The distribution of m(π+π−) in the decay η′ → π+π−γ has been measured with the Crystal Barrel detector. The results are based on a total of 7392 observed η′ decays. The box anomaly constant is extracted from this and its value is found to agree well with theoretical expectations. The pseudoscalar nonet parameters (f1, f8 and θPS) are determined. Finally, we find that there is a problem of consistency between QCD and the standard VDM assumption.
The distribution of m(π+π−) in the decay η′ → π+π−γ has been measured with the Crystal Barrel detector. The results are based on a total of 7392 observed η′ decays. The box anomaly constant is extracted from this and its value is found to agree well with theoretical expectations. The pseudoscalar nonet parameters (f1, f8 and θPS) are determined. Finally, we find that there is a problem of consistency between QCD and the standard VDM assumption.