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Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic B -Meson Decays at Belle IIAhmed, H.; Aloisio, A.; Anh Ky, N.; Atmacan, H.; Aushev, V.; Bae, H.; Bambade, P.; Bansal, S.; Barrett, M.; Bauer, M.; Bennett, J. V.; Bertacchi, V.; Bilka, T.; Briere, R. A.; Browder, T. E.; Cao, L.; Casarosa, G.; Cecchi, C.; Choudhury, S.; Cunliffe, S.; De Nuccio, M.; Dhamija, R.; Dong, T. V.; Dossett, D.; Fillinger, T.; Finocchiaro, G.; Frey, A.; Gabrielli, A.; Gaudino, G.; Gaur, V.; Giri, A.; Grammatico, T.; Granderath, S.; Graziani, E.; Gruberová, Z.; Halder, S.; Hayasaka, K.; Heredia de la Cruz, I.; Hsu, C.-L.; Iijima, T.; Itoh, R.; Iwasaki, M.; Jacobs, W. W.; Kim, C.-H.; Kinoshita, K.; Kohani, S.; Kojima, K.; Krokovny, P.; Kumar, R.; Lacaprara, S.; Liu, Z. Q.; Maharana, S. P.; Marinas, C.; Martini, A.; McKenna, J. A.; Meier, F.; Milesi, M.; Mussa, R.; Nakamura, I.; Narwal, D.; Natochii, A.; Nazaryan, G.; Nishida, S.; Ono, H.; Pakhlova, G.; Paladino, A.; Panta, A.; Patra, S.; Paul, S.; Peschke, R.; Pinna Angioni, G.; Podesta-Lerma, P. L. M.; Pokharel, S.; Praz, C.; Rad, N.; Raeuber, G.; Reif, M.; Roney, J. M.; Rostomyan, A.; Rout, N.; Senyo, K.; Shen, C. P.; Shiu, J.-G.; Skorupa, J.; Spataro, S.; Stroili, R.; Tamponi, U.; Tanida, K.; Urquijo, P.; van Tonder, R.; Vitale, L.; Wallner, S.; Wessel, C.; Won, E.; Yabsley, B. D.; Yin, J. H.; Zhai, Y.; Zhou, Q. D.; Zhou, X. Y.; Žlebčík, R.
Physical review letters, 08/2023, Volume: 131, Issue: 5Journal Article
We present the first measurement of the ratio of branching fractions of inclusive semileptonic $B$-meson decays, $R(X_{e/\mu}) = \mathcal{B}(B\to X \, e \, \nu) / \mathcal{B}(B\to X \, \mu \, \nu)$, a precision test of electron-muon universality, using data corresponding to $189\,\mathrm{fb}^{-1}$ from electron-positron collisions collected with the Belle II detector. In events where the partner $B$ meson is fully reconstructed, we use fits to the lepton momentum spectra above $1.3\,\mathrm{GeV}/c$ to obtain $R(X_{e/\mu}) = 1.007 \pm 0.009~(\mathrm{stat}) \pm 0.019~(\mathrm{syst})$, which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.
