Improved Branching Ratio Measurement for the Decay K_L⁰ → μ⁺μ^-

We report results from Experiment 871, performed at the BNL AGS, of a measurement of the branching ratio K_L⁰→μ⁺μ^- with respect to the CP-violating mode K_L⁰→π⁺π^-. This experiment detected over 6200 candidate μ⁺μ^- events, a factor of 6 more than that seen in all previous measurements combined. The resulting branching ratio Γ(K_L⁰→μ⁺μ^-)/Γ(K_L⁰→π⁺π^-)  =  (3.474±0.057)×10^-6 leads to a branching fraction B(K_L⁰→μ⁺μ^-)  =  (7.18±0.17)×10^-9, which is consistent with the current world average, and reduces the uncertainty in this decay mode by a factor of 3.