Recent $\bar{B} \rightarrow D^{(*)} \tau^- \bar{\nu}_\tau$ Studies at Belle

TL;DR

This paper reports a new measurement of the ratio R(D*) and tau polarization in B decays, providing further evidence of a deviation from the Standard Model predictions, which could indicate new physics.

Contribution

It presents a novel, statistically independent measurement of R(D*) and the first measurement of tau polarization using Belle data, refining the understanding of the R(D*) anomaly.

Findings

Measured R(D*) = 0.270 ± 0.035 (stat) +0.028/-0.025 (syst)

Measured tau polarization P_tau(D*) = -0.38 ± 0.51 (stat) +0.21/-0.16 (syst)

R(D*) anomaly increased to 3.4σ from the SM prediction.

Abstract

The semitauonic decay $\bar{B} \rightarrow D^* \tau^- \bar{\nu}_\tau$ is sensitive to new physics beyond the Standard Model (SM) that has an enhanced coupling to the $\tau$ lepton. In the ratio of branching fractions $R(D^*) = \mathcal{B}(\bar{B} \rightarrow D^* \tau^- \bar{\nu}_\tau) / \mathcal{B}(\bar{B} \rightarrow D^* \ell^- \bar{\nu}_\ell)$, where $\ell^- = e^-$ or $\mu^-$, a 3.3$\sigma$ anomaly was observed. In order to investigate the anomaly further, Belle performed a new $R(D^*)$ measurement using one-prong hadronic $\tau$ decays, which was statistically independent of the previous two measurements. This measurement included the first measurement of the $\tau$ polarization $P_\tau(D^*)$ using the kinematics of the two-body decays. The obtained results, $R(D^*) = 0.270 \pm 0.035 ({\rm stat}) ^{+0.028}_{-0.025} ({\rm syst})$ and $P_\tau(D^*) = -0.38 \pm 0.51 (stat) ^{+0.21}_{-0.16} (syst)$, were consistent both with the SM and the world-average $R(D^*)$. Including this result, the $R(D^*)$ anomaly became 3.4$\sigma$ away from the SM prediction.