Search for lepton-flavor-violating $\tau$ decays to a lepton and an invisible boson at Belle II

Abstract We search for lepton-flavor-violating $\tau^-\to e^-\alpha$ and $\tau^-\to\mu^-\alpha$ decays, where $\alpha$ is an invisible spin-0 boson. The search uses electron-positron collisions at $10.58$ GeV center-of-mass energy with an integrated luminosity of $62.8$ fb$^{-1}$, produced by the SuperKEKB collider and collected with the Belle II detector. We search for an excess in the lepton-energy spectrum of the known $\tau^-\to e^-\bar\nu_e \nu_\tau$ and $\tau^-\to \mu^-\bar\nu_\mu \nu_\tau$ decays. We report 95% confidence-level upper limits on the branching-fraction ratio ${\mathcal B}(\tau^-\to e^- \alpha) / {\mathcal B}(\tau^- \to e^- \bar\nu_e \nu_\tau)$ in the range $(1.1-9.7) \times 10^{-3}$ and on ${\mathcal B}(\tau^-\to \mu^- \alpha) / {\mathcal B}(\tau^- \to \mu^- \bar\nu_\mu \nu_\tau)$ in the range $(0.7-12.2) \times 10^{-3}$ for $\alpha$ masses between 0 and 1.6 GeV/c$^2$. These results provide the most stringent bounds on invisible boson production from $\tau$ decays.
Tags
Working group Tau
Principal authors @  Eduard Cruz Burelo, Alejandro De Yta Hernandez, Marcela Garcia Hernandez, Ivan Heredia de la Cruz, Thomas Kraetzschmar, Petar Rados, Armine Rostomyan, Francesco Tenchini
Reference Phys.Rev.Lett. 130, 181803 (2023), DOI: 10.1103/PhysRevLett.130.181803
Document BELLE2-PUB-PH-2022-009
Links arXiv:2212.03634, Inspire, PRL
Bibtex 
@article{Belle-II:2022heu,
    author = "Adachi, I. and others",
    collaboration = "Belle-II",
    title = "{Search for Lepton-Flavor-Violating {\ensuremath{\tau}} Decays to a Lepton and an Invisible Boson at Belle II}",
    eprint = "2212.03634",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2022-007, KEK Preprint 2022-39",
    doi = "10.1103/PhysRevLett.130.181803",
    journal = "Phys. Rev. Lett.",
    volume = "130",
    number = "18",
    pages = "181803",
    year = "2023"
}