Belle II results related to $b \to c$ anomalies

TL;DR

This paper presents precise measurements of lepton flavor universality in B decays at Belle II, finding results consistent with the Standard Model and no evidence of violation.

Contribution

First inclusive branching ratio measurement at Belle II testing lepton universality with high precision, and a comprehensive angular asymmetry analysis in B decays, both consistent with the Standard Model.

Findings

Inclusive branching ratio ratio $R(X_{e/})$ consistent with Standard Model.

Angular asymmetries agree with Standard Model predictions.

No evidence of lepton universality violation found.

Abstract

We report two new measurements for tests of lepton flavor universality between electrons and muons using semileptonic $B$ decays with a data set of $189~\mathrm{fb}^{-1}$ collected at the Belle II experiment between 2019 and 2021. Firstly, we find a ratio of inclusive branching fractions at $R(X_{e/\mu}) \equiv \mathcal{B}(\overline{B} \rightarrow Xe^{-}\overline{\nu}_{e})/\mathcal{B}(\overline{B} \rightarrow X\mu^{-}\overline{\nu}_{\mu}) = 1.007 \pm 0.009 (\mathrm{stat.}) \pm 0.019 (\mathrm{syst.})$. This inclusive measurement leads to the most precise universality test based on branching fractions. The measured $R(X_{e/\mu})$ is consistent with the Standard-Model prediction. Secondly, we measure a comprehensive set of five angular asymmetries of $\overline{B}^{0} \rightarrow {D^{*}}^{+}\ell^{-}\overline{\nu}_{\ell}$ decays and obtain agreements with the Standard Model for the measured asymmetries and their differences at a $p$-value above $13\%$. From both tests, no evidence of lepton universality violation is found.