Measurement of Γ(η→π+π−γ)/Γ(η→π+π−π0) with the KLOE detector

The ratio R-eta = Gamma(eta -> pi(+)pi(-)gamma)/Gamma(eta -> pi(+)pi(-)pi(0)) has been measured by analysing 22 million phi -> eta gamma decays collected by the KLOE experiment at DANE, corresponding to an integrated luminosity of 558 pb(-1). The eta -> pi(+)pi(-)gamma proceeds both via the rho resonant contribution, and possibly a non-resonant direct term, connected to the box anomaly. Our result, R-eta = 0.1856 +/- 0.0005(stat)+/- 0.0028(syst), points out a sizable contribution of the direct term to the total width. The di-pion invariant mass for the eta -> pi(+)pi(-)gamma decay could be described in a model-independent approach in terms of a(-) single free parameter, alpha. The determined value of the parameter alpha is alpha = (1.32 +/- 0.08(stat-0.09syst)(+0.10) +/- 0.02(theo)) GeV-2. 2012 Elsevier B.V. All rights reserved.