First observation of a baryonic $B_s^0$ decay

The first observation of a baryonic $B_s^0$ decay, $B^0_s \to p \bar{\Lambda} K^-$, is reported using proton-proton collision data corresponding to an integrated luminosity of 3.0 $\text {fb}^{-1}$recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The branching fraction is measured to be $$ {\cal B}(B^0_s \to p \bar{\Lambda} K^- + \bar{B}^0_s \to p \bar{\Lambda} K^-) = \left[5.48 \,^{+0.82}_{-0.80} \pm 0.60 \pm 0.51 ({\cal B}) \pm 0.32 (f_s/f_d)\right]\times 10^{-6}\,, $$ where the first uncertainty is statistical and the second systematic, the third uncertainty accounts for the experimental uncertainty on the branching fraction of the $B^0 \to p \bar{\Lambda} \pi^-$ decay used for normalisation, and the fourth uncertainty relates to the knowledge of the ratio of $b$-quark hadronisation probabilities $f_s/f_d$. The notation $B^0_s \to p \bar{\Lambda} K^- + \bar{B}^0_s \to p \bar{\Lambda} K^-$ emphasises the fact that both $B_s^0$ and $\bar{B}^0_s$ can decay to the $p \bar{\Lambda} K^-$ final state. This branching fraction measurement is not corrected for a possible bias due to the finite width difference of the $B_s^0$ system.