Characteristic Dirac Signature in Elastic Proton Scattering at Intermediate Energies

Nonrelativistic nucleon-nucleus first-order multiple-scattering calculations are extended to include virtual (Dirac) negative energy states of just the projectile. This effect may be thought of as virtual $N\overline{N}$ pair production and annihilation in the field of the nucleus. This extension leads to a parameter-free Dirac description of the projectile in elastic proton scattering which produces a characteristic effect in spin observables over a wide range of energies which is in agreement with experiment. This Dirac signature is extremely stable with respect to uncertainties in the microscopic input.