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 /μ}) =B (B →X e ν ) /B (B →X μ ν ) , a precision test of electron-muon universality, using data corresponding to 189 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 GeV /c to obtain R (X_{e /μ}) =1.007 ±0.009 (stat )±0.019 (syst ) , which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.