Measurement of the branching ratio for the rare decay [ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]]

We have measured the branching ratio for [ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]] using our full data set obtained during running periods in 1988, 1989, and 1990. The total number of [mu][sup +][mu][sup [minus]] candidates after a background subtraction is 707, which represents the largest sample to date of this rare decay mode. Our result is [ital B]([ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]])=(6.86[plus minus]0.37)[times]10[sup [minus]9], which is consistent with earlier results and very near the unitarity bound. The upper limit on [ital B]([ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]]) is used to set an upper limit on the real part of the amplitude [ital scrA]([ital K][sub [ital L]][sup 0][r arrow][mu][sup +][mu][sup [minus]]); this is subsequently used to constrain the top quark mass [ital m][sub [ital t]] and the Wolfenstein parameter [rho] of the CKM matrix.