Unified Statistical Thermal Freeze-out Model 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.

Energy Dependence of Particle Ratios in High Energy Nucleus-Nucleus Collisions: A USTFM Approach

We study the identified particle ratios produced at mid-rapidity (y&lt;0.5) in heavy-ion collisions, along with their correlations with the collision energy. We employ our earlier proposed unified statistical thermal freeze-out model (USTFM), which incorporates the effects of both longitudinal and transverse hydrodynamic flow in the hot hadronic system. A fair agreement seen between the experimental data and our model results confirms that the particle production in these collisions is of statistical nature. The variation of the chemical freeze-out temperature and the baryon chemical potential with respect to collision energies is studied. The chemical freeze-out temperature is found to be almost constant beyond the RHIC energy and is found to be close to the QCD predicted phase-transition temperature suggesting that the chemical freeze-out occurs soon after the hadronization takes place. The vanishing value of chemical potential at LHC indicates very high degree of nuclear transparency in the collision.

Centrality Dependence of and ϕ(1020) Production at LHC**Supported by Council of Scientific and Industrial Research, New Delhi for This Work

We study the centrality dependence of the mid-rapidity yields and transverse momentum distributions of and resonances produced in Pb + Pb collisions at . The mid-rapidity density and the shape of the transverse momentum spectra are well reproduced by an earlier proposed Unified Statistical Thermal Freeze-out Model (USTFM), which incorporates the effects of both longitudinal as well as transverse hydrodynamic flow. The freeze-out properties in terms of kinetic freeze-out temperature and transverse flow velocity parameter are extracted from the model fits to the transverse momentum data provided by the ALICE experiment at the LHC. The kinetic freeze-out temperature is found to increase with decreasing event centrality while the transverse flow velocity parameter shows a mild decrease on moving towards peripheral collisions. Moreover the centrality dependence of the mid-rapidity system size at freeze-out has also been studied in terms of transverse radius parameter.

Centrality dependence of particle yields and their ratios at RHIC experiments

The collision centrality dependence of the yields per unit rapidity dN/dy along with their ratios for various hadrons produced in Au+Au collisions at different collision energies have been studied within the framework of unified statistical thermal freeze-out model (USTFM) taking into account both longitudinal and transverse hydrodynamic flows. Bulk freeze-out properties in terms of the thermal parameters, temperature and mid-rapidity baryon chemical potential at chemical freeze-out, obtained within the proposed model, which are in agreement with experimental data. The extracted chemical freeze-out temperature is found to depend weakly on the collision centrality. It is also found that this temperature is almost independent of the collision energies considered in this work. The closeness of the freeze-out temperature to the predicted phase-transition temperature suggests that the chemical freeze-out happens near hadronization. Furthermore, the dependence of the mid-rapidity chemical potential on the collision energy at different centralities, as well as the centrality dependence of the mid-rapidity size of the system in terms of the transverse size of the system, has been studied at the RHIC. The effect of resonance decay contributions has also been taken into account.

Centrality and system size dependence of φ meson production at RHIC

We study the centrality and system size dependence of φ meson production by reproducing its transverse-momentum distributions and mid-rapidity distributions (dN/dy) for various collision systems of Au+Au, Cu+Cu, d+Au and p+p at various RHIC energies of , 130 GeV and 200 GeV by using Unified Statistical Thermal Freeze-out Model (USTFM). The calculated results are found to be in good agreement with the experimental data points taken from the STAR experiment. Freeze-out conditions in terms of transverse flow velocity and thermal freeze-out temperature are extracted from the fits of transverse-momentum spectra of the particles at different centrality classes and for different collision systems. The transverse size of the system in terms of transverse-radius parameter is determined from the fits of rapidity distributions at various centrality classes and for various collision systems.

Kinetic freeze-out spectra of identified particles produced in p–Pb collisions at $$\sqrt {s_{NN} }$$ s N N = 5.02 TeV

We study the transverse momentum spectra of identified pions (π– + π+), kaons ((K – + K +), K 0 ), protons (p + p) and lambda hyperons (Λ + Λ) produced at mid-rapidity (0 < y cm < 0.5) in most central (0‒5)% p–Pb collisions at $$\sqrt {s_{NN} }$$ = 5.02 TeV in comparison with a Unified Statistical Thermal Freeze-out Model (USTFM). The measurements for pions are reported upto p T = 3 GeV, the kaons (K – + K +) are reported upto p T = 2.5 GeV, K 0 is reported upto p T = 7 GeV, and the baryons (protons and lambda hyperons) are reported upto p T = 3.5 GeV. A good agreement is seen between the calculated results and the experimental data points taken from the ALICE experiment. The transverse momentum spectra are found to be flatter for heavy particles than for light particles. Bulk freeze-out properties in terms of kinetic freeze-out temperature and the transverse collective flow velocity are extracted from the fits of the transverse momentum spectra of these hadrons. The effect of resonance decay contributions has also been taken care of.

Chemical potential dependence of particle ratios within a unified thermal approach

A unified statistical thermal freeze-out model (USTFM) is used to study the chemical potential dependence of identified particle ratios at mid-rapidity in heavy-ion collisions. We successfully reproduce the experimental data ranging from SPS energies to LHC energies, suggesting the statistical nature of the particle production in these collisions and hence the validity of our approach. The behavior of the freeze-out temperature is studied with respect to chemical potential. The freeze-out temperature is found to be universal at the RHIC and LHC and is close to the QCD predicted phase transition temperature, suggesting that the chemical freeze-out occurs soon after the hadronization takes place.

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.

Identified charged particle distributions in Au + Au collisions at s NN = 9.2GeV

Midrapidity results on multiplicity density [Formula: see text] and transverse momentum distributions [Formula: see text] of pions, kaons and protons in Au + Au collisions at [Formula: see text] are reproduced by using our earlier proposed Unified Statistical Thermal Freeze-out Model (USTFM). The calculated results are found to be in fairly good agreement with the experimental data points taken from STAR experiment. Freeze-out conditions in terms of a transverse flow velocity parameter and the thermal freeze-out temperature are extracted from the fits of transverse momentum spectra of the particles at different collision centralities. A large value of midrapidity chemical potential reveals the effects of almost complete stopping in the center-of-mass frame of the system produced. The resonance decay contributions are found to have a negligible effect on the transverse momentum spectra of the particles while these are found to be significant for determining the shape of the rapidity spectra.

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.

Unified thermal freeze-out model and its parameters at RHIC

The variation of the parameters (thermal freeze-out temperature, transverse collective flow velocity parameter, distribution width and the nuclear transparency parameters a and b ) of the unified statistical thermal freeze-out model of multi-particle production with the center of mass energy, s NN have been studied at RHIC. We find that the transverse collective flow velocity parameter increases with the increase in s NN . The thermal/hydro-dynamical freeze-out temperature is found to decrease with the increase in s NN . It is found that the freeze-out of doubly and triply strange particles occur at somewhat earlier times compared to non-strange and singly strange hadronic species during the evolution of the fireball at each energy. The value of “ a ” is found to increase with the decrease in energy while as the value of “ b ” is found to increase with the increase in center of mass energy.