Test of lepton flavor universality and search for lepton flavor violation in decays

Choudhury, S.*; 谷田 聖; Belle Collaboration*; 他221名*

Journal of High Energy Physics (Internet), 2021(3), p.105_1 - 105_19, 2021/03

https://doi.org/10.1007/JHEP03(2021)105

We present measurements of the branching fractions for the decays and , and their ratio (), using a data sample of 711 that contains events. The data were collected at the resonance with the Belle detector at the KEKB asymmetric-energy collider. The ratio is measured in five bins of dilepton invariant-mass-squared (): , and (, along with the whole region. The value for is . The first and second uncertainties listed are statistical and systematic, respectively. All results for are consistent with Standard Model predictions. We also measure -averaged isospin asymmetries in the same bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the bin in the mode with muon final states. The measured differential branching fractions, , are consistent with theoretical predictions for charged decays, while the corresponding values are below the expectations for neutral decays. We have also searched for lepton-flavor-violating decays and set confidence-level upper limits on the branching fraction in the range of for , and modes.