Discrepancy between Soft-Pion Predictions and Experiment inNN→NNπ

Sources of a discrepancy between experiment and model-independent soft-pion predictions for the production of low-energy pions in the reaction $\mathrm{pp}\to \mathrm{np}{\pi }^{+}$ at 740 MeV are examined and discussed. These predictions deal only with the leading behavior of the production amplitude. However, higher-order terms, present because $\mu \ne 0$, seem to dominate because of resonance effects. An extended version of the Mandelstam resonance model appears sufficient to explain the experimental data, even for the lowest-energy pions. Ambiguities in pion production at threshold are also discussed.