Thermal Freeze-out Conditions Research Articles

Study of Pion Transverse Momentum Distributions in Ultra-Relativistic Heavy Ion Collisions: A USTFM Approach

The mid-rapidity (|y| < 0.5) transverse momentum distributions of identified charged pions produced in different collision systems at different centre-of-mass energies ( $${{\sqrt S }_{{NN}}}$$ ) ranging from RHIC to LHC have been studied using our early proposed unified statistical thermal freeze-out model (USTFM). We have also studied the effect of different collision centralities on the mid-rapidity (|y| < 0.5) production of pions. Our model results are found to be in good agreement with the experimental data. Different thermal freeze-out conditions which include transverse flow, thermal freeze-out temperature, and velocity profile have been extracted by providing the best fit to the experimental data points. We have incorporated the effects of transverse as well as longitudinal flow in this analysis. The effects of heavier decay resonances are also taken into account. Moreover, the strangeness conservation criteria have also been imposed in this analysis of pion distributions. The calculated freeze-out parameters indicates that the freeze-out temperature increases with decreasing centrality, while the transverse flow velocity increases by increase in the centrality of the colliding system.

Particle Transverse Momentum Distributions in p-p Collisions at √sNN=0.9 TeV

The midrapidity transverse momentum spectra of hadrons (p, K+, Ks0, ϕ, Λ, and (Ξ-+Ξ--)) and the available rapidity distributions of the strange hadrons (Ks0, (Λ+Λ-), (Ξ-+Ξ--)) produced in p-p collisions at LHC energy √sNN = 0.9 TeV have been studied using a Unified Statistical Thermal Freeze-out Model (USTFM). The calculated results are found to be in good agreement with the experimental data. The theoretical fits of the transverse momentum spectra using the model calculations provide the thermal freeze-out conditions in terms of the temperature and collective flow parameters for different hadronic species. The study reveals the presence of a significant collective flow and a well-defined temperature in the system thus indicating the formation of a thermally equilibrated hydrodynamic system in p-p collisions at LHC. Moreover, the fits to the available experimental rapidity distributions data of strange hadrons show the effect of almost complete transparency in p-p collisions at LHC. The model incorporates longitudinal as well as a transverse hydrodynamic flow. The contributions from heavier decay resonances have also been taken into account. We have also imposed the criteria of exact strangeness conservation in the system.

Centrality dependence of thermal freeze-out conditions at LHC in Pb+Pb collisions at sNN = 2.76TeV

We have analyzed the available midrapidity [Formula: see text] transverse momentum spectra of identified particles such as protons [Formula: see text], kaons [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] for different centralities of Pb+Pb collisions at the LHC energy [Formula: see text]. We have used our earlier proposed unified statistical thermal freeze-out model. The model incorporates the effect of nuclear transparency in such energetic collisions and the resulting asymmetry in the collective-flow profile along the longitudinal and the transverse directions. Our calculated results are found to be in good agreement with the experimental data measured by the ALICE experiment. The model calculation fits the experimental data for different particle species which provide thermal freeze-out conditions in terms of temperature and collective-flow parameters. The analysis shows a rise in the thermal freeze-out temperature and a mild decrease in the transverse collective-flow velocity as we go from central to the peripheral collisions. The baryon chemical potential is assumed to be nearly zero for the bulk of the matter [Formula: see text], a situation expected in the heavy ion collisions at LHC energies in the Bjorken approach owing to nearly complete nuclear transparency. The contributions from the decay of the heavier resonances are also taken into account in our calculations.

Transverse momentum distributions of strange hadrons produced in p–p collisions at √s NN = 200 GeV

The mid-rapidity transverse momentum spectra of strange hadrons (\(K^ + ,K^ - ,K^ - /K^ + ,\Lambda ,\bar \Lambda ,\Xi ^ - ,\Xi ^ + \), and Ω) produced in p–p collisions at the highest RHIC energy √sNN = 200 GeV have been studied using a statistical unified thermal freeze-out model. The calculated results are found to be in good agreement with the experimental data taken from STAR and BRAHMS experiments. The fits of the transverse momentum spectra to the model calculations provide the thermal freeze-out conditions in terms of the temperature and collective flow effect parameters for different particle species. The model incorporates a longitudinal and a transverse hydrodynamic flow. The rapidity distributions of kaons and their ratios are also reproduced successfully, which reveals the presence of partial nuclear transparency effects in p–p collisions at √sNN = 200 GeV. The contributions from heavier decay resonances are also taken into account.

