Measurement of the Electroweak Penguin Process $B\to X_s{\ell }^{+}{\ell }^{-}$

We report the first measurement of the branching fraction for the inclusive decay $B\to X_s{\ell }^{+}{\ell }^{-}$, where $\ell$ is either an electron or a muon, and $X_s$ is a hadronic recoil system that contains an $s$ quark. We analyzed a data sample of $65.4\times {10}^6$ $B$ meson pairs collected with the Belle detector at the KEKB $e^{+}{e}^{-}$ asymmetric-energy collider. We find $\mathcal{B}(B\to X_s{\ell }^{+}{\ell }^{-})=[6.1\pm 1.4(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}{)}_{-1.1}^{+1.4}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})]\times {10}^{-6}$ for dilepton masses greater than $0.2\mathrm{G}\mathrm{e}\mathrm{V}/c^2$.

Figure 1

SM expectations for the (a) dilepton and (b) recoil mass spectra; the observed (c) dilepton and (d) recoil mass spectra (circles). Inner and outer error bars indicate the statistical and total errors, respectively. The histograms in (c), (d) show the SM expectations after all the selections are applied; histograms are normalized to the expected branching fractions. The gaps in (c) are due to the $J/\psi$, ${\psi }^{\prime }$ vetoes. The dotted line in (d) indicates the $M_{X_s}<2.1\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ requirement.

Figure 2

Beam-energy constrained mass distributions for (a) $B\to X_s{e}^{+}{e}^{-}$, (b) $B\to X_s{\mu }^{+}{\mu }^{-}$, (c) $B\to X_s{\ell }^{+}{\ell }^{-}$, and (d) $B\to X_s{e}^{\pm }{\mu }^{\mp }$. The solid lines indicate the fit results and the dotted lines show the sum of the background components.