Measurement of the branching fraction ratios $R(D^{+})$ and $R(D^{*+})$ using muonic $\tau$ decays

TL;DR

This paper measures the ratios of branching fractions for specific B meson decays involving tau leptons compared to muons, using LHCb data, and finds results consistent with the Standard Model.

Contribution

First measurement of these branching fraction ratios using muonic tau decays at LHCb, providing data for testing lepton flavor universality.

Findings

Measured R(D+) = 0.249 ± 0.043 ± 0.047

Measured R(D*+) = 0.402 ± 0.081 ± 0.085

Results are compatible with Standard Model predictions

Abstract

The branching fraction ratios of $\overline{B}^0\to D^+\tau^-\overline{\nu}_{\tau}$ and $\overline{B}^0\to D^{*+}\tau^-\overline{\nu}_{\tau}$ decays are measured with respect to their muonic counterparts, using a data sample corresponding to an integrated luminosity of 2.0 fb$^{-1}$ collected by the LHCb experiment in proton-proton collisions at $\sqrt{s} = 13$ TeV. The reconstructed final states are formed by combining $D^+$ mesons with $\tau^-\to\mu^-\overline{\nu}_{\mu}\nu_{\tau}$ candidates, where the $D^+$ is reconstructed via the $D^+\to K^-\pi^+\pi^+$ decay. The results are \begin{align*} R(D^{+}) &= 0.249 \pm 0.043 \pm 0.047, R(D^{*+}) &= 0.402 \pm 0.081\pm 0.085, \end{align*} where the first uncertainties are statistical and the second systematic. The two measurements have a correlation coefficient of $-0.39$ and are compatible with the Standard Model.