Measurement of Time-Dependent CP-Violating Asymmetry in B0 -> Ks pi0 gamma Decay

We present a new measurement of CP-violation parameters in B0 -> Ks pi0 gamma decay based on a sample of 275 x 10^6 B\bar{B} pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB energy-asymmetric e^+e^- collider. One of the B mesons is fully reconstructed in the B0 -> Ks pi0 gamma decay. The flavor of the accompanying B meson is identified from its decay products. CP-violation parameters are obtained from the asymmetry in the distribution of the proper-time intervals between the two B decays. We obtain S_Kspi0gamma = -0.58 +0.46-0.38(stat) +-0.11(syst) and A_Kspi0gamma = +0.03 +-0.34(stat) +-0.11(syst), for the Kspi0 invariant mass covering the full range up to 1.8 GeV/c^2. We also measure the CP-violation parameters for the case B0 -> K*0(-> Kspi0) gamma and obtain S_K*0gamma = -0.79 +0.63-0.50(stat) +- 0.10(syst) for A_K*0gamma fixed at 0.