Initial and Final State Temperatures of Antiproton Emission Sources in High Energy Collisions

The momentum or transverse momentum spectra of antiprotons produced at mid-rapidity in proton-helium (p+He), gold-gold (Au+Au), deuton-gold (d+Au), and lead-lead (Pb+Pb) collisions over an energy range from a few GeV to a few TeV are analyzed by the Erlang distribution, the inverse power-law (the Hagedorn function), and the blast-wave fit, or the superposition of two-component step function. The excitation functions of parameters such as the mean transverse momentum, initial state temperature, kinetic freeze-out temperature, and transverse flow velocity increase (slightly) from a few GeV to a few TeV and from peripheral to central collisions. At high energy and in central collisions, large collision energy is deposited in the system, which results in high degrees of excitation and expansion.

     

Transverse momentum spectra of the produced hadrons at SPS energy and a random walk model

The transverse momentum spectra of the produced hadrons have been compared to a model, which is based on the assumption that a nucleus–nucleus collision is a superposition of isotropically decaying thermal sources at a given freeze-out temperature. The freeze-out temperature in nucleus–nucleus collisions is fixed from the inverse slope of the transverse momentum spectra of hadrons in nucleon–nucleon collision. The successive collisions in thessss nuclear reactions leads to gain in transverse momentum, as the nucleons propagate in the nucleus following a random walk pattern. The average transverse rapidity shift per collision is determined from the nucleon–nucleus collision data. Using this information, we obtain parameter-free result for the transverse momentum distribution of produced hadrons in nucleus–nucleus collisions. It is observed that such a model is able to explain the transverse mass spectra of the produced pions at SPS energies. However, it fails to satisfactorily explain the transverse mass spectra of kaons and protons. This indicates the presence of collective effect which cannot be accounted for, by the initial state collision broadening of transverse momentum of produced hadrons, the basis of random walk model.     

Hadron spectra from nuclear collisions

We describe high energy nuclear collisions by a superposition of isotropically decaying thermal sources (“fireballs”) of freeze-out temperature T = 0.15 GeV. The longitudinal fireball superposition is taken as boost-invariant, in a rapidity range determined by the average energy loss of nucleons in p?p collisions. The transverse fireball motion is assumed to be due to random walk initial state collisions; it is determined by p?A data and then extrapolated to central A?B interactions. We thus obtain parameter-free predictions for the rapidity and transverse momentum spectra of hadrons produced in high energy nucleus-nucleus collisions. The results account fully for the observed broadening of transverse momentum distributions, so that single-particle spectra require neither collective flow nor temperature increase.     

Jozsó's Legacy: Chemical and kinetic freeze-out in heavy-ion collisions

We review J. Zimányi's key contributions to the theoretical understanding of dynamical freeze-out in nuclear collisions and their subsequent applications to ultra-relativistic heavy-ion collisions, leading to the discovery of a freeze-out hierarchy where chemical freeze-out of hadron yields precedes the thermal decoupling of their momentum spectra. Following Zimányi's lines of reasoning we show that kinetic freeze-out necessarily leads to a dependence of the corresponding freeze-out temperature on collision centrality. This centrality dependence can be predicted within hydrodynamic models, and for Au+Au collisions at RHIC this prediction is shown to reproduce the experimentally observed centrality dependence of the thermal decoupling temperature, extracted from hadron momentum spectra. The fact that no such centrality dependence is observed for the chemical decoupling temperature, extracted from the hadron yields measured in these collisions, excludes a similar kinetic interpretation of the chemical decoupling process. We argue that the chemical decoupling data from Au+Au collisions at RHIC can only be consistently understood if the chemical freeze-out process is driven by a phase transition, and that the measured chemical decoupling temperature therefore measures the critical temperature of the quark-hadron phase transition. We propose additional experiments to further test this interpretation.     

Universal scaling of kinetic freeze-out parameters across different collision systems at LHC energies

In this study, we perform Tsallis Blast-Wave analysis on the transverse momentum spectra of identified hadrons produced in a wide range of collision systems at the Large Hadron Collider (LHC) including pp, pPb, XeXe, and PbPb collisions. The kinetic freeze-out properties varying with event multiplicity are investigated across these systems. We find that the extracted kinetic freeze-out temperature, radial flow velocity, and non-extensive parameter exhibit a universal scaling behavior for these systems with very different geometric sizes, especially when the independent baryon Tsallis non-extensive parameter is considered. This universality may indicate the existence of a unified partonic evolution stage in different collision systems at the LHC energies.     

