A test of light-lepton universality in the rates of inclusive semileptonic $B$-meson decays at Belle II

TL;DR

This paper reports the first precise measurement of the ratio of inclusive semileptonic B-meson decays to electrons and muons, testing electron-muon universality and finding results consistent with the Standard Model.

Contribution

It provides the first measurement of the ratio of inclusive semileptonic B-meson decay rates to electrons and muons with unprecedented precision.

Findings

Measured ratio R(X_{e/μ}) = 1.007 ± 0.009 (stat) ± 0.019 (syst)

Result agrees with Standard Model predictions

Most precise test of lepton universality in B decays to date

Abstract

We present the first measurement of the ratio of branching fractions of inclusive semileptonic $B$-meson decays, $R(X_{e/\mu}) = \mathcal{B}(B\to X \, e \, \nu) / \mathcal{B}(B\to X \, \mu \, \nu)$, a precision test of electron-muon universality, using data corresponding to $189\,\mathrm{fb}^{-1}$ from electron-positron collisions collected with the Belle II detector. In events where the partner $B$ meson is fully reconstructed, we use fits to the lepton momentum spectra above $1.3\,\mathrm{GeV}/c$ to obtain $R(X_{e/\mu}) = 1.007 \pm 0.009~(\mathrm{stat}) \pm 0.019~(\mathrm{syst})$, which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.