Viscosity and thermal conductivity effects at first-order phase transitions in heavy-ion collisions

Effects of viscosity and thermal conductivity on the dynamics of first-order phase transitions are studied. The nuclear gas-liquid and hadron-quark transitions in heavy-ion collisions are considered. We demonstrate that at non-zero thermal conductivity, $\kappa \neq 0$, onset of spinodal instabilities occurs on an isothermal spinodal line, whereas for $\kappa =0$ instabilities take place at lower temperatures, on an adiabatic spinodal.Comment: invited talk at 6th International Workshop on Critical Point and Onset of Deconfinment (CPOD2010), Dubna, August 22-28, 201

Mapping the phase diagram of strongly interacting matter

We employ a conformal mapping to explore the thermodynamics of strongly interacting matter at finite values of the baryon chemical potential $\mu$. This method allows us to identify the singularity corresponding to the critical point of a second-order phase transition at finite $\mu$, given information only at $\mu=0$. The scheme is potentially useful for computing thermodynamic properties of strongly interacting hot and dense matter in lattice gauge theory. The technique is illustrated by an application to a chiral effective model.Comment: 5 pages, 3 figures; published versio

Net-proton probability distribution in heavy ion collisions

We compute net-proton probability distributions in heavy ion collisions within the hadron resonance gas model. The model results are compared with data taken by the STAR Collaboration in Au-Au collisions at sqrt(s_{NN})= 200 GeV for different centralities. We show that in peripheral Au-Au collisions the measured distributions, and the resulting first four moments of net-proton fluctuations, are consistent with results obtained from the hadron resonance gas model. However, data taken in central Au-Au collisions differ from the predictions of the model. The observed deviations can not be attributed to uncertainties in model parameters. We discuss possible interpretations of the observed deviations.Comment: 4 pages, 2 figure

Magnetic properties of (Fe1−x_{1-x}Cox_x)2_2B alloys and the effect of doping by 5dd elements

We have explored, computationally and experimentally, the magnetic properties of \fecob{} alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing Co$_2$B, for which it is found that both full potential effects and electron correlations treated within dynamical mean field theory are of importance for a correct description. The material exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations between $x=0.1$ and $x=0.5$. A simple model for the temperature dependence of magnetic anisotropy suggests that the complicated non-monotonous temperature behaviour is mainly due to variations in the band structure as the exchange splitting is reduced by temperature. Using density functional theory based calculations we have explored the effect of substitutional doping the transition metal sublattice by the whole range of 5$d$ transition metals and found that doping by Re or W elements should significantly enhance the magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed in enhancing the magnetic anisotropy due to formation of other phases. On the other hand, doping by Ir and Re was successful and resulted in magnetic anisotropies that are in agreement with theoretical predictions. In particular, doping by 2.5~at.\% of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50\% compared to its parent (Fe$_{0.7}$Co$_{0.3}$)$_2$B compound, making this system interesting, for example, in the context of permanent magnet replacement materials or in other areas where a large magnetic anisotropy is of importance.Comment: 15 pages 17 figure

Fluctuations as probe of the QCD phase transition and freeze-out in heavy ion collisions at LHC and RHIC

We discuss the relevance of higher order moments of net baryon number fluctuations for the analysis of freeze-out and critical conditions in heavy ion collisions at LHC and RHIC. Using properties of O(4) scaling functions, we discuss the generic structure of these higher moments at vanishing baryon chemical potential and apply chiral model calculations to explore their properties at non-zero baryon chemical potential. We show that the ratios of the sixth to second and eighth to second order moments of the net baryon number fluctuations change rapidly in the transition region of the QCD phase diagram. Already at vanishing baryon chemical potential they deviate considerably from the predictions of the hadron resonance gas model which reproduce the second to fourth order moments of the net proton number fluctuations at RHIC. We point out that the sixth order moments of baryon number and electric charge fluctuations remain negative at the chiral transition temperature. Thus, they offer the possibility to probe the proximity of the thermal freeze-out to the crossover line.Comment: 24 pages, 12 EPS files, revised version, to appear in EPJ

Hydrodynamical description of a hadron-quark first-order phase transition

Solutions of hydrodynamical equations are presented for the equation of state of the Var der Waals type allowing for the first order phase transition. Attention is focused on description of the hadron-quark phase transition in heavy ion collisions. It is shown that fluctuations dissolve and grow as if the fluid is effectively very viscous. Even in spinodal region germs are growing slowly due to viscosity and critical slowing down. This prevents enhancement of fluctuations in the near-critical region, which is frequently considered as a signal of the critical point in heavy ion collisions.Comment: 4 pages, 4 figure

1/f noise from correlations between avalanches in self-organized criticality

We show that large, slowly driven systems can evolve to a self-organized critical state where long range temporal correlations between bursts or avalanches produce low frequency $1/f^{\alpha}$ noise. The avalanches can occur instantaneously in the external time scale of the slow drive, and their event statistics are described by power law distributions. A specific example of this behavior is provided by numerical simulations of a deterministic ``sandpile'' model.Comment: Completely revised version: 4 pages (revtex), 3 eps figure

Net-charge probability distributions in heavy ion collisions at chemical freeze-out

We explore net charge probability distributions in heavy ion collisions within the hadron resonance gas model. The distributions for strangeness, electric charge and baryon number are derived. We show that, within this model, net charge probability distributions and the resulting fluctuations can be computed directly from the measured yields of charged and multi-charged hadrons. The influence of multi-charged particles and quantum statistics on the shape of the distribution is examined. We discuss the properties of the net proton distribution along the chemical freeze-out line. The model results presented here can be compared with data at RHIC energies and at the LHC to possibly search for the relation between chemical freeze-out and QCD cross-over lines in heavy ion collisions.Comment: 21 pages, 6 figure