THEORY OF THE COUPLED NN - PI-NN SYSTEM

A review is presented of the current status of pi-NN theories that include the effect of pion absorption. We emphasize models that are unitary at the two and three-body level and that provide for a simultaneous description of all the processes NN --> NN, NN --> pi-NN, NN <-- --> pi-d, pi-d --> pi-d and pi-d --> pi-NN. Despite the availability of relatively sophisticated calculations, we conclude that none of the models are sufficiently accurate to make reliable predictions, and that at least a partial inclusion of four-body unitarity may be necessary.