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.
Categories: hep-ex