Phase structure of excited baryonic matter in the relativistic mean field theory.

Abstract

We analyze the phase structure of the nonlinear mean-field meson theory of baryonic matter (nucleons plus delta resonances). Depending on the choice of the coupling constants, we find three physically distinct phase transitions in this theory: a nucleonic liquid-gas transition in the low temperature, ${T}_{c}$20 MeV, low density, \ensuremath{\rho}\ensuremath{\simeq}0.5${\ensuremath{\rho}}_{0}$, regime, a high-temperature (T\ensuremath{\simeq}150 MeV) finite density transition from a gas of massive hadrons to a nearly massless baryon, antibaryon plasma, and, third, a strong phase transition from the nucleonic fluid to a resonance-dominated ``delta-matter'' isomer at \ensuremath{\rho}>2${\ensuremath{\rho}}_{0}$ and ${T}_{c}$50 MeV. All three phase transitions are of first order. It is shown that the occurrence of these different phase transitions depends critically on the coupling constants. Since the production of pions also depends strongly on the coupling constants, it is seen that the equation of state cannot be derived unambiguously from pion data.