A measurement of the branching ratio of long-lived neutral kaon going to positive muon negative muon

The branching ratio $B(K\sbsp{L}{0} \to \mu\sp+\mu\sp-)$ has been measured using data obtained during running periods in 1988, 1989 and 1990. The data obtained in the 1990 running period are the latest and last from BNL experiment E791. During the three running periods, 87, 274 and 346 respective candidate events were observed, forming an overall sample of 707 $\mu\sp+\mu\sp-$ events. This number represents the largest sample to-date of $K\sbsp{L}{0} \to \mu\sp+\mu\sp-$ events. The result for the branching fraction using the total data set is $B(K\sbsp{L}{0} \to \mu\sp+\mu\sp-)$ = (6.86 $\pm$ 0.37) $\times$ 10$\sp{-9}$. This result is very near the unitarity bound of 6.81 $\times$ 10$\sp{-9}$ and is consistent with earlier measurements