Measurement of B decays to $\varphi K\gamma$

We search for the decays $B^{-}\to \varphi K^{-}\gamma$ and ${\overline{B}}^0\to \varphi {\overline{K}}^0\gamma$ in a data sample of $228\times {10}^6$ $B\overline{B}$ pairs collected at the $\mathit{{\rm Y}}(4S)$ resonance with the BABAR detector. We measure the branching fraction $\mathcal{B}(B^{-}\to \varphi K^{-}\gamma )=(3.5\pm 0.6\pm 0.4)\times {10}^{-6}$ and set an upper limit $\mathcal{B}({\overline{B}}^0\to \varphi {\overline{K}}^0\gamma )<2.7\times {10}^{-6}$ at the 90% confidence level. We also measure the direct $CP$ asymmetry in $B^{-}\to \varphi K^{-}\gamma$, ${\mathcal{A}}_{CP}=(-26\pm 14\pm 5)\%$. The uncertainties are statistical and systematic, respectively.

Figure 1

Missing mass (a) and reconstructed mass (b) fits in the signal region for the charged mode and the neutral mode (c,d). The dotted curves show the background contribution while the solid curves show the sum of signal and background.

Figure 2

The background-subtracted and efficiency-corrected $\varphi K$ mass distributions (points with uncertainties) for the charged mode (a) and the neutral mode (b). The signal MC model for the mass spectrum, based on Ref. [18], is shown as a histogram without uncertainties and is normalized to the efficiency-corrected signal yield obtained in data.