$\chi$PT Calculations with two-pion Loops for S-wave $\pi^0$ Production   in pp Collision
  The total cross section for the $pp \to pp \pi^0$ reaction at energies close to threshold is calculated within the frame of a chiral perturbation theory, taking into account tree and one loop diagrams up to chiral order D=2. Two-pion loop contributions dominate $\pi^0$ production at threshold. The calculated cross section reproduces data, both scale and energy dependence, fairly well. 
E. Gedalin, A. Moalem and L. Razdolskaya