$\bar{B} \rightarrow D^{(*)} τ^- \barν_τ$ and Related Tauonic Topics at Belle

Abstract

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