Precision calculation of $\gamma d\to \pi^+ nn$ within chiral perturbation theory

Abstract

The reaction $\gamma d\to \pi^+ nn$ is calculated up to order $\chi^{5/2}$ in chiral perturbation theory, where $\chi$ denotes the ratio of the pion to the nucleon mass. Special emphasis is put on the role of nucleon--recoil corrections that are the source of contributions with fractional power in $\chi$. Using the known near threshold production amplitude for $\gamma p\to \pi^+ n$ as the only input, the total cross section for $\gamma d\to \pi^+ nn$ is described very well. A conservative estimate suggests that the theoretical uncertainty for the transition operator amounts to 3 % for the computed amplitude near threshold.