Measurement of the CP Asymmetry in B→X_{s}γ

Direct $CP$ violation in the $b\to s\gamma$ process is a sensitive probe of physics beyond the standard model. We report a measurement of the $CP$ asymmetry in $B\to X_s\gamma$, where the hadronic recoil system $X_s$ is reconstructed using a pseudoreconstruction technique. In this approach there is negligible contamination from $b\to d\gamma$ decays, which are expected to have a much larger $CP$ asymmetry. We find $A_{CP}=0.002\pm 0.050\mathrm{\left(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{\right)}\pm 0.030\mathrm{\left(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{\right)}$ for $B\to X_s\gamma$ events having recoil mass smaller than $2.1\mathrm{G}\mathrm{e}\mathrm{V}/c^2$. The analysis is based on a data sample of $140{\mathrm{f}\mathrm{b}}^{-1}$ recorded at the $\Upsilon \left(4S\right)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ storage ring.