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 p 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 states. We discuss the possibility of measurable signatures of the scalar-vector structure of the nucleon-nucleus potential for the p photoproduction. The p 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 annihilation within the nucleus. The inclusive cross section for -induced p creation and the resulting increase of charged pion production are probably too small as compared with the expected background to be measurable.