THE PROBLEM OF MATTER STABILITY IN THE NAMBU-JONA-LASINIO MODEL

We reinvestigate the conditions for stable matter solutions in the Nam bu-Jona-Lasinio (NJL) model, In mean field approximation the NJL model can be regarded as an extension of the Walecka mean field model to in clude negative energy fermion states, While this extension is necessar y to allow for a chiral phase transition, it was found some time ago t hat at the same time it destroys the wanted saturation properties of t he Walecka model. We reformulate this problem in terms of the thermody namic potential and find that there is indeed a connection between the se two features. We show that the minimum of the thermodynamic potenti al which corresponds to stable nuclear matter in the Walecka model is shifted from a finite to zero effective fermion mass in the chiral ML model, This shift is closely related to the chiral phase transition, U nder certain conditions the shifted minima may still lead to stable ma tter solutions but only in the chirally restored phase. We discuss a p ossible interpretation of these solutions as a schematic bag model des cription.