$\bar{B} \rightarrow D^{(*)} \tau^- \bar{\nu}_\tau$ and Related Tauonic Topics at Belle

Abstract

The decays $\bar{B} \rightarrow D^{(*)} \tau^- \bar{\nu}_\tau$ are good probes to new physics beyond the Standard Model. The ratios of branching fractions $R(D^{(*)}) \equiv \mathit{BF}(\bar{B} \rightarrow D^{(*)} \tau^- \bar{\nu}_\tau) / \mathit{BF}(\bar{B} \rightarrow D^{(*)} \ell^- \bar{\nu}_\ell)$ (where $\ell^- = e^-, \mu^-$) measured by Belle, BaBar and LHCb show 3.9$\sigma$ deviation from the SM expectations as of 2015. In 2016, the Belle collaboration has shown two new measurements for the $\bar{B} \rightarrow D^{(*)} \tau^- \bar{\nu}_\tau$ decay. These include the first application of the semileptonic tagging to the $R(D^*)$ measurement and the first measurement of the $\tau$ polarization using the hadronic $\tau$ decays. We also review the two measurements for $B^- \rightarrow \tau^- \bar{\nu}_\tau$ at Belle. Along with these results, compatibility with the type-II Two-Higgs-Doublet Model is discussed.