Henrik Junkerkalefeld

16 December 2022

Abstract: In this work a measurement of the ratio of branching fractions of inclusive semileptonic $B$ meson decays, $R(X_{e/\mu})=\mathcal{B}(B\to X e \nu)/\mathcal{B}(B\to X \mu \nu)$ 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 $R(X_{e/\mu}) = 1.033 \pm 0.010 \pm 0.019$. The first contribution to the uncertainty is statistical, the second systematic. This is the most precise branching-fraction based test of $e - \mu$ universality in semileptonic $B$ decays to date and agrees with the Standard Model expectation within $1.3\,\sigma$.