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Title: Chemical freeze-out in relativistic heavy-ion collisions

Abstract

One surprising result in relativistic heavy-ion collisions is that the abundance of various particles measured in experiments is consistent with the picture that they reach chemical equilibrium at a temperature much higher than the temperature they freeze out kinetically. Using a multiphase transport model to study particle production in these collisions, we find, as an example, that the effective pion to nucleon ratio, which includes those from resonance decays, indeed changes very little during the evolution of the hadronic matter from the chemical to the kinetic freeze-out, and it is also accompanied by an almost constant specific entropy. Finally, we further use a hadron resonance gas model to illustrate the results from the transport model study.

Authors:
ORCiD logo [1];  [2]
  1. Chinese Academy of Sciences (CAS), Beijing (China). Shanghai Inst. of Applied Physics
  2. Texas A&M Univ., College Station, TX (United States). Cyclotron Inst. and Dept. of Physics and Astronomy
Publication Date:
Research Org.:
Texas A&M Univ., College Station, TX (United States)
Sponsoring Org.:
USDOE; Welch Foundation; National Natural Science Foundation of China (NNSFC); Chinese Academy of Sciences (CAS)
OSTI Identifier:
1425948
Grant/Contract Number:
SC0015266; 2015CB856904; 2014CB845401; 11475243; 11421505; Y290061011; Y526011011; 15DZ2272100; 13PJ1410600; A-1358
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Physics Letters. Section B
Additional Journal Information:
Journal Volume: 772; Journal Issue: C; Journal ID: ISSN 0370-2693
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
74 ATOMIC AND MOLECULAR PHYSICS

Citation Formats

Xu, Jun, and Ko, Che Ming. Chemical freeze-out in relativistic heavy-ion collisions. United States: N. p., 2017. Web. doi:10.1016/j.physletb.2017.06.061.
Xu, Jun, & Ko, Che Ming. Chemical freeze-out in relativistic heavy-ion collisions. United States. doi:10.1016/j.physletb.2017.06.061.
Xu, Jun, and Ko, Che Ming. Mon . "Chemical freeze-out in relativistic heavy-ion collisions". United States. doi:10.1016/j.physletb.2017.06.061. https://www.osti.gov/servlets/purl/1425948.
@article{osti_1425948,
title = {Chemical freeze-out in relativistic heavy-ion collisions},
author = {Xu, Jun and Ko, Che Ming},
abstractNote = {One surprising result in relativistic heavy-ion collisions is that the abundance of various particles measured in experiments is consistent with the picture that they reach chemical equilibrium at a temperature much higher than the temperature they freeze out kinetically. Using a multiphase transport model to study particle production in these collisions, we find, as an example, that the effective pion to nucleon ratio, which includes those from resonance decays, indeed changes very little during the evolution of the hadronic matter from the chemical to the kinetic freeze-out, and it is also accompanied by an almost constant specific entropy. Finally, we further use a hadron resonance gas model to illustrate the results from the transport model study.},
doi = {10.1016/j.physletb.2017.06.061},
journal = {Physics Letters. Section B},
number = C,
volume = 772,
place = {United States},
year = {Mon Jun 26 00:00:00 EDT 2017},
month = {Mon Jun 26 00:00:00 EDT 2017}
}

