Observation and mass measurement of the baryon Xi(-)(b)

We report the observation and measurement of the mass of the bottom, strange baryon Xi(-)(b) through the decay chain Xi(-)(b)-->J/psi Xi(-), where J/psi-->mu(+)mu(-), Xi(-)-->Lambda pi(-), and Lambda-->p pi(-). A signal is observed whose probability of arising from a background fluctuation is 6.6 x 10(-15), or 7.7 Gaussian standard deviations. The Xi(-)(b) mass is measured to be 5792.9 +/- 2.5(stat) +/- 1.7(syst) MeV/c(2).