Study of the decay φ with the KLOE detector The KLOE Collaboration

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 f 0 mass and coupling constants to the φ , to π + π − and to K ¯ K .