Measurement of the branching ratio of $\bar{B} \to D^{(\ast)} \tau^- \bar{\nu}_\tau$ relative to $\bar{B} \to D^{(\ast)} \ell^- \bar{\nu}_\ell$ decays with hadronic tagging at Belle

TL;DR

This paper measures the ratios of branching fractions for B meson decays involving tau leptons compared to lighter leptons, using the full Belle dataset, and finds results consistent with the Standard Model predictions.

Contribution

First measurement of R(D) and R(D*) ratios using hadronic tagging and leptonic tau decays at Belle with full dataset, providing precise tests of lepton flavor universality.

Findings

Results are consistent with Standard Model predictions.

Measured R(D)= 0.375 ± 0.064(stat.) ± 0.026(syst.)

Measured R(D*)= 0.293 ± 0.038(stat.) ± 0.015(syst.)

Abstract

We report a measurement of the branching fraction ratios R(D(*)) of Bbar -> D(*) tau- nubar_tau relative to Bbar -> D()* l- nubar_l (where l = e or mu) using the full Belle data sample of 772 x 10^6 BBbar pairs collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. The measured values are R(D)= 0.375 +- 0.064(stat.) +- 0.026(syst.) and R(D*) = 0.293 +- 0.038(stat.) +- 0.015(syst.). The analysis uses hadronic reconstruction of the tag-side B meson and purely leptonic tau decays. The results are consistent with earlier measurements and do not show a significant deviation from the standard model prediction.