Chiral Perturbation Theory Calculations For S-wave $pi^0$ production In pp Collision

The total cross section for the pp --> pp $pi^0$ reaction at energies close to threshold is calculated within the frame of a chiral perturbation theory. The tree and one loop diagrams up to chiral order D=2 contributions are taken into account. The $L^{(0)}$ isoscalar-scalar part of t-channel two-pion exchange loop diagrams enhances the production amplitude strongly. The calculated cross section scale and energy dependence are very close to data.