Kinetic Freeze-out Temperature Research Articles

Elliptic flow of charged and strange hadrons in PbAu collisions at 158 AGeV/c measured in CERES experiment

Differential elliptic flow of v2(pT ) for π − , K 0 , p and Λ is measured at center- of-mass energy of √ sNN=17.3 GeV near the mid-rapidity region in rather central PbAu collisions collected by the CERES/NA45 experiment at CERN. The proton v2(pT )i s ex- tracted from π + sample and particle ratios measured by NA49 experiment adapted to CERES conditions. The proton v2(pT ) data show a downward swing towards low pT with excursions into negative v2 values which was not observed earlier. The results are com- pared with corresponding measurements performed at NA49 and STAR experiments as well as with theoretical predictions from ideal relativistic hydrodynamics. The obtained results for baryons are below hydrodynamic predictions even at the kinetic freeze-out temperature of T f =160 MeV which needs introducing of a viscous hydrodynamics at the late hadronic phase.

FUGACITY AND REHEATING OF PRIMORDIAL NEUTRINOS

We clarify in a quantitative way the impact that distinct chemical Tc and kinetic Tk freeze-out temperatures have on the reduction of the neutrino fugacity ϒν below equilibrium, i.e. ϒν<1, and the increase of the neutrino temperature Tν via partial reheating. We establish the connection between ϒν and Tk via the modified reheating relation Tν(ϒν)/Tγ, where Tγ is the temperature of the background radiation. Our results demonstrate that one must introduce the chemical nonequilibrium parameter, i.e. the fugacity, ϒν, as an additional standard cosmological model parameter in the evaluation of CMB fluctuations as its value allows measurement of Tk.

Asymptotic Regge trajectories of non-strange mesons

We analyze the asymptotic behavior of Regge trajectories of non-strange mesons. In contrast to an existing belief, it is demonstrated that for the asymptotically linear Regge trajectories the width of heavy hadrons cannot linearly depend on their mass. Using the data on masses and widths of rho_J, omega_J, a_J and f_J mesons for the spin values J \leq 6, we extract the parameters of the asymptotically linear Regge trajectory predicted by the finite width model of quark gluon bags. As it is shown the obtained parameters for the data set B correspond to the cross-over temperature lying in the interval 170.9-175.3 MeV which is consistent with the kinetic freeze-out temperature of early hadronizing particles found in relativistic heavy ion collisions at and above the highest SPS energy.

J/ψproduction and elliptic flow in relativistic heavy-ion collisions

Using the two-component model for charmonium production, which includes contributions from both initial hard nucleon-nucleon scattering and from regeneration in the quark-gluon plasma, we study the nuclear modification factor ${R}_{\mathit{AA}}$ and elliptic flow ${v}_{2}$ of $J/\ensuremath{\psi}$ in relativistic heavy-ion collisions. For the expansion dynamics of produced hot, dense matter, we introduce a schematic fireball model with its transverse acceleration determined from the pressure gradient inside the fireball and azimuthally anisotropic expansion parametrized to reproduce measured ${v}_{2}$ of light hadrons. We assume that light hadrons freeze out at the temperature of 120 MeV while charmonia freeze out at 160 MeV, similar to the kinetic and chemical freeze-out temperatures in the statistical model, respectively. For the properties of charmonia in the quark-gluon plasma, we use the screening mass between their charm and anticharm quarks and their dissociation cross sections given by the perturbative quantum chromodynamical (pQCD) calculations in the leading order and up to the next-to-leading order, respectively. For the relaxation time of charm and anticharm quarks in the quark-gluon plasma, we also use the one calculated in the leading-order pQCD. Modeling the effect of higher-order corrections in pQCD by introducing multiplicative factors to the dissociation cross sections of charmonia and the elastic scattering cross sections of charm and anticharm quarks, we find that this effect is small for the ${R}_{\mathit{AA}}$ of $J/\ensuremath{\psi}$ as they suppress the number of initially produced $J/\ensuremath{\psi}$ but enhance the number of regenerated ones. The higher-order corrections increase, however, the ${v}_{2}$ of $J/\ensuremath{\psi}$. Our results suggest that the ${v}_{2}$ of $J/\ensuremath{\psi}$ can play an important role in discriminating between $J/\ensuremath{\psi}$ production from initial hard collisions and from regeneration in the quark-gluon plasma.

