Radiative Transitions of Charmoniumlike Exotics in the Dynamical Diquark Model

Using the dynamical diquark model, we calculate the electric-dipole radiative decay widths to $X(3872)$ of the lightest negative-parity exotic candidates, including the four $I=0$, $J^{PC} \! = \! 1^{--}$ ("$Y$") states. The $O$(100--1000 keV) values obtained test the hypothesis of a common substructure shared by all of these states. We also calculate the magnetic-dipole radiative decay width for $Z_c(4020)^0 \! \to \! \gamma X(3872)$, and find it to be rather smaller ($<$~10 keV) than its predicted value in molecular models.