We present an improved measurement of the branching fraction for the electroweak penguin process $B\to X_s{\ell }^{+}{\ell }^{-}$, where $\ell$ is an electron or a muon and $X_s$ is a hadronic system containing an $s$-quark. The measurement is based on a sample of $152\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ events collected with the Belle detector at the KEKB energy asymmetric $e^{+}{e}^{-}$ collider. The $X_s$ hadronic system is reconstructed from one $K^{\pm }$ or $K_{\mathrm{S}}^0$ and up to four pions, where at most one pion can be neutral. Averaging over both lepton flavors, the inclusive branching fraction is measured to be $\mathcal{B}(B\to X_s{\ell }^{+}{\ell }^{-})=(4.11\pm 0.83(\mathrm{stat}{)}_{-0.81}^{+0.85}(\mathrm{syst}))\times {10}^{-6}$ for $M_{{\ell }^{+}{\ell }^{-}}>0.2\mathrm{GeV}/c^2$.

• Received 24 March 2005

DOI:https://doi.org/10.1103/PhysRevD.72.092005