LHCb measures R(D**), the Lambda_b semileptonic branching fraction, and B to D* mu nu form factors to test lepton universality.
Branching fraction measurement of the $\mathit{\Lambda} \to p \mu^- \overline{\nu}_{\mu}$ decay
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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.
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Wave functions fixed from a mass-spectrum fit in the relativistic three-quark model are used to compute branching fractions, LFU ratios, and the complete set of octet hyperon semileptonic transition form factors without extra parameters, agreeing with data and lattice results where available.
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- Scalar and Tensor Form Factors for $\Lambda \rightarrow p\ell \bar{\nu}_\ell$ from Lattice QCD