Time-Dependent {\boldmath $CP$}-Violating Asymmetry in $B^0\to\rho^0\gamma$ Decays

We report the first measurement of CP-violation parameters in $B^0\to\rho^0\gamma$ decays based on a data sample of $657\times 10^6 B\bar{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The flavor of the $B$ meson is identified from the decay products of the accompanying $B$ meson. We obtain the time-dependent and direct CP-violating parameters, $\cals_{\rhoz\gamma} = -0.83\pm 0.65(stat)\pm 0.18(syst)$ and $\cala_{\rhoz\gamma} = -0.44\pm 0.49(stat)\pm 0.14(syst)$, respectively.