Cross section of the reaction $e^+ e^- \to π^+π^-π^+π^- $ below 1 GeV at CMD-2

Abstract

Using 3.07 ${pb}^{-1}$ of data collected in the energy range 0.60-0.97 GeV by CMD-2, about 150 events of the process $\epm \to \pch$ have been selected. The energy dependence of the cross section agrees with the assumption of the $a_1(1260) π$ intermediate state which is dominant above 1 GeV. For the first time \fourpi events are observed at the $ρ$ meson energy. Under the assumption that all these events come from the $ρ$ meson decay, the value of the cross section at the $ρ$ meson peak corresponds to the following decay width: Γ(ρ^0 \to \fourpi) = (2.8 \pm 1.4 \pm 0.5) {keV} or to the branching ratio B(ρ^0 \to \fourpi) = (1.8 \pm 0.9 \pm 0.3) \cdot 10 ^{-5}.