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

TL;DR

This paper reviews recent measurements of B meson decays involving tau leptons at Belle, highlighting potential deviations from the Standard Model and exploring implications for new physics such as the Two-Higgs-Doublet Model.

Contribution

It reports new Belle measurements of B to D(*) tau nu decays, including the first use of semileptonic tagging and tau polarization analysis, advancing the experimental study of lepton flavor universality.

Findings

Observed deviations from Standard Model predictions in R(D*) ratios.

First measurement of tau polarization in B decays at Belle.

Compatibility of results with type-II Two-Higgs-Doublet Model discussed.

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.