Measurement of $\mathcal{R}(D)$ and $\mathcal{R}(D^*)$ with a semileptonic tagging method

TL;DR

This paper measures the ratios of branching fractions for B meson decays involving tau leptons using a semileptonic tagging method, providing results consistent with Standard Model predictions.

Contribution

It introduces a semileptonic tagging approach to measure al R(D) and al R(D^*) with high precision using Belle data.

Findings

al R(D) = 0.307 .037 .016

al R(D^*)= 0.283 .018 .014

Results agree with Standard Model within 1.1 standard deviations

Abstract

We report a measurement of the ratios of branching fractions $\mathcal{R}(D^{(*)}) = {\cal B}(\bar{B} \to D^{(*)} \tau^- \bar{\nu}_{\tau})/{\cal B}(\bar{B} \to D^{(*)} \ell^- \bar{\nu}_{\ell})$ where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772 \times 10^6$ $B\bar{B}$ events recorded at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. The tag-side B meson is reconstructed in a semileptonic decay mode, and the signal-side ${\tau}$ is reconstructed in a purely leptonic decay. The results are $\mathcal{R}(D) = 0.307 \pm 0.037 \pm 0.016$ and $\mathcal{R}(D^*)=0.283 \pm 0.018 \pm 0.014$, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the Standard Model predictions within 0.2 and 1.1 standard deviations, respectively.