Charge asymmetry of the nuclear interaction and neutron-neutron scattering parameters

Analyses of bound systems and low-energy scattering experiments give clear evidence of a departure from charge symmetry in the nuclear interaction. Furthermore, this effect could be enhanced in a nuclear medium thereby solving the Nolen-Schiffer anomaly observed in mirror nuclei. Since the extraction of the n-n scattering parameters is tied to the theoretical analysis of particular nuclear reactions, we have examined some theoretical treatments and approximations used in the study of the reaction ${\pi }^{-}\mathrm{d}\to \gamma \mathrm{nn}$ which is ideal for this purpose. In particular, we consider the neutron-neutron enhancement in higher n-n partial waves by introducing a simplified approach to the eigenamplitude and multipole expansions. We also give a description of the methods used in the calculation of the final state interactions in the ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ n-n state to determine the low-energy n-n scattering parameters.