Freeze-out Curve Research Articles

The Sixth-Order Cumulants of Net-proton Multiplicity Distributions Measured in STAR at RHIC

Abstract The ratios of different-order cumulants of conserved quantities have become important observables for probing the Quantum Chromodynamics (QCD) phase diagram, due to their sensitivity to the correlation length of the system near the phase transition. It has been predicted that a sixth-order cumulant may change its sign if the freeze-out curve is close to the phase transition temperature. In this paper, preliminary results are presented on the ratios of sixth-order to second-order cumulants ( C 6 / C 2 ) of the net-proton multiplicity distributions at mid-rapidity ( | y | 0.5 ) measured in Au + Au collisions at s N N = 7.7 , 11.5 , 19.6 , 27 , 39 , 62.4 and 200 GeV .

On the Higher Moments of Particle Multiplicity, Chemical Freeze-Out, and QCD Critical Endpoint

We calculate the first six nonnormalized moments of particle multiplicity within the framework of the hadron resonance gas model. In terms of the lower order moments and corresponding correlation functions, general expressions of higher order moments are derived. Thermal evolution of the first four normalized moments and their products (ratios) are studied at different chemical potentials, so that it is possible to evaluate them at chemical freeze-out curve. It is found that a nonmonotonic behaviour reflecting the dynamical fluctuation and strong correlation of particles starts to appear from the normalized third order moment. We introduce novel conditions for describing the chemical freeze-out curve. Although the hadron resonance gas model does not contain any information on the criticality related to the chiral dynamics and singularity in the physical observables, we are able to find out the location of the QCD critical endpoint atμ~350 MeV and temperatureT~162 MeV.

Finite size scaling on the phase diagram of QCD

Abstract In the last few years there has been remarkable progress in the comparison of experimental data on the shape of event-by-event distributions of conserved quantities and lattice thermodynamic predictions based on the grand canonical ensemble. In this talk we discuss how the QCD crossover temperature and the freezeout curve are extracted from the analysis of fluctuations. We report that one can also go further and locate the QCD critical point at µ }~ 2T c. We also list the systematics which must be brought under control in future.

Hadronization and hadronic freeze-out in relativistic nuclear collisions

We analyze hadrochemical freeze-out in central Pb+Pb collisions at CERN SPS energies, employing the hybrid version of the Ultrarelativistic Quantum Molecular Dynamics model, which describes the transition from a hydrodynamic stage to hadrons by the Cooper-Frye mechanism, and matches to a final hadron-resonance cascade. We fit the results both before and after the cascade stage using the Statistical Model, to assess the effect of the cascade phase. We observe a strong effect on antibaryon yields except anti-Omega, resulting in a shift in T and mu_B of the freeze-out curve. We discuss indications of a similar effect in SPS and RHIC data, and propose a method to recover the bulk hadron freeze-out conditions.

Chiral condensate and chemical freeze-out

We consider a chemical freeze-out mechanism which is based on a strong medium dependence of the rates for inelastic flavor-equilibrating collisions based on the delocalization of hadronic wave functions and growing hadronic radii when approaching the chiral restoration. We investigate the role of mesonic (pion) and baryonic (nucleon) fluctuations for melting the chiral condensate in the phase diagram in the (T,mu)-plane. We apply the PNJL model beyond mean-field and present an effective generalization of the chiral perturbation theory result which accounts for the medium dependence of the pion decay constant while preserving the GMOR relation. We demonstrate within a schematic resonance gas model consisting of a variable number of pionic and nucleonic degrees of freedom that within the above model a quantitative explanation of the hadonic freeze-out curve and its phenomenological conditions can be given.

Validity of the hadronic freeze-out curve

We analyze hadrochemical freeze-out in central Pb+Pb collisions at CERN SPS energies, employing the hybrid version of UrQMD which models hadronization from a hydrodynamic phase by the Cooper–Frye mechanism, and matches to a final hadron-resonance cascade. We fit the results both before and after the cascade stage using the statistical hadronization model to assess the effect of the cascade phase. We observe a strong effect on antibaryon yields except anti-Ω, resulting in a shift in T and μB. We discuss the implications for the freeze-out curve.

Hints from Lattice for QCD Critical Point Search

The freeze-out curve in the QCD phase diagram embodies a substantial amount of precise experimental data in heavy ion collisions. We present our lattice QCD results along the freeze-out curve. The variance, skew and kurtosis of the event distribution of baryon number are studied at several energies of interest through Padé resummations. A smooth behaviour is predicted for three ratios of these quantities at current RHIC and future LHC energies. Any deviations from these at the RHIC energy scan would signal the presence of a nearby critical point. Our lattice results on the critical point do show such a behaviour.

THE HAGEDORN TEMPERATURE REVISITED

The Hagedorn temperature, T H is determined from the number of hadronic resonances including all mesons and baryons. This leads to a stable result T H = 174 MeV consistent with the critical and the chemical freeze-out temperatures at zero chemical potential. We use this result to calculate the speed of sound and other thermodynamic quantities in the resonance hadron gas model for a wide range of baryon chemical potentials following the chemical freeze-out curve. We compare some of our results to those obtained previously in other papers.

Lattice QCD predictions for shapes of event distributions along the freezeout curve in heavy-ion collisions

We present lattice QCD results along the freezeout curve of heavy-ion collisions. The variance, skew and kurtosis of the event distribution of baryon number are studied through Padé resummations. We predict smooth behaviour of three ratios of these quantities at current RHIC and future LHC energies. Deviations from this at lower energies signal the presence of a nearby critical point.

Probing freeze-out conditions in heavy ion collisions with moments of charge fluctuations

We calculate the first four moments of baryon number, electric charge and strangeness fluctuations within the hadron resonance gas model. Different moments and their ratios as well as skewness and kurtosis are evaluated on the phenomenologically determined freeze-out curve in the temperature, baryon chemical potential plane. The model results and its predictions as well as relations between different moments are compared with the first data on net proton fluctuations in Au–Au collisions obtained at RHIC by the STAR Collaboration. We find good agreement between the model calculations and experimental results. We also point out that higher order moments should be more sensitive to critical behavior and will also distinguish hadron resonance gas model calculations from results obtained from lattice QCD.