Phase transitions in warm, asymmetric nuclear matter.

A relativistic meaneld model of nuclear matter with arbitrary proton fraction is studied at nite temperature. An analysis is performed of the liquid{ gas phase transition in a system with two conserved charges (baryon number and isospin) using the stability conditions on the free energy, the conservation laws, and Gibbs' criteria for phase equilibrium. For a binary system with two phases, the coexistence surface (binodal) is two-dimensional. The Maxwell construction through the phase-separation region is discussed, and it is shown that the stable con guration can be determined uniquely at every density. Moreover, because of the greater dimensionality of the binodal surface, the liquid{gas phase transition is continuous (second order by Ehrenfest's de nition), rather than discontinuous ( rst order), as in familiar one-component systems. Using a meaneld equation of state calibrated to the properties of nuclear matter and nite nuclei, various phase-separation scenarios are considered. The model is then applied to the liquid{gas phase transition that may occur in the warm, dilute matter produced in energetic heavy-ion collisions. In asymmetric matter, instabilities that produce a liquid{gas phase separation arise from uctuations in the proton concentration (chemical instability), rather than from uctuations in the baryon density (mechanical instability).