Dynamical calculation of the decay psi to psi +2 pi

The authors calculate the psi ' to psi +2 pi decay width on the basis of partial wave dispersion relations in the pi pi channel. This is basically a coupled channel problem involving pi pi , DD, etc. intermediate states. The calculation acquires anomalous thresholds due to the fact that the vertex psi ' psi (2 pi ) is internally unstable. It is found that anomalous singularity contributions provide by far the most dominant contribution, which explains the rather large psi ' to psi =2 pi decay mode. Very good agreement with the experimental width and mass distribution is achieved. (21 refs).