The reactions pp→ppπ0 and pp→dπ+ at threshold: The role of the isoscalar πN scattering amplitude

We examine the role of the elementary isoscalar pion–nucleon scattering amplitude in the description of the processes pp→ppπ0 and pp→dπ+ at threshold. We argue that the presently used tree level dimension two approximation used in chiral perturbation theory is insufficient as input by direct comparison with the πN scattering data. We also show that a successful semi-phenomenological boson-exchange model does better in the description of these data. The influence of the violation of crossing symmetry in the meson-exchange model has to be studied in more detail. We stress that further investigations of the process pp→dπ+ can pave the way to a deeper understanding of the pion dominated part of the transition operator.