A measurement of the ratio of light-lepton inclusive semileptonic \(B\) decay rates \(R(X_{e/\mu})\) = \(\mathcal{B}(B \rightarrow X e \nu)\) / \(\mathcal{B}(B \rightarrow X \mu \nu) \) using \(189 \mathrm{fb}^{-1}\) of data at Belle II

Abstract

In this work a measurement of the ratio of branching fractions of inclusive semileptonic 𝐵 meson decays, 𝑅(𝑋 𝑒/𝜇 ) = B (𝐵 → 𝑋𝑒𝜈)/B (𝐵 → 𝑋 𝜇𝜈) is reported as a probe of 𝑒 − 𝜇 universality. Electron-positron collisions collected at the Υ(4𝑆) resonance with the Belle II detector corresponding to 189 fb −1 of integrated luminosity are used. One 𝐵 meson is reconstructed in its fully hadronical decays, while the semileptonic 𝐵 meson decay is selected by demanding the reconstructed lepton to have a momentum in the 𝐵 rest frame above 1.3 GeV/𝑐. We find 𝑅(𝑋 𝑒/𝜇 ) = 1.033 ± 0.010 ± 0.019. The first contribution to the uncertainty is statistical, the second systematic. This is the most precise branching-fraction based test of 𝑒 − 𝜇 universality in semileptonic 𝐵 decays to date and agrees with the Standard Model expectation within 1.3 𝜎.