Measurement of the branching ratio of $\bar{B}^0 \rightarrow D^{*+} \tau^- \bar{\nu}_{\tau}$ relative to $\bar{B}^0 \rightarrow D^{*+} \ell^- \bar{\nu}_{\ell}$ decays with a semileptonic tagging method

TL;DR

This paper measures the ratio of branching fractions of B meson decays involving tau leptons versus lighter leptons, using a large data sample from the Belle experiment, providing results consistent with the Standard Model.

Contribution

First measurement of the ratio ${ m R}(D^*)$ using a semileptonic tagging method with a large Belle dataset, improving precision over previous results.

Findings

Measured ${ m R}(D^*)=0.302 \

stat

syst

Abstract

We report a measurement of ratio ${\cal R}(D^*) = {\cal B}(\bar{B}^0 \rightarrow D^{*+} \tau^- \bar{\nu}_{\tau})/{\cal B}(\bar{B}^0 \rightarrow D^{*+} \ell^- \bar{\nu}_{\ell})$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times10^6$ $B\bar{B}$ pairs recorded at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. We select a sample of $B^0 \bar{B}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*\mp} \ell^{\pm}$. We measure ${\cal R}(D^*)= 0.302 \pm 0.030({\rm stat)} \pm 0.011({\rm syst)}$, which is within $1.6 \sigma$ of the Standard Model theoretical expectation, where $\sigma$ is the standard deviation including systematic uncertainties.