Progress in NN --> NNpi

Abstract

We survey the recent developments in the reaction $NN \to NN\pi$ in effective field theory. We show that the proper construction of the production operator needs a careful separation of irreducible pieces from reducible ones. The result of this consideration is a complete cancellation of all loops in the production operator at NLO. Moreover, we show that this procedure brings the leading Weinberg-Tomozawa vertex on-shell, thus enhancing the corresponding contribution to the transition amplitude by a factor of 4/3 as compared to the commonly used value. We also discuss the role of the $\Delta(1232)$ for the s-wave pion production. Being relatively sizable individually the direct and rescattering mechanisms of the $\Delta$ excitation at NLO cancel each other to a large extent. Thus, we conclude that the net effect of the $\Delta$ at NLO is very small.