Observation of $X(3872)\to J/\psi \gamma$ and search for $X(3872)\to\psi'\gamma$ in B decays

We report a study of $B\to (J/\psi \gamma) K$ and $B\to (\psi' \gamma)K$ decay modes using $772\times 10^{6}$ $B\bar{B}$ events collected at the \Upsilon(4S)$ resonance with the Belle detector at the KEKB energy-asymmetric $e^+ e^-$ collider. We observe $X(3872) \to J/\psi \gamma$ and report the first evidence for $\chi_{c2} \to J/\psi \gamma$ in $B\to (X_{c\bar{c}}\gamma) K$ decays, while in a search for $X(3872) \to \psi' \gamma$ no significant signal is found. We measure the branching fractions, $\mathcal{B}(B^{\pm} \to X(3872) K^{\pm}) \mathcal{B}(X(3872) \to J/\psi\gamma)$ $=$ $(1.78^{+0.48}_{-0.44}\pm 0.12)\times 10^{-6}$, $\mathcal{B} (B^{\pm} \to\chi_{c2} K^{\pm})$$=$ $(1.11^{+0.36}_{-0.34} \pm 0.09) \times 10^{-5}$, $\mathcal{B}(B^{\pm} \to X(3872) K^{\pm}) \mathcal{B}(X(3872) \to \psi'\gamma)$ $<$ $3.45\times 10^{-6}$ (upper limit at 90% C.L.) and also provide upper limits for other searches.