Intermediate-energy-proton scattering, the Dirac equation, and nuclear structure

The Dirac equation employing an impulse approximation optical potential based on $\mathrm{NN}$ phase shifts has been shown to provide a description of the intermediate energy proton-nucleus spin-dependent elastic scattering data not yet found in the context of a Schrödinger approach. Using analytic methods generalized to the Dirac equation, we trace this success to the nonlinear density contributions which arise as a natural consequence of the Dirac approach. This new understanding resurrects the hope of using intermediate energy protons as a probe of nuclear structure details.