Centrality dependence of charged hadron transverse momentum spectra in Au+Au collisions from sqrt[s(NN)]=62.4 to 200 GeV

We have measured transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt[s(NN)]=62.4 GeV. The spectra are presented for transverse momenta 0.25相似文献   

System size and centrality dependence of charged hadron transverse momentum spectra in Au + Au and Cu + Cu collisions at square root of SNN = 62.4 and 200 GeV

We present transverse momentum distributions of charged hadrons produced in Cu + Cu collisions at square root of SNN = 62.4 and 200 GeV. The spectra are measured for transverse momenta of 0.25 < pT < 5.0 GeV/c at square root of SNN = 62.4 GeV and 0.25 < pT < 7.0 GeV/c at square root of SNN = 200 GeV, in a pseudorapidity range of 0.2 < eta < 1.4. The nuclear modification factor R(AA) is calculated relative to p + p data at both collision energies as a function of collision centrality. At a given collision energy and fractional cross section, R(AA) is observed to be systematically larger in Cu + Cu collisions compared to Au + Au. However, for the same number of participating nucleons, R(AA) is essentially the same in both systems over the measured range of pT, in spite of the significantly different geometries of the Cu + Cu and Au + Au systems.     

Low mass dimuons produced in relativistic nuclear collisions

The NA60 experiment has measured low mass muon pair production in In-In collisions at 158A GeV with unprecedented precision. We show that these data are reproduced very well by a dynamical model with parameters scaled from fits to measurements of hadronic transverse mass spectra and Hanbury Brown-Twiss correlations in Pb-Pb and Pb-Au collisions at the same energy. The data are consistent with in-medium properties of rho and omega mesons at finite temperature and density as deduced from empirical forward-scattering amplitudes. Inclusion of the vacuum decay of the rho meson after freeze-out is necessary for an understanding of the mass and transverse momentum spectrum of dimuons with M less similar 0.9 GeV/c(2).     

Nuclear modification factors for hadrons at forward and backward rapidities in deuteron-gold collisions at sqrt[s(NN)]=200 GeV

We report on charged hadron production in deuteron-gold reactions at sqrt[s(NN)]=200 GeV. Our measurements in the deuteron direction cover 1.4相似文献   

Centrality dependence of pi(+/-), K(+/-), p,and (-)p production from sqrt[s(NN)] = 130 GeV Au + Au collisions at RHIC

Identified pi(+/-), K(+/-), p, and (-)p transverse momentum spectra at midrapidity in sqrt[s(NN)] = 130 GeV Au+Au collisions were measured by the PHENIX experiment at RHIC as a function of collision centrality. Average transverse momenta increase with the number of participating nucleons in a similar way for all particle species. Within errors, all midrapidity particle yields per participant are found to be increasing with the number of participating nucleons. There is an indication that K(+/-), p, and (-)p yields per participant increase faster than the pi(+/-) yields. In central collisions at high transverse momenta (p(T) > or =2 GeV/c), (-)p and p yields are comparable to the pi(+/-) yields.     

Hadronization in heavy-ion collisions: recombination and fragmentation of partons

We argue that the emission of hadrons with transverse momentum up to about 5 GeV/c in central relativistic heavy ion collisions is dominated by recombination, rather than fragmentation of partons. This mechanism provides a natural explanation for the observed constant baryon-to-meson ratio of about one and the apparent lack of a nuclear suppression of the baryon yield in this momentum range. Fragmentation becomes dominant at higher transverse momentum, but the transition point is delayed by the energy loss of fast partons in dense matter.     

Covariant Non-Equilibrium Transport Theory Solutions for RHIC

New numerical solutions of 3+1D covariant kinetic theory are reported for nuclear collisions in the energy domain E_cm200 AGeV. They were obtained using the MPC 0.1.2 parton transport code employing high parton subdivision to retain Lorentz covariance. The solutions are compared to those of relativistic hydrodynamics employing Cooper–Frye isotherm freeze-out. The transport solutions follow a different dynamical path than hydrodynamics due to large dissipative effects when pQCD scattering rates and HIJING initial conditions are assumed. The transport freeze-out four-volume is sensitive to the reaction rates. The final transverse momentum distributions are found to deviate by up to an order of magnitude from those of Cooper–Frye frozen hydrodynamics.     

Highlights from the STAR experiment at RHIC

Ultra-relativistic heavy-ion collisions produced at RHIC differ significantly from a superposition of proton-proton collisions. Evidence of collective expansion has been gathered. The yield of high transverse momentum particles has been found to be lower in head-on Au?Au collisions than is expected by scaling p-p collisions. Di-jet processes, which are frequent in p-p collisions, are almost absent in head-on Au?Au collisions. The current results from RHIC indicate that Au?Au collisions at $\sqrt {S_{NN} } = 130$ GeV and $\sqrt {S_{NN} } = 200$ GeV yield an expanding system that is opaque to high momentum partons.     

Centrality dependence of charm production from a measurement of single electrons in Au+Au collisions at sqrt[s(NN)]=200 GeV

The PHENIX experiment has measured midrapidity transverse momentum spectra (0.4相似文献   

Reconstruction of hadronization stage in Pb+Pb collisions at 158 A GeV/c

Recent data on hadron multiplicities in central Pb+Pb collisions at 158 A GeV/c at mid-rapidity are analyzed within the concept of chemical freeze-out. A non-uniformity of the baryon chemical potential along the beam axis is taken into account. An approximate analytical solution of the hydrodynamic equations for a chemically frozen Boltzmann-like gas is found. The Cauchy conditions for hydrodynamic evolution of the hadron resonance gas are fixed at the thermal freeze-out hypersurface from analysis of one-particle momentum spectra and HBT correlations. The proper time of chemical freeze-out and physical conditions at the hadronization stage, such as energy density and averaged transverse velocity, are found.     

