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

Phys Rev Lett. 2023 Aug 4;131(5):051804. doi: 10.1103/PhysRevLett.131.051804.

Abstract

We present the first measurement of the ratio of branching fractions of inclusive semileptonic B-meson decays, R(X_{e/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(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.