Test of lepton flavour universality with a measurement of $R(D^{*})$ using hadronic $B$ tagging at the Belle II experiment

Abstract The ratio of branching fractions $R(D^{*}) = \mathcal{B}(\overline{B} \rightarrow D^{*} \tau^{-} \overline{\nu}_{\tau})$/$\mathcal{B} (\overline{B} \rightarrow D^{*} \ell^{-} \overline{\nu}_{\ell})$, where $\ell$ is an electron or muon, is measured using a Belle~II data sample with an integrated luminosity of $189~\mathrm{fb}^{-1}$ at the SuperKEKB asymmetric-energy $e^{+} e^{-}$ collider. Data is collected at the $\Upsilon(\mathrm{4S})$ resonance, and one $B$ meson in the $\Upsilon(\mathrm{4S})\rightarrow B\overline{B}$ decay is fully reconstructed in hadronic decay modes. The accompanying signal $B$ meson is reconstructed as $\overline{B}\rightarrow D^{*} \tau^{-}\overline{\nu}_{\tau}$ using leptonic $\tau$ decays. The normalization decay, $\overline{B}\rightarrow D^{*} \ell^{-} \overline{\nu}_{\ell}$, where $\ell$ is an electron or muon, produces the same observable final state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields $R(D^{*}) = 0.262~_{-0.039}^{+0.041}(\mathrm{stat})~_{-0.032}^{+0.035}(\mathrm{syst})$. This result is consistent with the current world average and with standard model predictions.
Tags
Working group Semileptonic and Missing Energy Decay
Principal authors @  Racha Cheaib, Koji Hara, Toru Iijima, Taichiro Koga, Kazuki Kojima, Kodai Matsuoka, Katsuro Nakamura, Hannah Wakeling, Qi-Dong Zhou
Reference Phys.Rev.D 110, 072020 (2024), DOI: 10.1103/PhysRevD.110.072020
Document BELLE2-PUB-PH-2023-013
Links arXiv:2401.02840, Inspire, PRD
Bibtex 
@article{Belle-II:2024ami,
    author = "Adachi, I. and others",
    collaboration = "Belle-II",
    title = "{Test of lepton flavor universality with a measurement of R(D*) using hadronic B tagging at the Belle II experiment}",
    eprint = "2401.02840",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "BELLE2-PUB-PH-2024-001 https://docs.belle2.org/record/4057/ ; Belle
  II Preprint 2024-001; KEK Preprint 2023-47",
    doi = "10.1103/PhysRevD.110.072020",
    journal = "Phys. Rev. D",
    volume = "110",
    number = "7",
    pages = "072020",
    year = "2024"
}