A Measurement of the Branching Ratio and Form Factor of K_L->mu mu gamma

We report on a measurement of the decay $K_Lrightarrowmu^+mu^-gamma$ from Fermilab experiment E799. We observe $207$ candidate signal events with an estimated background of $10.5 pm 4.0$ events and establish $B(K_Lrightarrowmu^+mu^-gamma) = (3.23pm 0.23(stat) pm 0.19(sys))times 10^{-7}$. This provides the first measurement of the $Kgammagamma^*$ form factor in the muonic Dalitz decay mode of the $K_L$.