Meson Exchange and Pion Rescattering Contributions to the Cross Section   for $pp\to pp\pi^0$
  Short range meson-exchange mechanisms such as $\rho\to\omega\pi^0$ contribute significantly to the amplitude for $pp\rightarrow pp\pi^0$ near threshold in addition to pion rescattering. The uncertainty of the latter contribution, which depends on poorly determined parameters, is estimated. Allowing for this uncertainty and taking the short range meson-exchange contributions into account makes it possible to reproduce the measured cross section. 
U. van Kolck, G.A. Miller, and D.O. Riska