  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.
