a Measurement of the Rate at which the Long-Lived Neutral Kaon Decays to Two Muons.

The decay K_sp{rm L }{circ} to mu^{+ }mu^{-} is an example of a flavor-changing neutral current. The famous GIM mechanism accounted for its low rate of occurrence. Unitarity requirements fix lower bound on this rate at (1.2 times 10^{-5})Gamma(K _sp{rm L}{circ} to gammagamma), which equals, given the present experimental value of the two-gamma rate, 0.132 sec^{-1}, for a branching fraction of 6.8 times 10 ^{-9}. Brookhaven National Laboratory Experiment 791, in a search for unobserved rare kaon decays, collected a sample of two-muon decay candidates. The background-subtracted number in this sample was 87.9 +/- 9.6 for a decay rate of 0.123 +/- 0.015(stat.) +/- 0.005(syst.) sec^{-1}. This is equivalent to a branching ratio of 6.3 times 10^{-9}, 0.625 standard deviation below the unitarity bound.