NN→NN_π⁺ reaction near threshold in a chiral power counting approach

Power-counting arguments are used to organize the interactions contributing to the NN→dπ,pnπ reactions near threshold. We estimate the contributions from the three formally leading mechanisms: the Weinberg-Tomozawa (WT) term, the impulse term, and the Δ-excitation mechanism. Subleading but potentially large mechanisms, including S-wave pion rescattering, the Galilean correction to the WT term, and short-ranged contributions are also examined. The WT term is shown to be numerically the largest, and the other contributions are found to approximately cancel. Similarly to the reaction pp→ppπ⁰, the computed cross sections are considerably smaller than the data. We discuss possible origins of this discrepancy.