Box Anomaly and $\eta' \to \pi ^+ \pi ^- \gamma$ Decay

Abstract

In the framework of the "current scheme" of $\eta - \eta'$ mixing we calculate the amplitude of the decay $\eta' \to \pi ^+ \pi ^- \gamma $ in the soft limit and compare our results with recently reported values of the phenomenological additive terms to the $\rho $--meson background.