Evidence for rho^0(770)-f_0(980) mixing in pi(-)p->pi(-)pi(+)n from CERN   measurements on polarized target
  We present model independent high resolution amplitude analyses of CERN data on pi(-)p->pi(-)pi(+)n at 17.2 GeV/c for dipion masses 580-1080 MeV at small momentum transfers on polarized target (Analysis I) and unpolarized target (Analysis II). The results for S- and P-wave transversity amplitudes from the Analysis I are used in a model independent determination of helicity amplitudes. All three analyses provide similar evidence for rho^0(770)-f_0(980) mixing in S- and P-wave amplitudes. In Analyses I and II the S-wave amplitudes |S_d|^2 peak at rho^0(770) mass while the P-wave amplitudes |L_d|^2 dip at f_0(980) mass. In both analyses the S-wave amplitudes S_tau are nearly in phase with amplitudes L_tau for both transversitities tau=u(spin up) and d(spin down), indicating resonant phases of rho^0(770) also in the S-wave. The S-wave single flip amplitudes |S_1|^2 and the relative phases Phi(S_1)-Phi(L_1) show the same pattern. While the non-flip amplitudes |L_0|^2 are small below 960 MeV, there is a sudden rise above this mass that may reflect the presence of f_0(980) resonance. We show that the apparent dependence of rho^0(770) width on helicity in amplitudes |U_tau|^2 and |N_tau|^2 results from the interference of amplitudes with helicities lambda=+/-1 and not from the violation of rotational/Lorentz symmetry. This suggests that the production mechanism and the mechanism responsible for rho^0(770)-f_0(980) mixing have independent dynamical origins. 
Miloslav Svec
hep-ph