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

TL;DR

This paper reports the first observation of the radiative decay B^0 ->  K^0  and measures its time-dependent CP violation parameters, providing new insights into rare B meson decays and CP violation phenomena.

Contribution

It presents the first observation of B^0 ->  K^0  decay and first measurements of its CP violation parameters, expanding understanding of radiative B decays.

Findings

First observation of B^0 ->  K^0  decay with 5.4 sigma significance.

Measured branching fraction: (2.74 1.0 0.3) ^{-6}.

CP violation parameters: S_{ K_S^0 } = +0.74^{+0.72}_{-1.05}, A_{ K_S^0 } = +0.35 0.58.

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 ${\cal B}(B^0 -> \phi K^0 \gamma) = (2.74\pm 0.60 \pm 0.32) \times 10^{-6}$, where the uncertainties are statistical and systematic, respectively. We also report the first measurements of time-dependent CP violation parameters: ${\mathcal S}_{\phi K_S^0 \gamma} = +0.74^{+0.72}_{-1.05} (stat)^{+0.10}_{-0.24} (syst)$ and ${\mathcal A}_{\phi K_S^0 \gamma} = +0.35 +/- 0.58 (stat)^{+0.23}_{-0.10} (syst)$. Furthermore, we measure ${\mathcal B}(B^+ -> \phi K^+ \gamma) = (2.48 +/- 0.30 +/- 0.24) x 10^{-6}$, ${\mathcal A}_{CP} = -0.03 +/- 0.11 +/- 0.08$ and find that the signal is concentrated in the M_{\phi K} mass region near threshold.