The σ pole in J/ ψ→ ωπ + π −

Abstract Using a sample of 58 million J / ψ events recorded in the BESII detector, the decay J / ψ → ω π + π − is studied. There are conspicuous ω f 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 ± 39 ) − i ( 252 ± 42 ) MeV from the mean of six analyses. The errors, which are dominated by the systematic errors, cover the statistical and systematic errors in the six analyses, as well as the observed variation of the six analyses.