Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic <i>B</i>-Meson Decays at Belle II

Abstract

We present the first measurement of the ratio of branching fractions of inclusive semileptonic B-meson decays, R(Xe/μ)=B(B→Xeν)/B(B→Xμν), 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(Xe/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.