Observation of the decay ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{K}^{-}$

Decays of the ${\mathrm{\Xi }}_b^{-}$ and ${\mathrm{\Omega }}_b^{-}$ baryons to the charmless final states $p{h}^{-}{h}^{\mathrm{\prime }-}$, where $h^{(\mathrm{\prime })}$ 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$8 TeV$,correspondingtoanintegratedluminosityof$3 {\rm fb}^{-1}$. The decay ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{K}^{-}$ is observed with a significance of $8.7$ standard deviations, and evidence at the level of $3.4$ standard deviations is found for the ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{\pi }^{-}$ 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 ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{\pi }^{-}$ and ${\mathrm{\Xi }}_b^{-}\to p{\pi }^{-}{\pi }^{-}$ relative to $\Xi_b^- \to p K^-K^-$ decays are also measured.