Test of lepton-flavor universality in $B\to K^{\ast }{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays at Belle

We present a measurement of $R_{K^{\ast }}$, the branching fraction ratio $\mathcal{B}(B\to K^{\ast }{\mu }^{+}{\mu }^{-})$/ $\mathcal{B}(B\to K^{\ast }{e}^{+}{e}^{-})$, for both charged and neutral $B$ mesons. The ratio for the charged case, $R_{K{}^{\ast +}}$, 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, $q^2$. The analysis is based on a data sample of $711{\mathrm{f}\mathrm{b}}^{-1}$, containing $772\times {10}^6$ $B\bar{B}$ events, recorded at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The obtained results are consistent with Standard Model expectations.