First observation of the charmless decay of $\Xi_b^- \to pK^-K^-$ at LHCb

We present here the search for ${Ξ}^−_b$ and ${Ω}^−_b$ baryon decays to the charmless hadronic final states ph−h′− at the LHCb detector, where h(′) denotes a kaon or pion. The analysis is based on a sample of proton-proton collision data collected at centre-of-mass energies √s= 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb−1. We observe the decay of ${Ξ}^−_b→pK^−K^−$ with a significance of 8.7 standard deviations, and evidence at the level of 3.4 standard deviations is found for the ${Ξ}^−_b→pK^−π^−$ decay. This result is the first ever observation of a ${Ξ}^−_b$ baryon decaying into charmless final state.