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, \(R(X_{e/\mu})\) = \(\mathcal{B}(B \rightarrow X e \nu)\) / \(\mathcal{B}(B \rightarrow X \mu \nu) \) is reported as a probe of \(e-\mu\) universality. Electron-positron collisions collected at the \(\Upsilon(4S)\) resonance with the Belle II detector corresponding to \(189\,\mathrm{fb}^{-1}\) 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 \(1.3\,\mathrm{GeV}/c\). We find \(R(X_{e/\mu}) = 1.033 \pm 0.010 \pm 0.020\). The first contribution to the uncertainty is statistical, the second systematic. This is the most precise test of \(e - \mu\) universality in semileptonic \(B\) decays based on a branching fraction ratio. The result agrees with the Standard Model expectation within \(1.3\,\sigma\).