  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.
