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.