Evidence of $σ$ particle in $J/ψ\to ωππ$

Based on a sample of 7.8 million BESI $J/ψ$ events, the decay of $J/ψ\to ωπ^+ π^-$ is studied. A low mass enhancement in the $π^+ π^-$ invariant mass spectrum recoiling against $ω$ particle is clearly seen which does not come from the phase space effect and the background. According to PWA analysis, this low mass enhancement is a broad $0^{++}$ resonance, the $σ$ particle. If a Breit-Winger function of constant width is used to fit the $σ$ signal, its mass and width are $384 \pm 66$ MeV and $458 \pm 100$ MeV respectively, which correspond to the pole position at (434 $\pm$ 78) - $i$ (202 $\pm$ 43) MeV.

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