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

Abstract

We present the first measurement of the ratio of branching fractions of inclusive semileptonic $B$-meson decays, $R(X_{e/\hat{I}¼})=\mathcal{B}(B\to Xe\hat{I}½)/\mathcal{B}(B\to X\hat{I}¼\hat{I}½)$, 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 $R(X_{e/\hat{I}¼})=1.007\pm 0.009(\mathrm{stat})\pm 0.019(\mathrm{syst})$, which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.