We report the measurement of the direct $CP$ asymmetry in the radiative $\overline{B}\to X_{s+d}\gamma$ decay using a data sample of $(772\pm 11)\times {10}^6$ $B\overline{B}$ pairs collected at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The $CP$ asymmetry is measured as a function of the photon energy threshold. For $E_{\gamma }^{*}\ge 2.1\mathrm{GeV}$, where $E_{\gamma }^{*}$ is the photon energy in the center-of-mass frame, we obtain ${\mathcal{A}}_{CP}(\overline{B}\to X_{s+d}\gamma )=(2.2\pm 3.9\pm 0.9)\%$, consistent with the standard model prediction.

• Received 7 January 2015

DOI:https://doi.org/10.1103/PhysRevLett.114.151601