Abstract
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 at 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 0.2, 1.1, and 0.8 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.
- Received 13 October 2019
- Accepted 10 March 2020
DOI:https://doi.org/10.1103/PhysRevLett.124.161803
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.
Published by the American Physical Society