Direct Test of the Scalar-Vector Lorentz Structure of the Nucleon- and Antinucleon-Nucleus Potential

Quantum Hadrodynamics in mean field approximation describes the effective nucleon-nucleus potential (about -50 MeV deep) as resulting from a strong repulsive vector (about 400 MeV) and a strong attractive scalar (about -450 MeV) contribution. This scalar-vector Lorentz structure implies a significant lowering of the threshold for $p\bar{p}$ photoproduction on a nucleus by about 850 MeV as compared to the free case since charge conjugation reverses the sign of the vector potential contribution in the equation of motion for the $\bar{p}$ states. It also implies a certain size of the photon induced $p\bar{p}$ pair creation cross section near threshold which is calculated for a target nucleus $^{208}$Pb. We also indicate a measurable second signature of the $p\bar{p}$ photoproduction process by estimating the increased cross section for emission of charged pions as a consequence of $\bar{p}$ annihilation within the nucleus.