Extraction ofannfrom the reactionπ−d→γnn

Extraction of a value for the neutron-neutron scattering length by an analysis of the reaction ${\pi }^{-}+d\to \gamma +n+n$ necessitates making assumptions about the reaction amplitude. We have studied the sensitivity of the extracted value of $a_{\mathrm{nn}}$ to such an analysis by performing a series of theoretical experiments in which a standard set of assumptions is used to analyze data generated by variations in one or more of these assumptions. For kinematically complete experiments the major sources of uncertainty lie in the energy dependence of the short-range characterization of the $n-n$ final-state wave function and to a lesser extent the treatment of the pion wave function in the initial state. We conclude that a value of $a_{\mathrm{nn}}$ can be extracted from this experiment with a theoretical uncertainty of less than 0.3 fm.