R. Aaij, A. S. W. Abdelmotteleb, Carlos Abellán Beteta, C. Abellán Beteta, T. Ackernley, A. A. Adefisoye, B. Adeva, M. Adinolfi, P. Adlarson, C. Agapopoulou, C. Aidala, Z. Ajaltouni, S. Akar, K. Carvalho Akiba, P. Albicocco, J. Albrecht, R. Aleksiejūnas, F. Alessio, P. Álvarez Cartelle, R. Amalric, S. Amato, S. Amato, Y. Amhis, L. An, L. Anderlini, M. Andersson, P. Andreola, M. Andreotti, Sergio Andres Estrada, Alessia Anelli, Dong Ao, C. Arata, F. Archilli, Zaratsyan Areg, Matteo Argenton, S. Arguedas Cuendis, Lisa Arnone, A. Artamonov, M. Artuso, E. Aslanides, R. Ataíde Da Silva, M. Atzeni, B. Audurier, Juliette Authier, D. Bacher, I. Bachiller, S. Bachmann, Marie Bachmayer, J. J. Back, P. Baladrón Rodríguez, V. Balagura, A Balboni, W. Baldini, Z. Baldwin, Luca Balzani, H. Bao, J. Baptista de Souza Leite, C. Barbero Pretel, M. Barbetti, I. R. Barbosa, R. J. Barlow, Mikhail Barnyakov, S. Barsuk, W. Barter, W. Barter, B. Batsukh, B. Batsukh, Piera Battista, A. Bay, A. Beck, M. Becker, F. Bedeschi, I. Bediaga, Noah Albin Behling, S. Belin, Alberto Bellavista, K. Belous, I. Belov, I. Belyaev, G. Benane, G. Bencivenni, E. Ben-Haim, A. Berezhnoy, R. Bernet, Sergi Bernet Andres, A. Bertolin, C. Betancourt, F. Betti, J. Bex, Ia. Bezshyiko, O. Bezshyyko, J. Bhom, M. S. Bieker, N. V. Biesuz, P. Billoir, A. Biolchini, M. Birch, F. C. R. Bishop, A. Bitadze, A. Bizzeti
A bstract A measurement of the branching fraction for the decay $$ \Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu } $$ Λ → p μ − ν ¯ μ is presented using 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 fb − 1 . The result is obtained using Λ → pμ − decays as a normalisation channel. The measured branching fraction is $$ \mathcal{B}\left(\Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu}\right)=\left(1.462\pm 0.016\pm 0.100\pm 0.011\right)\times {10}^{-4} $$ B Λ → p μ − ν ¯ μ = 1.462 ± 0.016 ± 0.100 ± 0.011 × 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 → u quark transitions. It is consistent with previous measurements, and the extracted lepton flavour universality test observable, $$ {R}^{\mu e}=\frac{\Gamma \left(\Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu}\right)}{\Gamma \left(\Lambda \to p{e}^{-}{\overline{\nu}}_e\right)}=0.175\pm 0.012 $$ R μe = Γ Λ → p μ − ν ¯ μ Γ Λ → p e − ν ¯ e = 0.175 ± 0.012 , agrees with the Standard Model prediction.