Measurements of Branching Ratios for K+/- -> pi+/- mu+ mu-

The branching ratio for K+ -> pi+ mu+ mu- decay was measured recently by
two BNL experiments, E787 and E865. Their results disagreed by more than
three standard deviations.
In two fixed-target runs at Fermilab the HyperCP (E871) experiment
collected the largest sample of hyperon and charged-kaon decays in the
world. Based on an analysis of the 1997 data, we observe the decay K- ->
pi- mu+ mu- for the first time and measure the branching ratios for K+/-
-> pi+/- mu+ mu-.