A New Measurement of the Rare Decay $K^+\to \pi^+ \mu^+ \mu^-$

TL;DR

This paper reports a new measurement of the rare decay $K^+ o \pi^+ \mu^+ \mu^-$, providing a more precise branching ratio that aligns with similar decays but differs from earlier results.

Contribution

The study presents the first measurement of the $K^+ o \pi^+ \\mu^+ \\mu^-$ decay branching ratio using over 400 events, improving precision and resolving previous discrepancies.

Findings

Branching ratio: (9.22 ± 0.60 (stat) ± 0.49 (syst))×10^{-8}

Agreement with $K^+ o \\pi^+ e^+ e^-$ decay measurements

Significant deviation from earlier measurements

Abstract

More than 400 $K^{+}\to\pi^{+}\mu^+\mu^-$ events were observed in a rare $K^+$ decay experiment at the AGS. Normalized to the $K^{+}\to\pi^{+}\pi^+\pi^-$ decay, the branching ratio is determined to be $(9.22 \pm 0.60 (stat) \pm 0.49 (syst))\times 10^{-8}$. This branching ratio and the $\mu\mu$ mass spectrum is in very good agreement with the measurement of the $K^{+}\to\pi^{+}e^+e^-$ decay, but deviates significantly from the previous measurement.