Test of light-lepton universality in decays with the Belle II experiment

We present a measurement of the ratio R_ = B (^- ^-__) / B (^- e^-ₑ_) of branching fractions B of the lepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKB e^+e^- collider. The sample has an integrated luminosity of 362\!\!2\, fb^-1 at a centre-of-mass energy of 10. 58\, GeV. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result, R_ = 0. 9675 0. 0007 0. 0036, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to the W boson in decays of 0. 9974 0. 0019, in agreement with the standard model expectation of unity.