Observation of the Decay $K^{-} \to π^{-} μ^{+} μ^{-}$ and Measurements of the Branching Ratios for $K^{\pm} \to π^{\pm} μ^{+} μ^{-}$

Using data collected with the HyperCP (E871) spectrometer during the 1997 fixed-target run at Fermilab, we report the first observation of the decay

K^{-} \to π^{-} μ^{+} μ^{-}

and new measurements of the branching ratios for

K^{\pm} \to π^{\pm} μ^{+} μ^{-}

. By combining the branching ratios for the decays

K^{+} \to π^{+} μ^{+} μ^{-}

and

K^{-} \to π^{-} μ^{+} μ^{-}

, we measure

Γ (K^{\pm} \to π^{\pm} μ^{+} μ^{-}) / Γ (K^{\pm} \to all) = (9.8 \pm 1.0 \pm 0.5) \times 10^{- 8}

. The

CP

asymmetry between the rates of the two decay modes is

[Γ (K^{+} \to π^{+} μ^{+} μ^{-}) - Γ (K^{-} \to π^{-} μ^{+} μ^{-})] / [Γ (K^{+} \to π^{+} μ^{+} μ^{-}) + Γ (K^{-} \to π^{-} μ^{+} μ^{-})] = - 0.02 \pm 0.11 \pm 0.04

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Observation of the DecayK−→π−μ+μ−and Measurements of the Branching Ratios forK±→π±μ+μ−