Peking Univ, Dept Phys, Beijing 100871, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
The pole structure of the low energy pi pi scattering amplitudes is studied using a proper chiral unitarization method combined with crossing symmetry and the low energy phase shift data. It is found that the sigma pole position is at M-sigma = 470 +/- 50 MeV, Gamma(sigma) = 570 +/- 50 MeV. The existence of the virtual state pole in the IJ=20 channel is reconfirmed. Various threshold parameters are estimated and are found in general in good agreement with the results obtained from the Roy equation analyses.