-6}$ in good agreement with the experimental upper bound. Its cousin $\bar B_s^0$ mode, $\bar B_s^0\to\Lambda^+_c\bar p$, is a purely $W$-exchange decay, hence is naturally suited for the study of the role of the $W$-exchange topology. We predict ${\cal B}(\bar B_s^0\to\Lambda^+_c\bar p)=(0.8\pm 0.3)\times 10^{-6}$, a relatively large branching ratio to be tested with a future measurement by the LHCb collaboration. Other predictions, such as ${\cal B}(\bar B^0\to\Xi_c^+\bar\Sigma^-)=(1.1\pm 0.4)\times 10^{-5}$, can be tested with future Belle II measurements.