Coupled channel effects in pion pion S-wave interaction
  We study coupled channel effects upon isospin I=2 and I=0 $\pi\pi$ S-wave interaction. With introduction of the $\pi\pi\to\rho\rho\to\pi\pi$ coupled channel box diagram contribution into $\pi\pi$ amplitude in addition to $\rho$ and $f_2 (1270)$ exchange, we reproduce the $\pi \pi$ I=2 S-wave and D-wave scattering phase shifts and inelasticities up to 2 GeV quite well in a K-matrix formalism. For I=0 case, the same $\pi\pi\to\rho\rho\to\pi\pi$ box diagram is found to give the largest contribution for the inelasticity among all possible coupled channels including $\pi\pi\to\omega\omega\to\pi\pi$, $\pi\pi\to K \bar{K}\to\pi\pi$. We also show why the broad $\sigma$ appears narrower in production processes than in $\pi\pi$ scattering process. 
F.Q.Wu and B.S.Zou