Decay of a bound state of A $\mu^+\mu^-$ pair into an $e^+e^-$ Dalitz pair and A photon

The form factor of the decay of a bound $\mu^+\mu^-$ system in a state with total spin equal to zero into a $e^+e^-$ pair and a $\gamma$ photon is investigated. The probability of decay of the $\mu^+\mu^-$ bound state into two photons is calculated. If the mass of the $\mu$ meson in the obtained expression is replaced by the electron mass, the resulting probability obtained for the decay of parapositronium into two photons agrees with the experimental data.