Study of the decay ϕ→f0(980)γ→π+π−γ with the KLOE detector

We measured, with the KLOE detector, the spectrum of π + π − invariant mass in a sample of 6.7 × 10 5 e + e − → π + π − γ events with the photon at large polar angle ( θ γ > 45 ° ) at a centre of mass energy s around the ϕ mass. We observe in this spectrum a clear contribution from the intermediate process ϕ → f 0 ( 980 ) γ . A sizeable effect is also observed in the distribution of the pion forward–backward asymmetry. We use different theoretical models to fit the spectrum and we determine the f0 mass and coupling constants to the ϕ, to π + π − and to K K ¯ .