First Measurement of the Absolute Branching Fraction of $\Lambda \to p \mu^- \bar{\nu}_{\mu}$

TL;DR

This paper reports the first measurement of the absolute branching fraction of the decay $oldsymbol{ ext{Lambda} o p oldsymbol{ ext{mu}}^- ar{ u}_{ ext{mu}}}$, providing a precise value that tests lepton flavor universality and searches for CP violation.

Contribution

The first direct measurement of the $ ext{Lambda} o p ext{mu}^- ar{ u}_{ ext{mu}}$ branching fraction with improved precision and its implications for lepton universality and CP symmetry.

Findings

Branching fraction measured as (1.48 ± 0.21 (stat) ± 0.08 (syst)) × 10^{-4}

Ratio of muonic to electronic decay widths is 0.178 ± 0.028

No evidence of CP violation observed in decay asymmetries

Abstract

The absolute branching fraction of $\Lambda \to p \mu^- \bar{\nu}_{\mu}$ is reported for the first time based on an $e^+e^-$ annihilation sample of ten billion $J/\psi$ events collected with the BESIII detector at $\sqrt{s}=3.097$ GeV. The branching fraction is determined to be ${\mathcal B}(\Lambda \to p\mu^- \bar{\nu}_{\mu}) = [1.48\pm0.21(\rm stat) \pm 0.08(\rm syst)]\times 10^{-4}$, which is a significant improvement in precision over the previous indirect measurements. Combining this result with the world average of ${\mathcal B}(\Lambda \to p e^- \bar{\nu}_{e})$, we obtain the ratio, $\frac{\Gamma(\Lambda \to p \mu^- \bar{\nu}_{\mu})}{\Gamma(\Lambda \to p e^- \bar{\nu}_{e})}$, to be $0.178 \pm 0.028$, which agrees with the standard model prediction assuming lepton flavor universality. The asymmetry of the branching fractions of $\Lambda \to p \mu^- \bar{\nu}_{\mu}$ and $\bar{\Lambda} \to \bar{p} \mu^+ \nu_{\mu}$ is also determined, and no evidence for $CP$ violation is found.