"The ratio Rη = Γ(η → π+π−γ)\/Γ(η → π+π−π0) has been measured by analysing 22 million φ → ηγ decays collected by the KLOE experiment at DAΦNE, corre- sponding to an integrated luminosity of 558 pb−1. The η → π+π−γ proceeds both via the ρ resonant contribution, and possibly a non-resonant direct term, connected to the box anomaly. Our result, Rη = 0.1856 ± 0.0005stat ± 0.0028syst, points out a sizable contribution of the direct term to the total width. The di-pion invariant mass for the η → π+π−γ decay could be described in a model-independent approach in terms of a single free parameter, α. The determined value of the parameter α is α = (1.32 ± 0.08stat+0.10syst±0.02theo) GeV−2."
DI MICCO, B., Ceradini, F., PRADO LONGHI, I. (2013). Measurement of Γ(η→π+π−γ)/Γ(η→π+π−π0) with the KLOE detector. PHYSICS LETTERS. SECTION B, 718, 910-914 [10.1016/j.physletb.2012.11.032].