Observation of the decay $Ξ_b^- \to p K^-K^-$

Abstract: Decays of the $\Xi_b^-$ and $\Omega_b^-$ baryons to the charmless final states $p h^{-}h^{\prime -}$, where $h^{(\prime)}$ denotes a kaon or pion, are searched for with the LHCb detector. The analysis is based on a sample of proton-proton collision data collected at centre-of-mass energies $\sqrt{s} = 7$ and $8 TeV$, corresponding to an integrated luminosity of $3 {\rm fb}^{-1}$. The decay $\Xi_b^- \to p K^-K^-$ is observed with a significance of $8.7$ standard deviations, and evidence at the level of $3.4$ standard deviations is found for the $\Xi_b^- \to p K^-\pi^-$ decay. Results are reported, relative to the $B^-\to K^+K^-K^-$ normalisation channel, for the products of branching fractions and $b$-hadron production fractions. The branching fractions of $\Xi_b^- \to p K^-\pi^-$ and $\Xi_b^- \to p \pi^-\pi^-$ relative to $\Xi_b^- \to p K^-K^-$ decays are also measured.