Measurement of the branching ratio of the decay ${\mathrm{\Xi }}^0\to {\mathrm{\Sigma }}^{+}{\mu }^{-}{\bar{\nu }}_{\mu }$

From the 2002 data taking with a neutral kaon beam extracted from the CERN-SPS, the NA48/1 experiment observed 97 $\Xi^{0}\rightarrow \Sigma^{+} \mu^{-} \bar{\nu}{\mu}$candidateswithabackgroundcontaminationof$30.8 \pm 4.2$ events. From this sample, the BR($\Xi^{0}\rightarrow \Sigma^{+} \mu^{-} \bar{\nu}{\mu}$)ismeasuredtobe$(2.17 \pm 0.32_{$\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}$}\pm 0.17_{$\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}$})\times10^{-6}$.