Instability of asymmetric nuclear matter in the extended Zimanyi-Moszkowski model

We have investigated the unstable spinodal region of asymmetric nuclear matter in the extended Zimanyi-Moszkowski (EZM) model. It is found that the model produces larger spinodal region, higher upper-density boundary and maximum asymmetry, than the familiar nonlinear Walecka model. This is due to the renormalized vector-meson coupling constants in the EZM model. Recent experimental developments of nuclear multifragmentation reaction have provided valuable information [1] on the liquid-gas phase transition and the critical phenomena in nuclear matter. The nuclear multifragmentation is considered as the spinodal decomposition [2-4]. A hot homogeneous nuclear matter, which is produced by high-energy heavy-ion reaction, expands due to thermal pressure and cools o¤ below the critical temperature, and then falls into unstable spinodal region. Because of density ‡uctuations, a homogeneous system converts into a mixed-phase state that consists of droplets surrounded by nuclear gas. In this respect, there are renewed theoretical interests [5-10] on the spinodal instability in nuclear matter. The investigations of Refs. [5], [7] and [9] are based on the nonlinear Walecka (NLW) model of relativistic mean-…eld (RMF) theory [11]. The NLW model has been most widely used in the investigations of nuclear matter, …nite nuclei, strange hadronic matter and neutron stars. Most of their calculations did not take into account the isovector-scalar meson that has signi…cant e¤ect to split the e¤ective masses of proton and neutron in the medium and alter the symmetry energy of asymmetric nuclear matter [12], although the recent works [7] and [9] considered the e¤ect. We further have to stress that the NLW model [5] cannot reproduce the empirical value of the critical temperature [13] for nuclear matter. Recently, the author has developed the extended Zimanyi-Moszkowski (EZM) model [14] and applied it to warm symmetric nuclear matter [15]. The model has succeeded to reproduce the empirical values of the critical temperature [13] and critical exponents [2] of nuclear matter in contrast to the NLW model. It is therefore valuable to investigate the spinodal instability of asymmetric nuclear matter in the EZM model. E-mail : [email protected]