Branching fraction measurement of the $\mathit{\Lambda} \to p \mu^- \overline{\nu}_{\mu}$ decay

TL;DR

This paper reports a precise measurement of the branching fraction for the decay Lambda to proton, muon, and neutrino, using LHCb data, improving previous measurements and testing lepton flavor universality.

Contribution

The study provides the most precise measurement to date of the Lambda decay branching fraction and confirms consistency with the Standard Model's lepton flavor universality.

Findings

Measured branching fraction: (1.462 ± 0.016 ± 0.100 ± 0.011) × 10^{-4}

Improved measurement precision by a factor of two

Lepton flavor universality ratio R^{μe} = 0.175 ± 0.012 agrees with Standard Model

Abstract

A measurement of the branching fraction for the decay $\mathit{\Lambda} \to p \mu^- \overline{\nu}_{\mu}$ is presented using $\textit{pp}$ collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV. The analysis is based on data recorded between 2016 and 2018, corresponding to an integrated luminosity of $5.4 \ \text{fb}^{-1}$. The result is obtained using $\mathit{\Lambda} \to p \pi^-$ decays as a normalisation channel. The measured branching fraction is $B(\mathit{\Lambda} \to p \mu^- \overline{\nu}_{\mu})= (1.462 \pm 0.016 \pm 0.100 \pm 0.011 ) \times 10^{-4}$, where the uncertainties are statistical, systematic, and due to the limited knowledge of the normalisation mode branching fraction, respectively. This result improves the precision of the branching fraction measurement by a factor of two compared to the previous best measurement and sets a more stringent bound on lepton flavour universality in $s \to u$ quark transitions. It is consistent with previous measurements, and the extracted lepton flavour universality test observable, $R^{\mu e} = \frac{\Gamma(\mathit{\Lambda} \to p \mu^- \overline{\nu}_\mu)}{\Gamma(\mathit{\Lambda} \to p e^- \overline{\nu}_e)} = 0.175 \pm 0.012$, agrees with the Standard Model prediction.