Y(4260) → γ + X(3872) in the diquarkonium picture

The observed Y(4260) o gamma + 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, Gamma_m rad, by a non-relativistic calculation of the electric dipole term of a diquarkonium bound state. We compute Gamma_m rad for generic composition values of the isospin of X and Y. Specializing to I=0 for X(3872), we find Gamma_m rad= 496~keV for Y(4260) with I=0 and Gamma_m rad= 179~keV for I=1. Combining with BESIII data, we derive upper bounds to B(Y o J/Psi+pi+pi) and to Gamma(Y o mu^+ mu^-). We expect to confront these results with forthcoming data from electron-positron and hadron colliders.