Observation and Mass Measurement of the Baryon ${\mathrm{\Xi }}_b^{-}$

We report the observation and measurement of the mass of the bottom, strange baryon ${\mathrm{\Xi }}_b^{-}$ through the decay chain ${\mathrm{\Xi }}_b^{-}\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-}$, where $J\mathrm{/}\psi \to {\mu }^{+}{\mu }^{-}$, ${\mathrm{\Xi }}^{-}\to \mathrm{\Lambda }{\pi }^{-}$, and $\mathrm{\Lambda }\to p{\pi }^{-}$. Evidence for observation is based on a signal whose probability of arising from the estimated background is 6.6 x 10^{-15}, or 7.7 Gaussian standard deviations. The ${\mathrm{\Xi }}_b^{-}$ mass is measured to be $5792.9\pm 2.5$ (stat.) $\pm 1.7$(syst\mathrm{.})MeV\mathrm{/}$c^2$.