ChPT Calculation for Two Pion Production in NN Collision Reaction at Threshold

We investigate the two pion production in NN collision reaction at threshold with Chiral Perturbation Theory (ChPT). The leading-order diagrams are presented and as an example we give the results of complete calculation of the reaction pn → d ππ at threshold. It is found that the leading order diagrams are of equal importance as expected by power counting. The role of initial, intermediate and final state interactions in nucleon-nucleon system is also investigated and found to be very important. The calculations also show that the non-resonant contributions near threshold in the two-pion production in NN collision reaction is sizable and important.