Observation of Radiative $B\to \varphi K\gamma$ Decays

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

Figure 1

A “penguin” diagram for $B^{-}\to \varphi K^{-}\gamma$ with $s\overline{s}$ pair creation.

Figure 2

The (a) $M_{\mathrm{b}\mathrm{c}}$, (b) $\Delta E$, (c) $M(K^{+}{K}^{-})$, (d) $E_{\gamma }^{*}$, and (e) $M(\varphi K^{-})$ distributions for the $B^{-}\to \varphi K^{-}\gamma$ decay mode and the (f) $M_{\mathrm{b}\mathrm{c}}$ distribution for the ${\overline{B}}^0\to \varphi {\overline{K}}^0\gamma$ decay mode are shown by histograms. The solid histograms are obtained with events from the $B$ mass sideband. The curves show the results of the fit described in the text. The measured $M(\varphi K^{-})$ distribution is compared to those obtained from the MC simulation, with a three-body phase-space model (circles with error bars) or adjusted to follow the data (squares connected by a line).