Measurement of and with a semileptonic tagging method

Caria, G.*; 谷田 聖; Belle Collaboration*; 他202名*

Physical Review Letters, 124(16), p.161803_1 - 161803_8, 2020/04

https://doi.org/10.1103/PhysRevLett.124.161803

The experimental results on the ratios of branching fractions and , where denotes an electron or a muon, show a long-standing discrepancy with the Standard Model predictions, and might hint to a violation of lepton flavor universality. We report a new simultaneous measurement of and , based on a data sample containing events recorded at the resonance with the Belle detector at the KEKB collider. In this analysis the tag-side meson is reconstructed in a semileptonic decay mode and the signal-side is reconstructed in a purely leptonic decay. The measured values are and , where the first uncertainties are statistical and the second are systematic. These results are in agreement with the Standard Model predictions within , and standard deviations for , and their combination, respectively. This work constitutes the most precise measurements of and performed to date as well as the first result for based on a semileptonic tagging method.