Test of light-lepton universality in $\tau$ decays with Belle II

Abstract We present a measurement of the ratio $R_\mu = \mathcal{B}(\tau^-\to \mu^-\bar\nu_\mu\nu_\tau) / \mathcal{B}(\tau^-\to e^-\bar\nu_e\nu_\tau)$ of branching fractions $\mathcal{B}$ of the $\tau$ lepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKB $e^+e^-$ collider. The sample has an integrated luminosity of $362\!\pm\!2\,\text{fb}^{-1}$ at a centre-of-mass energy of $10.58\,\text{GeV}$. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result, $R_\mu = 0.9675 \pm 0.0007 \pm 0.0036$, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to the $W$ boson in $\tau$ decays of $0.9974 \pm 0.0019$, in agreement with the standard model expectation of unity.
Tags
Working group Tau
Principal authors @  Paul Feichtinger, Zuzana Gruberova, Michel Hernandez Villanueva, Philipp Horak, Gianluca Inguglia, Alberto Martini, Petar Rados, Geraldine Raeuber, Armine Rostomyan
Reference JHEP 08, 205 (2024), DOI: 10.1007/JHEP08(2024)205
Document BELLE2-PUB-PH-2023-005
Links arXiv:2405.14625, Inspire, JHEP
Bibtex 
@article{Belle-II:2024vvr,
    author = "Adachi, Ichiro and others",
    collaboration = "Belle-II",
    title = "{Test of light-lepton universality in {\ensuremath{\tau}} decays with the Belle II experiment}",
    eprint = "2405.14625",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2024-002, KEK Preprint 2023-49",
    doi = "10.1007/JHEP08(2024)205",
    journal = "JHEP",
    volume = "08",
    pages = "205",
    year = "2024"
}