Progress in NN --> NNpi

Understand

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.