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

We present the first measurement of the ratio of branching fractions of inclusive semileptonic $B$-meson decays, $R(X_{e\mathrm{/}\mu })=\mathcal{B}(B\to Xe\nu )\mathrm{/}\mathcal{B}(B\to X\mu \nu )$, a precision test of electron-muon universality, using data corresponding to $189{\mathrm{f}\mathrm{b}}^{-1}$ 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 $1.3\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}c$ to obtain $R(X_{e\mathrm{/}\mu })=1.007\pm 0.009(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.019(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$, which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.