Phase Transition in the chiral $σ$-$ω$ model with dilatons

We investigate the properties of different modifications to the linear $\sigma$-model (including a dilaton field associated with broken scale invariance) at finite baryon density $\rho$ and nonzero temperature $T$. The explicit breaking of chiral symmetry and the way the vector meson mass is generated are significant for the appearance of a phase of nearly vanishing nucleon mass besides the solution describing normal nuclear matter. The elimination of the abnormal solution prohibits the onset of a chiral phase transition but allows to lower the compressibility to a reasonable range. The repulsive contributions from the vector mesons are responsible for the wide range of stability of the normal phase in the $(\mu,T)$-plane. The abnormal solution becomes not only energetically preferable to the normal state at high temperature or density, but also mechanically stable due to the inclusion of dilatons.