Charmonium production and elliptic flow in relativistic heavy-ion collisions

Using the two-component model for charmonium production, which includes contributions from both initial hard nucleon-nucleon scattering and from regeneration in the quark-gluon plasma, we have studied the nuclear modification factor RAA and elliptic flow v2 of J= in relativistic heavy-ion collisions. For the expansion dynamics of produced hot dense matter, we have introduced a schematic fire-cylinder model with its transverse acceleration determined from the pressure gradient inside the fire-cylinder and azimuthally anisotropic expansion parameterized to reproduce measuredv2 of light hadrons. We have assumed that light hadrons freeze out at the temperature of 120 MeV while charmonia at 160 MeV, similar to the kinetic and chemical freeze-out temperatures in the statistical model, respectively. For the properties of charmonia in the quark-gluon plasma, we have used the screening mass between their charm and anticharm quarks and their dissociation cross sections given by the perturbative QCD (pQCD) in the leading order and up to the next-to-leading order, respectively. For the relaxation time of charm and anticharm quarks in the quark-gluon plasma, we have also used the one calculated in the leading order pQCD. Modeling the effect of higher-order corrections in pQCD by introducing multiplicative factors to the dissociation cross sections of charmonia and the elastic scattering cross sections of charm and anticharm quarks, we have found that this effect is small for the RAA of J= as they suppress the number of initially produced J= but enhance the number of regenerated ones. The higher-order corrections increase, however, the v2 of J= . Our results thus suggest that thev2 of J= can play an important role in discriminating between J= production from initial hard collisions and from regeneration in the quark-gluon plasma.

Antiproton evolution in little bangs and in the Big Bang

The abundances of antiprotons and protons are considered within momentum-integrated Boltzmann equations describing Little Bangs, i.e., fireballs created in relativistic heavy-ion collisions. Despite a large annihilation cross section we find a small drop of the ratio of antiprotons to protons from 170 MeV (chemical freeze-out temperature) to 100 MeV (kinetic freeze-out temperature) for CERN-SPS and BNL-RHIC energies thus corroborating the solution of the previously exposed antiproton puzzle. In contrast, the Big Bang evolves so slowly that the antibaryons are kept for a long time in equilibrium resulting in an exceedingly small fraction. The adiabatic path of cosmic matter in the phase diagram of strongly interacting matter is mapped out.

Systematic measurements of identified particle spectra inpp,d+Au, andAu+Aucollisions at the STAR detector

Identified charged-particle spectra of pi(+/-), K(+/-), p, and (p) over bar at midrapidity (vertical bar y vertical bar < 0.1) measured by the dE/dx method in the STAR (solenoidal tracker at the BNL Relativistic Heavy Ion Collider) time projection chamber are reported for pp and d + Au collisions at root s(NN) = 200 GeV and for Au + Au collisions at 62.4, 130, and 200 GeV. Average transverse momenta, total particle production, particle yield ratios, strangeness, and baryon production rates are investigated as a function of the collision system and centrality. The transverse momentum spectra are found to be flatter for heavy particles than for light particles in all collision systems; the effect is more prominent for more central collisions. The extracted average transverse momentum of each particle species follows a trend determined by the total charged-particle multiplicity density. The Bjorken energy density estimate is at least several GeV/fm(3) for a formation time less than 1 fm/c. A significantly larger net-baryon density and a stronger increase of the net-baryon density with centrality are found in Au + Au collisions at 62.4 GeV than at the two higher energies. Antibaryon production relative to total particle multiplicity is found to be constant over centrality, but increases with the collision energy. Strangeness production relative to total particle multiplicity is similar at the three measured RHIC energies. Relative strangeness production increases quickly with centrality in peripheral Au + Au collisions, to a value about 50% above the pp value, and remains rather constant in more central collisions. Bulk freeze-out properties are extracted from thermal equilibrium model and hydrodynamics-motivated blast-wave model fits to the data. Resonance decays are found to have little effect on the extracted kinetic freeze-out parameters because of the transverse momentum range of our measurements. The extracted chemical freeze-out temperature is constant, independent of collision system or centrality; its value is close to the predicted phase-transition temperature, suggesting that chemical freeze-out happens in the vicinity of hadronization and the chemical freeze-out temperature is universal despite the vastly different initial conditions in the collision systems. The extracted kinetic freeze-out temperature, while similar to the chemical freeze-out temperature in pp, d + Au, and peripheral Au + Au collisions, drops significantly with centrality in Au + Au collisions, whereas the extracted transverse radial flow velocity increases rapidly with centrality. There appears to be a prolonged period of particle elastic scatterings from chemical to kinetic freeze-out in central Au + Au collisions. The bulk properties extracted at chemical and kinetic freeze-out are observed to evolve smoothly over the measured energy range, collision systems, and collision centralities.

