An empirical model of low-energy np scattering: Insights beyond ERT

A model is presented of s-wave np elastic scattering as proceeding through an intermediate, off-shell dibaryon d*, with corrections to the npd vertex and d* propagator. The model relies on plausible conjectures and hypotheses to match identically the form of the simple amplitude given by the shape-independent effective range theory (SI ERT), which exactly describes the extant np elastic data (within errors) to at least 3 MeV. The result provides insight into the mechanisms involved in np scattering, which go beyond what ERT can reveal because ERT is a product of wave-mechanics and is therefore generally independent of mechanism. For example, in this model, the signs of the triplet and singlet scattering lengths are determined by the (opposite) spatial symmetries of the triplet and singlet np wavefunctions and pion exchange in the vertex corrections.