Test of Lepton-Flavor Universality in B → K ∗ ℓ + ℓ − Decays at Belle

Abstract

We present a measurement of RK , the branching fraction ratio BðB → K μþμ−Þ=BðB → K eþe−Þ, for both charged and neutral B mesons. The ratio for the charged case RK þ is the first measurement ever performed. In addition, we report absolute branching fractions for the individual modes in bins of the squared dilepton invariant mass q2. The analysis is based on a data sample of 711 fb−1, containing 772 × 106 BB¯ events, recorded at the ϒð4SÞ resonance with the Belle detector at the KEKB asymmetricenergy eþe− collider. The obtained results are consistent with standard model expectations.