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Y(4260) --> gamma + X(3872) in the diquarkonium picture

The observed Y(4260)\to γ+ X(3872) decay is a natural consequence of the diquark-antidiquark description of Y and X resonances. In this note we attempt an estimate of the transition rate, Γ_{\rm rad}, by a non-relativistic calculation of the electric dipole term of a diquarkonium bound state. We compute Γ_{\rm rad} for generic composition values of the isospin of X and Y. Specializing to I=0 for X(3872), we find Γ_{\rm rad}= 496~keV for Y(4260) with I=0 and Γ_{\rm rad}= 179~keV for I=1. Combining with BESIII data, we derive upper bounds to B(Y\to J/Ψ+π+π) and to Γ(Y\to μ^+ μ^-). We expect to confront these results with forthcoming data from electron-positron and hadron colliders.