Can one directly test 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 pp-bar photoproduction on a nucleus by about 850 MeV as compared with the free case since charge conjugation reverses the sign of the vector potential contribution in the equation of motion for the p-bar states. We discuss the possibility of measurable signatures of the scalar-vector structure of the nucleon-nucleus potential for the pp-bar photoproduction. The pp-bar photoproduction cross section is calculated for the target nucleus 208Pb near threshold. We indicate the increased cross section for emission of charged pions as a consequence of the subsequent p-bar annihilation within the nucleus. The inclusive cross section for γ-induced pp-bar creation and the resulting increase of charged pion production are probably too small as compared with the expected background to be measurable. (author)