The $σ$ pole in $J/ψ\to ωπ^+ π^-$

Using a sample of 58 million $J/ψ$ events recorded in the BESII detector, the decay $J/ψ\to ωπ^+ π^-$ is studied. There are conspicuous $ωf_2(1270)$ and $b_1(1235)π$ signals. At low $ππ$ mass, a large broad peak due to the $σ$ is observed, and its pole position is determined to be $(541 \pm 39)$ - $i$ $(252 \pm 42)$ MeV from the mean of six analyses. The errors are dominated by the systematic errors.

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