Observation of radiative B -> phi K gamma decays

The radiative decay $B \to \phi K \gamma$ is observed for the first time. The branching fraction for the charged $B^- \to \phi K^- \gamma$ decay mode is measured to be ${\cal B}(B^- \to \phi K^- \gamma) = (3.4 \pm 0.9 \pm 0.4) \times 10^{-6}$. The photon energy distribution for the $B^- \to \phi K^- \gamma$ decay is presented. The signal for the neutral $\bar{B}^0 \to \phi \bar{K}^0 \gamma$ decay mode is not statistically significant and an upper limit, ${\cal B}(\bar{B}^0 \to \phi \bar{K}^0 \gamma) < 8.3 \times 10^{-6}$ at $90% {\rm CL}$, is set. The analysis is based on a dataset of 90 fb$^{-1}$ collected by the Belle experiment at the $e^+ e^-$ asymmetric collider KEKB.