A test of light-lepton universality in the rates of inclusive semileptonic $B$-meson decays at Belle II

Abstract 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.
Tags
Working group Semileptonic and Missing Energy Decay
Principal authors @  Florian Bernlochner, Jochen Dingfelder, Marcel Hohmann, Henrik Junkerkalefeld, Peter Lewis, Marco Milesi, Paolo Rocchetti, Phillip Urquijo
Reference Phys.Rev.Lett. 131, 051804 (2023), DOI: 10.1103/PhysRevLett.131.051804
Document BELLE2-PUB-PH-2022-002
Links arXiv:2301.08266, Inspire, PRL
Bibtex 
@article{Belle-II:2023qyd,
    author = "Aggarwal, L. and others",
    collaboration = "Belle-II",
    title = "{Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic B-Meson Decays at Belle II}",
    eprint = "2301.08266",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2022-009 KEK Preprint 2022-45",
    doi = "10.1103/PhysRevLett.131.051804",
    journal = "Phys. Rev. Lett.",
    volume = "131",
    number = "5",
    pages = "051804",
    year = "2023"
}