Transverse Momentum Distributions of Hadrons Produced in Pb-Pb Collisions at LHC EnergysNN=2.76TeV

The transverse momentum spectra of several types of hadrons,p,p̅,K+,K-,Ks0,Λ,Ω,Ω̅,Ξ-, andΞ̅produced in most central Pb-Pb collisions at LHC energysNN=2.76 TeV have been studied at midrapidity (|y|&lt;0.5) using an earlier proposed unified statistical thermal freeze-out model. The calculated results are found to be in good agreement with the experimental data measured by the ALICE experiment at LHC. The model calculation fits provide the thermal freeze-out conditions in terms of the temperature and collective flow effect parameters for different particle species. Interestingly the model parameter fits to the experimental data reveal stronger collective flow in the system and lesser freeze-out temperatures of the different particle species as compared to Au-Au collisions at RHIC. The strong increase of the collective flow appears to be a consequence of the increasing particle density at LHC. The model used incorporates a longitudinal as well as transverse hydrodynamic flow. The chemical potential has been assumed to be nearly equal to zero for the bulk of the matter owing to high degree of nuclear transparency effect at such collision energies. The contributions from heavier decay resonances are also taken into account.

Relativistic hydrodynamics with strangeness production

The relativistic hydrodynamic approach is used to describe the production of strangeness and/or heavy quarks in ultrarelativistic heavy ion reactions. Production processes are important ingredients of dissipative effects in the hadronic liquid. Beyond viscosity also chemo- and thermo-diffusion processes are considered. This also allows us to specify chemical and thermal freeze-out conditions.

Strange Baryon Resonance Production insNN=200 GeVp+pandAu+AuCollisions

We report the measurements of Σ(1385) and Λ(1520) production in p+p and Au+Au collisions at sNN=200 GeV from the STAR Collaboration. The yields and the pT spectra are presented and discussed in terms of chemical and thermal freeze-out conditions and compared to model predictions. Thermal and microscopic models do not adequately describe the yields of all the resonances produced in central Au+Au collisions. Our results indicate that there may be a time span between chemical and thermal freeze-out during which elastic hadronic interactions occur.Received 27 April 2006DOI:https://doi.org/10.1103/PhysRevLett.97.132301©2006 American Physical Society

What do We Learn from Resonance Production in Heavy Ion Collisions?

The resonance production of $\Delta$, K(892), $\Sigma$(1385), $\Lambda$(1520) and $\phi$ from elementary p+p and Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200 GeV from the STAR experiment at RHIC is presented. Yields and spectra are discussed in terms of chemical and thermal freeze-out conditions. Thermal models do not describe sufficiently the yields of the resonance production in central Au+Au collisions. The approach which includes pseudo-elastic and elastic hadronic interactions after chemical freeze-out until thermal freeze-out suggests a time span of $\Delta \tau>$5 fm/c.

Strange resonance production in p + p and Au + Au collisions at RHIC energies

Resonance yields and spectra from elementary p+p and Au+Au collisions at from the STAR experiment at RHIC are presented and discussed in terms of chemical and thermal freeze-out conditions. Thermal models do not adequately describe the yields of the resonance production in central Au+Au collisions. The approach to include elastic hadronic interactions between chemical freeze-out and thermal freeze-out suggests a time of Δτ> 5 fm/c.

Probing thermal freeze-out conditions via multi-strange baryons in Au+Au collisions at

We report preliminary STAR results on multi-strange baryon transverse momentum distributions from Au+Au collisions at . No apparent increase of the average transverse momentum ⟨p⊥⟩ for Ξ baryons is observed over the measured centrality range. In addition, for the most central collisions the spectra indicate that these multi-strange particles are emitted from a thermalized system and experience a significant transverse flow effect but that the system is hotter and the flow is smaller than that encountered by the π, K, p and Λ.

Search for QGP and thermal freeze-out of strange hadrons

After reviewing the observables of QGP we perform an analysisof m⊥ spectra of strange hadrons measured as functionof centrality in 156 A GeV Pb-Pb interactions. We show that there is a good agreement between the chemical and thermalfreeze-out conditions, providing additional evidence for the formation and sudden disintegration of a supercooled QGP fireball.