Analysis of Δ<Superscript>0</Superscript>(1232) production in collisions of protons with carbon nuclei at 4.2 GeV/c

The production of Δ⁰(1232)-resonances in p+¹²C collisions at 4.2 GeV/c was analyzed with 4π acceptance. The mass distribution of Δ⁰(1232) was reconstructed using an angular criterion. The fraction of charged π ⁻-mesons coming from Δ⁰(1232) decay was estimated and compared to those obtained in earlier works. The momentum, transverse momentum, kinetic energy, and rapidity distributions as well as invariant cross sections of Δ⁰(1232)-resonances were reconstructed in the laboratory frame. The mean kinematical characteristics of the reconstructed Δ⁰(1232) were compared to those of participant protons in experiment and within some of the models. The freeze-out temperature of Δ⁰(1232) estimated in the present analysis was compared with those obtained using different methods for Δ(1232) produced with other sets of colliding nuclei at various incident energies. The relative number of nucleons excited to Δ⁰ (1232) at freeze-out conditions in p+¹²C collisions was estimated.     

Identified particle distributions in pp and Au+Au collisions at square root of (sNN)=200 GeV

Transverse mass and rapidity distributions for charged pions, charged kaons, protons, and antiprotons are reported for square root of [sNN]=200 GeV pp and Au+Au collisions at Relativistic Heary Ion Collider (RHIC). Chemical and kinetic equilibrium model fits to our data reveal strong radial flow and long duration from chemical to kinetic freeze-out in central Au+Au collisions. The chemical freeze-out temperature appears to be independent of initial conditions at RHIC energies.     

The three-dimensional (2 + 1)-fluid model for relativistic nuclear collisions

A new multi-fluid model is constructed for describing high-energy heavy-ion collisions. It is assumed that two baryonic fluids formed by the projectile and target nucleons produce a third hadronic fluid via inelastic nucleon-nucleon collisions. The production and expansion dynamics of the hadronic fluid are investigated in detail. Two equations of state for this fluid are considered: one corresponding to an ideal gas of pions and resonances and another one corresponding to an interacting hadron gas described by the relativistic mean-field model. The effects of freeze-out and non-zero pion chemical potential are investigated. The rapidity and transverse momentum spectra of secondary pions are compared with the experimental data forS + S collisions at 200 GeV/nucleon.The authors thank J. Schaffner for his most valuable assistance in the application of the mean-field model. The authors are also grateful to H. Sorge and A. Jahns for fruitful discussions. This work was supported by the Gesellschaft für Schwerionenforschung (GSI) and the Bundesministerium für Forschung und Technologie (BMFT).     

Analyzing Transverse Momentum Spectra of Pions,Kaons and Protons in p–p,p–A and A–A Collisions via the Blast-Wave Model with Fluctuations

The transverse momentum spectra of different types of particles, π±, K±, p and p¯, produced at mid-(pseudo)rapidity in different centrality lead–lead (Pb–Pb) collisions at 2.76 TeV; proton–lead (p–Pb) collisions at 5.02 TeV; xenon–xenon (Xe–Xe) collisions at 5.44 TeV; and proton–proton (p–p) collisions at 0.9, 2.76, 5.02, 7 and 13 TeV, were analyzed by the blast-wave model with fluctuations. With the experimental data measured by the ALICE and CMS Collaborations at the Large Hadron Collider (LHC), the kinetic freeze-out temperature, transverse flow velocity and proper time were extracted from fitting the transverse momentum spectra. In nucleus–nucleus (A–A) and proton–nucleus (p–A) collisions, the three parameters decrease with the decrease of event centrality from central to peripheral, indicating higher degrees of excitation, quicker expansion velocities and longer evolution times for central collisions. In p–p collisions, the kinetic freeze-out temperature is nearly invariant with the increase of energy, though the transverse flow velocity and proper time increase slightly, in the considered energy range.     

Unified description of transverse momentum spectrums contributed by soft and hard processes in high-energy nuclear collisions

The transverse momentum spectrums of π ^?, π ⁺, K ^?, K ⁺, \(\bar p\) , and p produced in p-Pb collisions at √s _NN = 502 TeV measured by the CMS Collaboration and in Pb-Pb collisions at √s _NN = 2.76 TeV measured by the ALICE Collaboration are described by a two-component Erlang distribution. The first component corresponds to “soft” excitation process and contributes in the low transverse momentum region, which is contributed by 2–5 partons (sea quarks and gluons) with strong interactions. The second component corresponds to “hard” scattering process and contributes in the high transverse momentum region, which is contributed by 2 partons (valent quarks) with violent head-on collision. Each parton source contributes an exponential transverse momentum spectrum. Both the soft and hard processes result in an Erlang distribution. The transverse momentum spectrums of final-state charged particles are then described by the two-component Erlang distribution. The contribution ratio (30–40%) of the hard process extracted from nuclear collisions at the Large Hadron Collider is consistent with that (17–46%) obtained from other methods.