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, $R(X_{e/\mu })=\mathcal{B}(B\to Xe\nu )/\mathcal{B}(B\to X\mu \nu )$, 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 $R(X_{e/\mu })=1.007\pm 0.009(\mathrm{stat})\pm 0.019(\mathrm{syst})$, which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.

Figure 1

Same-charge control channel (left) and opposite-charge signal (right) spectra of the lepton momentum in the $B_{\mathrm{sig}}$ rest frame $p_{\ell }^B$ with the fit results overlaid. The background component mostly contains events with fake or secondary leptons. The last bin contains overflow events. The hatched area shows the total statistical plus systematic uncertainty, added in quadrature in each bin.