Phase transition of the nucleon-antinucleon plasma in a relativistic mean-field theory

Studying Walecka's mean-field theory we find that one can reproduce the observed binding energy and density of nuclear matter within experimental precision in an area characterized by a line in the coupling-constant plane. A part of this line defines systems which exhibit a phase transition around $T_c\sim 200$ MeV for zero baryon density. The rest corresponds to such systems where the phase transition is absent; in that case a peak appears in the specific heat around $T\sim 200$ MeV. We interpret these results as indicating that the hadron phase of nuclear matter alone indicates the occurrence of an abrupt change in the bulk properties around ${\rho }_V\sim 0$ and $T\sim 200$ MeV.