Response of Charged Hadrons to a Coulomb Potential in Heavy-Ion Collisions at the RHIC

Response of Charged Hadrons to a Coulomb Potential in Heavy-Ion Collisions at the RHIC Byungsik Hong Department of Physics, Korea University, Seoul 136-701 (Received 27 August 2007, in nal form 19 November 2007) We have analyzed the charged hadron spectra at the Relativistic Heavy Ion Collider (RHIC) in terms of the Coulomb interaction between the hadrons and the thermally expanding reball. Fitting the pion, kaon, and proton spectra simultaneously by using the boost-invariant thermal blast wave formula gives a common kinetic freeze-out temperature of 130 MeV and an expansion velocity of 0.67c, which are quite di erent from the previous estimates. Considering the Coulomb interaction, the yield ratios of negative to positive hadrons are expected to increase signi cantly in the low-momentum region. For the published hadron spectra at the RHIC, we determine the upper limit of the magnitude for the Coulomb potential as 10 MeV. Such a relatively small value of the Coulomb potential is consistent with the large nuclear transparency observed at the RHIC. PACS numbers: 25.75.Dw, 25.75.Ld

Freeze-out Dynamics at RHIC

Investigation of the final hadronic state properties of ultra-relativistics pp and Au+Au collisions supplies information on freeze-out conditions at RHIC and possible insights into early stages of these collisions. A variety of particle spectra measured by STAR are studied within the framework of chemical and local kinetic equlibrium models. Here we present the extracted chemical and final kinetic freeze-out temperatures, strangeness saturation factor, final collective flow velocity, and the inferred flow velocity at chemical freeze-out. In light of those measurements we disscuss dynamical evoution of the collission system.

Hydrodynamical evolution near the QCD critical end point

Hydrodynamical calculations have been successful in describing global observables in ultrarelativistic heavy ion collisions, which aim to observe the production of the quark-gluon plasma. On the other hand, recently, a lot of evidence that there exists a critical end point (CEP) in the QCD phase diagram has been accumulating. Nevertheless, so far, no equation of state with the CEP has been employed in hydrodynamical calculations. In this paper, we construct the equation of state with the CEP on the basis of the universality hypothesis and show that the CEP acts as an attractor of isentropic trajectories. We also consider the time evolution in the case with the CEP and discuss how the CEP affects the final state observables, such as the correlation length, fluctuation, chemical freezeout, kinetic freezeout, and so on. Finally, we argue that the anomalously low kinetic freezeout temperature at the BNL Relativistic Heavy Ion Collider suggests the possibility of the existence of the CEP.

Kinetic freeze-out and radial flow in 11.6 A GeV Au+Au collisions

We study the kinetic freeze-out conditions of hadrons in Au+Au collisions at 11.6 A GeV/ c using different parametrizations of an expanding thermal fireball. We take into account the available double differential momentum spectra of a variety of particle species, covering a large fraction of the total momentum space. The overall fit to the data is very good and indicates a relatively low kinetic freeze-out temperature of about 90 MeV with an average transverse expansion velocity at midrapidity of about 0.5 c .