Possible links between the liquid-gas and deconfinement-hadronization phase transitions

It is commonly accepted that strongly interacting matter has several phase transitions in different domains of temperature and baryon density. In this contribution I discuss two most popular phase transitions which in principle can be accessed in nuclear collisions. One of them, the liquid-gas phase transition, is well established theoretically and studied experimentally in nuclear multifragmentation reactions at intermediate energies. The other one, the deconfinement-hadronization phase transition, is at the focus of present and future experimental studies with relativistic heavy-ion beams at SPS, RHIC and LHC. Pssible links between these two phase transitions are identified from the viewpoint of their manifestation in violent nuclear collisions.Comment: 15 pages in revtex, 2 figures, to be published in the book "Dynamics and Thermodynamics with Nuclear Degrees of Freedom" by Springe

Studying Phase Transitions in Nuclear Collisions

In this talk I discuss three main topics concerning the theoretical description and observable signatures of possible phase transitions in nuclear collisions. The first one is related to the multifragmentation of thermalized sources and its connection to a liquid-gas phase transition in finite systems. The second one is dealing with the Coulomb excitation of ultrarelativistic heavy ions resulting in their deep disintegration. The third topic is devoted to the description of a first order phase transition in rapidly expanding matter. The resulting picture is that a strong collective flow of matter will lead to the fragmentation of a metastable phase into droplets. If the transition from quark-gluon plasma to hadron gas is of the first order it will manifest itself by strong nonstatistical fluctuations in observable hadron distributions.Comment: Invited talk presented at the International Conference "Physics with Storage Rings" (Bloomington, USA, 12-16 Sep. 1999), 16 pages in LaTeX including 4 eps figures, fig. 1 in colo

Equilibrium nuclear ensembles taking into account vaporization of hot nuclei in dense stellar matter

We investigate the high-temperature effect on the nuclear matter that consists of mixture of nucleons and all nuclei in the dense and hot stellar environment. The individual nuclei are described within the compressible liquid-drop model that is based on Skyrme interactions for bulk energies and that takes into account modifications of the surface and Coulomb energies at finite temperatures and densities. The free-energy density is minimized with respect to the individual equilibrium densities of all heavy nuclei and the nuclear composition. We find that their optimized equilibrium densities become smaller and smaller at high temperatures because of the increase of thermal contributions to bulk free energies and the reduction of surface energies. The neutron-rich nuclei become unstable and disappear one after another at some temperatures. The calculations are performed for two sets of model parameters leading to different values of the slope parameter in the nuclear symmetry energy. It is found that the larger slope parameter reduces the equilibrium densities and the melting temperatures. We also compare the new model with some other approaches and find that the mass fractions of heavy nuclei in the previous calculations that omit vaporization are underestimated at $T \lesssim 10$MeV and overestimated at $T\gtrsim10$~MeV. The further sophistication of calculations of nuclear vaporization and of light clusters would be required to construct the equation of state for explosive astrophysical phenomena.Comment: 27pages, 8 figures, accepted for publication in Phys. Rev.

Nonlinear oscillations of compact stars in the vicinity of the maximum mass configuration

We solve the dynamical GR equations for the spherically symmetric evolution of compact stars in the vicinity of the maximum mass, for which instability sets in according to linear perturbation theory. The calculations are done with the analytical Zeldovich-like EOS P=a(rho-rho_0) and with the TM1 parametrisation of the RMF model. The initial configurations for the dynamical calculations are represented by spherical stars with equilibrium density profile, which are perturbed by either (i) an artificially added inward velocity field proportional to the radial coordinate, or (ii) a rarefaction corresponding to a static and expanded star. These configurations are evolved using a one-dimensional GR hydro code for ideal and barotropic fluids. Depending on the initial conditions we obtain either stable oscillations or the collapse to a black hole. The minimal amplitude of the perturbation, needed to trigger gravitational collapse is evaluated. The approximate independence of this energy on the type of perturbation is pointed out. At the threshold we find type I critical behaviour for all stellar models considered and discuss the dependence of the time scaling exponent on the baryon mass and EOS.Comment: 15 pages, 8 figures, accepted for publication in EP

Collective mechanism of dilepton production in high-energy nuclear collisions

Collective bremsstrahlung of vector meson fields in relativistic nuclear collisions is studied within the time-dependent Walecka model. Mutual deceleration of the colliding nuclei is described by introducing the effective stopping time and average rapidity loss of baryons. It is shown that electromagnetic decays of virtual omega-mesons produced by bremsstrahlung mechanism can provide a substantial contribution to the soft dilepton yield at the SPS bombarding energies. In particular, it may be responsible for the dilepton enhancement observed in 160 AGev central Pb+Au collisions. Suggestions for future experiments to estimate the relative contribution of the collective mechanism are given.Comment: 6 page

The nuclear liquid-gas phase transition at large NcN_c in the Van der Waals approximation

We examine the nuclear liquid-gas phase transition at large number of colors ($N_c$) within the framework of the Van Der Waals (VdW) model. We argue that the VdW equation is appropriate at describing inter-nucleon forces, and discuss how each parameter scales with $N_c$. We demonstrate that $N_c=3$ (our world) is not large with respect to the other dimensionless scale relevant to baryonic matter, the number of neighbors in a dense system $N_N$. Consequently, we show that the liquid-gas phase transition looks dramatically different at $N_c \to \infty$ with respect of our world: The critical point temperature becomes of the order of \lqcd rather than below it. The critical point density becomes of the order of the baryonic density, rather than an order of magnitude below it. These are precisely the characteristics usually associated with the "Quarkyonic phase". We therefore conjecture that quarkyonic matter is simply the large $N_c$ limit of the nuclear liquid, and the interplay between $N_c$ and $N_N$ is the reason why the nuclear liquid in our world is so different from quarkyonic matter. We conclude by suggesting ways our conjecture can be tested in future lattice measurements.Comment: Version accepted for publication, Phys.Rev.

Radial oscillations of neutral and charged hybrid stars

We construct stellar models of hadron stars and hybrid stars and calculate the frequencies of their lowest radial mode of vibration. Chandrasekhar's equation for radial oscillations is generalized for stars with internal electric fields and earlier versions of that generalization are simplified. For the hybrid stars a Gibbs construction is employed. It is found that the softening of the equation of state associated with the presence of deconfined quarks reduces the oscillation frequency. We show that a slight charge inbalance should lead to increased maximum mass, decreased central density and lower oscillation frequencies