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

Abstract

We present measurements of the branching fractions for the decays B → Kμ⁺μ⁻ and B → Ke⁺e⁻, and their ratio (R_K), using a data sample of 711 fb⁻¹ that contains 772 × 10⁶$B\overline{B}$ events. The data were collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e⁻ collider. The ratio R_K 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 R_K 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 R_K 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, $d\mathrm{ℬ}$/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.