Heavy Meson exchange in pion production and absorption

The conventional approach to calculate the s-wave cross section for pion production in nucleon-nucleon collisions involves direct production and on-shell rescattering. It is now known that such calculations underestimate the empirical cross section. It has been suggested that including the contributions of heavy mesons in the axial exchange current could explain the discrepancy. More recently, large effects have also been attributed to the off-shell pion rescattering process. A precise measurement of the pp → ppπ0 cross section very close to threshold, carried out at the Indiana University Cooler Facility, has originally drawn attention to this problem. However, similar discrepancies have also been found in other pion production and absorption processes. I will review the current status of the available experimental information which bears on this question, and compare it with theoretical attempts to interpret the data