PHYSICAL REVIEW LETTERS 131, 051804 (2023)

ISSN: 0031-9007, SCIE

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

The Belle II collaboration (L. Aggarwal, ..., N. Anh Ky, ...)

We present the first measurement of the ratio of branching fractions of inclusive semileptonic B-meson decays, RðX e=μÞ ¼ BðB → XeνÞ=BðB → XμνÞ, 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 RðX e=μÞ ¼ 1.007  0.009ðstatÞ  0.019ðsystÞ, which is the most precise lepton- universality test of its kind and agrees with the standard-model expectation.

DOI: DOI: 10.1103/PhysRevLett.131.051804