| Abstract: | We present a measurement of $R_{K^{ast}}$, the ratio of the branching
fractions ${cal B}(B o K^ast mu^+ mu^-)$ and ${cal B}(B o K^ast e^+
e^-)$, for both charged and neutral $B$ mesons. The ratio for charged $B$
mesons, $R_{K{^{ast +}}}$, is the first measurement ever performed. The
analysis is based on a data sample of $711~mathrm{fb}^{-1}$, containing
$772 imes 10^{6}$ $Bar B$ events, recorded at the $Upsilon(4S)$ resonance
with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. |
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