In the framework of the "current scheme" of η−η′ mixing we calculate the amplitude of the decay η′→π+π−γ in the soft limit and compare our results with recently reported values of the phenomenological additive terms to the ρ--meson background.
@article{arxiv.hep-ph/9803411,
title = {Box Anomaly and $\eta' \to \pi ^+ \pi ^- \gamma$ Decay},
author = {A. V. Kisselev and V. A. Petrov},
journal= {arXiv preprint arXiv:hep-ph/9803411},
year = {2009}
}