ROLE OF HEAVY-MESON EXCHANGE IN PION-PRODUCTION NEAR-THRESHOLD

Recent calculations of s-wave pion production have severely underestimated the accurately known pp --> pppi0 total cross section near threshold. In these calculations, only the single-nucleon axial-charge operator is considered. We have calculated, in addition to the one-body term, the two-body contributions to this reaction that arise from the exchange of mesons. We find that the inclusion of the scalar sigma-meson exchange current (and lesser contributions from other mesons) increases the cross section by about a factor of 5 and leads to excellent agreement with the data. The results are sensitive neither to changes in the distorting potential that generates the NN wave functions nor to different choices for the meson-nucleon form factors.