First observation of the φ→π+π−γ decay

Abstract Radiative decays of the φ meson have been studied using a data sample of about 20 million φ decays collected by the CMD-2 detector at VEPP-2M collider in Novosibirsk. From selected e + e − → π + π − γ events the φ → π + π − γ decay has been observed for the first time. Under the assumption that the intermediate f 0 (980) γ state dominates in the φ → π + π − γ decay, the corresponding branching ratio is Br ( φ → f 0 (980) γ )=(1.93±0.46±0.50)×10 −4 . Selected e + e − → μ + μ − γ events were used to obtain Br ( φ → μ + μ − γ )=(1.43±0.45±0.14)×10 −5 for E γ >20 MeV. Using the same data sample, upper limits at 90% CL have been obtained for the C-violating decay of the φ : Br ( φ → ργ ) −5 ; and for the P- and CP-violating decay of the η : Br ( η → π + π − ) −4 .