Measurement of the branching ratio for the ${\mathrm{\Xi }}^{\mathrm{-}}$→${\mathrm{\Sigma }}^{\mathrm{-}}$γ radiative decay

We have measured the branching ratio for the hyperon radiative decay ${\mathrm{\Xi }}^{\mathrm{-}}$→${\mathrm{\Sigma }}^{\mathrm{-}}$γ from a sample of 211±33 events obtained in the polarized 375 GeV/c charged hyperon beam at Fermilab. We find B(${\mathrm{\Xi }}^{\mathrm{-}}$→${\mathrm{\Sigma }}^{\mathrm{-}}$γ/${\mathrm{\Xi }}^{\mathrm{-}}$→${\mathrm{\Lambda }}^0$${\mathrm{\pi }}^{\mathrm{-}}$)=(1.22±0.23±0.06)×${10}^{\mathrm{-}4}$ where the quoted errors are statistical and systematic, respectively. We have also obtained an indication that the sign of the asymmetry parameter of this decay is positive.