Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic B -Meson Decays at Belle II

• Published in: Physical review letters Vol. 131; no. 5; p. 051804
• Main Authors: Aggarwal, L., Ahmed, H., Aihara, H., Akopov, N., Aloisio, A., Anh Ky, N., Asner, D. M., Atmacan, H., Aushev, T., Aushev, V., Bae, H., Bahinipati, S., Bambade, P., Banerjee, Sw, Bansal, S., Barrett, M., Baudot, J., Bauer, M., Baur, A., Beaubien, A., Becker, J., Bennett, J. V., Bernieri, E., Bernlochner, F. U., Bertacchi, V., Bertemes, M., Bertholet, E., Bessner, M., Bettarini, S., Bhardwaj, V., Bhuyan, B., Bianchi, F., Bilka, T., Bilokin, S., Biswas, D., Bodrov, D., Borah, J., Bozek, A., Bračko, M., Briere, R. A., Browder, T. E., Budano, A., Bussino, S., Campajola, M., Cao, L., Casarosa, G., Cecchi, C., Cerasoli, J., Chang, M.-C., Cheaib, R., Cheema, P., Chekelian, V., Cheon, B. G., Chilikin, K., Chirapatpimol, K., Cho, H.-E., Cho, K., Cho, S.-J., Choi, S.-K., Choudhury, S., Cochran, J., Corona, L., Cunliffe, S., Dattola, F., De La Cruz-Burelo, E., De La Motte, S. A., De Nardo, G., De Nuccio, M., De Pietro, G., de Sangro, R., Destefanis, M., Dhamija, R., Di Capua, F., Dingfelder, J., Doležal, Z., Dong, T. V., Dorigo, M., Dossett, D., Dreyer, S., Dubey, S., Dujany, G., Ecker, P., Eliachevitch, M., Feichtinger, P., Ferber, T., Ferlewicz, D., Fillinger, T., Finocchiaro, G., Fodor, A., Forti, F., Frey, A., Fulsom, B. G., Gabrielli, A., Ganiev, E., Garcia-Hernandez, M., Gaudino, G., Gaur, V., Gaz, A., Gellrich, A., Ghevondyan, G.
• Format: Journal Article
• Language: English
• Published: United States American Physical Society 02-08-2023; American Physical Society (APS)
• Subjects: Flavor symmetries; Heavy mesons; Lepton colliders; Leptonic, semileptonic & radiative decays; Particle data analysis; Physics; PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; Tau leptons
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.131.051804

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.