Observation of the Decay K- -> pi- mu+ mu- and Measurements of the Branching Ratios for K+/- -> pi+/- mu+ mu-

TL;DR

This paper reports the first observation of the decay K- -> pi- mu+ mu- and provides new measurements of the branching ratios for K+/- -> pi+/- mu+ mu-, including a ratio of decay rates and a CP asymmetry measurement.

Contribution

It presents the first observation of a rare kaon decay and new precise measurements of its branching ratios and CP asymmetry, expanding understanding of kaon decay processes.

Findings

First observation of K- -> pi- mu+ mu- decay.

Measured branching ratio ratio (K+/- -> pi+/- mu+ mu-)/(K+/- -> all) = (9.8 +/- 1.0 +/- 0.5)x10^(-8).

CP asymmetry between K+ and K- decay modes = -0.02 +/- 0.11 +/- 0.04.

Abstract

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- -> pi- mu+ mu- and new measurements of the branching ratios for K+/- -> pi+/- mu+ mu- . By combining the branching ratios for the K+ and K- decays, we measure the ratio (K+/- -> pi+/- mu+ mu-)/(K+/- -> all) = (9.8 +/- 1.0 +/- 0.5)x10^(-8). The CP asymmetry between the K+ and K- decay modes = -0.02 +/- 0.11 +/- 0.04.