Kavli Affiliate: T. Higuchi
| First 5 Authors: Belle II Collaboration, I. Adachi, K. Adamczyk, L. Aggarwal, H. Ahmed
| Summary:
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 $Upsilon(mathrm{4S})rightarrow Boverline{B}$ 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.
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