S-wave $\pi^0$ production In pp Collision In A Covariant OBE Model
  The total cross section for the pp to pp $\pi^0$ reaction at energies close to threshold is calculated in a covariant one-boson-exchange model. The amplitudes for the elementary BN to N$\pi^0$ processes are taken to be the sum of s, u and t pole terms. The main contributions to the primary productionamplitude is due to a sigma meson exchange. Both the scale and energy dependence of the cross section are perfectly reproduced. 
E. Gedalin, A. Moalem and L. Razdolskaja (Ben Gurion University)