Describing radiation decay using the instant form of relativistic quantum mechanics

For a transition between states with different total angular momentum values, we develop a method for constructing an electromagnetic current operator that satisfies the Lorentz-covariance and current-conservation conditions. Our method is realized in the framework of an instant form of relativistic quantum mechanics using the general method of relativistic parameterization of matrix elements of local operators. We illustrate the method by calculating the radiation decay of a $\rho$-meson into a pion. In the modified impulse approximation, we obtain an analytic expression for the transition form factor of this process in the form of an integral representation of dispersion type. Results of the numerical calculation agree with the results of calculating the light-front dynamics and with the model of vector meson dominance.