S-wave $\pi^0$ Production in pp Collision in a Covariant OBE Model
  The total cross section for the pp -> pp pi^0 reaction at energies close to threshold is calculated using a covariant one-boson-exchange model, where a boson B created on one of the incoming protons is converted into a neutral pion on the second. The amplitudes for the conversion processes, B N -> N pi^0, are taken to be the sum of s, u and t-channel pole terms. The main contributions to the primary production amplitude is due to an effective isoscalar sigma meson pole in a t-channel, which is enhanced strongly due to offshellness. With this contribution included the model reproduces, both the scale and energy dependence of the cross section. 
E.Gedalin, A.Moalem and L.Razdolskaya