000003406 001__ 3406
000003406 005__ 20230201093410.0
000003406 037__ $$aBELLE2-CONF-PROC-2023-006
000003406 041__ $$aeng
000003406 100__ $$aHenrik Junkerkalefeld
000003406 245__ $$aA measurement of the ratio of light-lepton inclusive semileptonic $B$ decay rates $R(X_{e/\mu})$ = $\mathcal{B}(B\to X e \nu)$ / $\mathcal{B}(B\to X \mu \nu)$ using $189\,\mathrm{fb}^{-1}$ of data at Belle~II
000003406 260__ $$a$$c2022-12-16
000003406 300__ $$amult. p
000003406 520__ $$aIn 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 $1.3\,\sigma$. 
000003406 8560_ $$fjunkerkalefeld@physik.uni-bonn.de
000003406 8564_ $$uhttps://docs.belle2.org/record/3406/files/BELLE2-CONF-PROC-2023-006.pdf