Kazuki Kojima ; Toru Iijima

2024
Nagoya University Nagoya

Abstract: In the standard model of particle physics (SM), lepton flavor universality is an axiom that states that the coupling constants in the interaction between leptons and gauge particles are the same regardless of the lepton flavor $e$, $\mu$, or $\tau$. This universality has been demonstrated by various experiments. On the other hand, in the semi-leptonic $B$ decay, the ratio of the 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 $\ell$ represents an electron or muon, show a tensions with the SM expectations at a significance of $3.2\sigma$. This deviation could be a sign of physics beyond the SM (BSM). We search for lepton flavor universality violation by measuring $R(D^{*})$ using $189~\mathrm{fb}^{-1}$ data collected between 2019 and 2021 at the Belle II experiment. We find \begin{align} R(D^{*}) = 0.262~{}_{-0.039}^{+0.041}~(\mathrm{stat.})~{}_{-0.032}^{+0.033}~(\mathrm{syst.}). \nonumber \end{align} This result is consistent with the SM predictions. Therefore, no significant violation of lepton flavor universality is observed. The results also agree with the previous measurements within the uncertainty. The world average of the $R(D^{(*)})$ measurements, including our measurement, shows a slight increase in the deviation from the SM prediction from $3.2\sigma$ to $3.3\sigma$. Finally, we discuss possible BSM contributions in $\overline{B} \rightarrow D^{*}\tau^{-}\overline{\nu}_{\tau}$ based on the new world average of $R(D^{(*)})$ and prospects of sensitivity on $R(D^{*})$ at the Belle~II experiment.

Note: Presented on 25 03 2024
Note: PhD