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

Abstract

We report the first observation of the radiative decay B^0 -> \phi K^0 \gamma using a data sample of 772 x 10^6 B B-bar pairs collected at the \Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We observe a signal of 37+/-8 events with a significance of 5.4 standard deviations including systematic uncertainties. The measured branching fraction is $\mathcal{B}(B^0->\varphi K^0\gamma )=(2.74\pm 0.60\pm 0.32)\times 10^{-6}$, where the uncertainties are statistical and systematic, respectively. We also report the first measurements of time-dependent CP violation parameters: ${\mathcal{S}}_{\varphi K_S^0\gamma }=+0.74_{-1.05}^{+0.72}(stat{)}_{-0.24}^{+0.10}(syst)$ and ${\mathcal{A}}_{\varphi K_S^0\gamma }=+0.35+/-0.58(stat{)}_{-0.10}^{+0.23}(syst)$. Furthermore, we measure $\mathcal{B}(B^{+}->\varphi K^{+}\gamma )=(2.48+/-0.30+/-0.24)x10^{-6}$, ${\mathcal{A}}_{CP}=-0.03+/-0.11+/-0.08$ and find that the signal is concentrated in the M_{\phi K} mass region near threshold.

Comments: 6 pages, 3 figures, Modified version is to be published in PRD(RC)