Observation of Radiative B^0 -> ϕK^0 γDecays and Measurements of Their Time-Dependent CP Violation

Abstract

We report the first observation of the radiative decay B^0->ϕK^0γusing 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 $\mathcal{B}(B^0->\varphi K^0\gamma )=(2.66\pm 0.60\pm 0.32)\times 10^{-6}$, where the uncertainties are statistical and systematic, respectively. We also report the first measurement of time-dependent CP violation parameters: ${\mathcal{S}}_{\varphi K_S^0\gamma }=+0.74_{-1.05}^{+0.72}(stat{)}_{-0.24}^{+0.10}(syst)$ and ${\mathcal{A}}_{\varphi K_S^0\gamma }=+0.35\pm 0.58(stat{)}_{-0.10}^{+0.23}(syst)$. We also precisely measure $\mathcal{B}(B^{+}->\varphi K^{+}\gamma )=(2.34\pm 0.29\pm 0.23)\times 10^{-6}$. The observed $M_{\varphi K}$ mass spectrum differs significantly from that expected in a three-body phase-space decay. These results are preliminary.

arXiv comment4 pages, 6 figures, To be published in the proceedings of 35th International Conference of High Energy Physics (ICHEP 2010), Paris, France, July 22-28, 2010