Observation of Radiative B^0 -> \phi K^0 \gamma Decays and Measurements of Their Time-Dependent CP Violation

We report the first observation of the radiative decay B^0->\phi K^0\gamma using a data sample of 772x10^6 BBar pairs collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We observe a signal of 35+/-8 events with a significance of 5.4 standard deviations including systematic uncertainties. The measured branching fraction is ${\cal B}(B^0->\phi K^0\gamma) = (2.66\pm 0.60 \pm 0.32) \times 10^{-6}$, where the uncertainties are statistical and systematic, respectively. We also report the first measurement of time-dependent CP violation parameters: ${\mathcal S}{\phi K_S^0 \gamma} = +0.74^{+0.72}{-1.05} (stat)^{+0.10}{-0.24} (syst)$and${\mathcal A}{\phi K_S^0 \gamma} = +0.35 \pm 0.58 (stat)^{+0.23}{-0.10} (syst)$\mathrm{.}Wealsopreciselymeasure${\mathcal B}(B^±>\phi K^+\gamma) = (2.34\pm 0.29 \pm 0.23) \times 10^{-6}$\mathrm{.}Theobserved$M{\phi K}$ mass spectrum differs significantly from that expected in a three-body phase-space decay. These results are preliminary.