Journal Article:
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  • On the basis of a nine-parameter expanding source model that includes special relativity, quantum statistics, resonance decays, and freeze-out on a realistic hypersurface in spacetime, we analyze in detail invariant {pi}{sup +}, {pi}{sup {minus}}, K{sup +}, and K{sup {minus}} one-particle multiplicity distributions and {pi}{sup +} and K{sup +} two-particle correlations in nearly central collisions of Si+Au at p{sub lab}/A=14.6thinspGeV/c. By considering separately the one-particle data and the correlation data, we find that the central baryon density, nuclear temperature, transverse collective velocity, longitudinal collective velocity, and source velocity are determined primarily by one-particle multiplicity distributions and that the transverse radius, longitudinalmore » proper time, width in proper time, and pion incoherence fraction are determined primarily by two-particle correlations. By considering separately the pion data and the kaon data, we find that although the pion freeze-out occurs somewhat later than the kaon freeze-out, the 99{percent} confidence-level error bars associated with the two freeze-outs overlap. By constraining the transverse freeze-out to the same source time for all points with the same longitudinal position and by allowing a more flexible freeze-out in the longitudinal direction, we find that the precise shape of the freeze-out hypersurface is relatively unimportant. By regarding the pion and kaon one-particle data to be unnormalized, we find that the nuclear temperature increases slightly, but that its uncertainty increases substantially. By including proton one-particle data (which are contaminated by spectator protons), we find that the nuclear temperature increases slightly. These detailed studies confirm our earlier conclusion based on the simultaneous consideration of the pion and kaon one-particle and correlation data that the freeze-out temperature is less than 100 MeV and that both the longitudinal and transverse collective velocities{emdash}which are anticorrelated with the temperature{emdash}are substantial. We also discuss the flaws in several previous analyses that yielded a much higher freeze-out temperature of approximately 140 MeV for both this reaction and other reactions involving heavier projectiles and/or higher bombarding energies. thinsp {copyright} {ital 1998} {ital The American Physical Society}« less
  • Identified charged pion, kaon, and proton spectra are used to explore the system size dependence of bulk freeze-out properties in Cu+Cu collisions at {radical}s{sub NN} = 200 and 62.4 GeV. The data are studied with hydrodynamically motivated blast-wave and statistical model frameworks in order to characterize the freeze-out properties of the system. The dependence of freeze-out parameters on beam energy and collision centrality is discussed. Using the existing results from Au + Au and pp collisions, the dependence of freeze-out parameters on the system size is also explored. This multidimensional systematic study furthers our understanding of the QCD phase diagrammore » revealing the importance of the initial geometrical overlap of the colliding ions. The analysis of Cu+Cu collisions expands the system size dependence studies from Au+Au data with detailed measurements in the smaller system. The systematic trends of the bulk freeze-out properties of charged particles is studied with respect to the total charged particle multiplicity at midrapidity, exploring the influence of initial state effects.« less
  • A quantum chromodynamics phase transition may reflect in a inhomogeneous decoupling surface of hadrons produced in relativistic heavy-ion collisions. We show that because of the nonlinear dependence of the particle densities on the temperature and baryon-chemical potential such inhomogeneities should be visible even in the integrated, inclusive abundances. We analyze experimental data from Pb+Pb collisions at the Super Proton Synchrotron at Centre Europeen de Recherches Nucleaires and Au+Au collisions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory to determine the amplitude of inhomogeneities.
  • An extension of the single-freeze-out model with thermal and geometric parameters dependent on the spatial rapidity, {alpha}{sub parallel}, is used to describe the rapidity and transverse-momentum spectra of pions, kaons, protons, and antiprotons measured at the Relativistic Heavy Ion Collider at {radical}(s{sub NN})=200 GeV by the BRAHMS Collaboration. THERMINATOR is used to perform the necessary simulation, which includes all resonance decays. The result of the fit to the rapidity spectra in the range of the BRAHMS data is the expected growth of the baryon and strange chemical potentials with the magnitude of {alpha}{sub parallel}, whereas the freeze-out temperature is keptmore » fixed. The value of the baryon chemical potential at {alpha}{sub parallel}{approx}3, which is the relevant region for particles detected at the BRAHMS forward rapidity y{approx}3, is about 200 GeV, i.e., lies in the range of the values obtained for the highest SPS energy. The chosen geometry of the fireball has a decreasing transverse size as the magnitude of {alpha}{sub parallel} is increased, which also corresponds to decreasing transverse flow. This feature is verified by reproducing the transverse momentum spectra of pions and kaons at various rapidities. The strange chemical potential obtained from the fit to the K{sup +}/K{sup -} ratio is such that the local strangeness density in the fireball is compatible with zero. The resulting rapidity spectra of net protons are described qualitatively in the model. As a result of the study, the knowledge of the 'topography' of the fireball is achieved, making other calculations possible. As an example, we give predictions for the rapidity spectra of hyperons.« less
  • We introduce a new scenario for heavy ion collisions that could solve the lingering problems associated with the so-called Hanbury Brown-Twiss (HBT) puzzle. We postulate that the system starts expansion as the perfect quark-gluon fluid but close to freeze-out it splits into clusters, due to a sharp rise of bulk viscosity in the vicinity of the hadronization transition. We then argue that the characteristic cluster size is determined by the viscosity coefficient and the expansion rate. Typically it is much smaller and at most weakly dependent of the total system volume (hence reaction energy and multiplicity). These clusters maintain themore » pre-existing outward-going flow, as a spray of droplets, but develop no flow of their own, and hadronize by evaporation. We provide an ansatz for converting the hydrodynamic output into clusters.« less