Test of lepton flavor universality and search for lepton flavor violation in B → Kℓℓ decays

The Belle Collaboration

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102 Scopus citations


We present measurements of the branching fractions for the decays B → Kμ+μ and B → Ke+e, and their ratio (RK), using a data sample of 711 fb−1 that contains 772 × 106BB¯ events. The data were collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e collider. The ratio RK is measured in five bins of dilepton invariant-mass-squared (q2): q2 ∈ (0.1, 4.0), (4.00, 8.12), (1.0, 6.0), (10.2, 12.8) and (> 14.18) GeV2/c4, along with the whole q2 region. The RK value for q2 ∈ (1.0, 6.0) GeV2/c4 is 1.03−0.24+0.28 ± 0.01. The first and second uncertainties listed are statistical and systematic, respectively. All results for RK are consistent with Standard Model predictions. We also measure CP-averaged isospin asymmetries in the same q2 bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the q2 ∈ (1.0, 6.0) GeV2/c4 bin in the mode with muon final states. The measured differential branching fractions, dℬ /dq2, are consistent with theoretical predictions for charged B decays, while the corresponding values are below the expectations for neutral B decays. We have also searched for lepton-flavor-violating B → Kμ±e decays and set 90% confidence-level upper limits on the branching fraction in the range of 10−8 for B+ → K+μ±e, and B0 → K0μ±e modes. [Figure not available: see fulltext.]

Original languageEnglish
Article number105
JournalJournal of High Energy Physics
Issue number3
StatePublished - Mar 2021
Externally publishedYes


  • B physics
  • FCNC Interaction
  • Flavor physics
  • Flavour Changing Neutral Currents
  • e-e Experiments


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