Universal chemical freeze-out as a signature of a phase transition

Heinz, Ulrich; Kestin, Gregory

doi:10.22323/1.029.0038

Cited by 28 publications

(57 citation statements)

References 45 publications

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“…In high-energy collisions the choice of plotting variables-y t vs p t , y vs x p or ξ p , ν vs n part , v 2 vs ∆ρ [2]strongly affects the visibility of important structure and its physical interpretation. Conventional momentumspace variables are y z for longitudinal momentum and p t for transverse momentum.…”

Section: B Plotting Formatsmentioning

confidence: 99%

“…Claims for achievement of the hydro limit [63] and so-called constituent-quark scaling [64,65] have been interpreted as evidence for a thermalized QGP at RHIC. Fig. 14 (left panel) shows per-particle azimuth quadrupole measure ∆ρ [2]/ √ ρ ref on centrality measure ν [26]. The quadrupole amplitude can be obtained from fits to 2D angular autocorrelations [16,24,34], and is related to v 2 by…”

Section: A Pt-integral V2mentioning

confidence: 99%

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Examination of the relevance of hydrodynamics for data measured at the BNL relativistic heavy ion collider

Trainor¹

2010

J. Phys. G: Nucl. Part. Phys.

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The hydrodynamic (hydro) model applied to heavy ion data from the relativistic heavy ion collider (RHIC) in the form of single-particle spectra and correlations seems to indicate that a dense QCD medium nearly opaque to partons, a strongly-coupled quark-gluon plasma (sQGP), is formed in more-central Au-Au collisions, and that the sQGP may have a very small viscosity ("perfect liquid"). Measurements of radial and elliptic flows, with possible coalescence of "constituent quarks" to form hadrons, seem to support the conclusion. However, other measurements provide contradictory evidence. Unbiased angular correlations indicate that a large number of back-to-back jets from initial-state scattered partons with energies as low as 3 GeV survive as "minijet" hadron correlations even in central Au-Au collisions, suggesting near transparency. Two-component analysis of single-particle hadron spectra reveals a corresponding spectrum hard component (parton fragment distribution described by pQCD) which can masquerade as "radial flow" in some spectrum analysis. Reinterpretation of "elliptic flow" as a QCD scattering process resulting in fragmentation is also possible. In this paper I review analysis methods and results in the context of two paradigms: the conventional hydrodynamics/hard-probes paradigm and an alternative quadrupole/minijets paradigm. Based on re-interpretation of fiducial data I argue that hydrodynamics may not be relevant to RHIC collisions. Collision evolution may be dominated by parton scattering and fragmentation, albeit the fragmentation process is strongly modified in more-central A-A collisions.

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Section: B Plotting Formatsmentioning

confidence: 99%

Section: A Pt-integral V2mentioning

confidence: 99%

Examination of the relevance of hydrodynamics for data measured at the BNL relativistic heavy ion collider

Trainor¹

2010

J. Phys. G: Nucl. Part. Phys.

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“…These discrepancies suggest that hadrons are "born" into equilibrium, i.e., the system is already in a chemically frozen out state at the end of the phase transition [7,8].…”

Section: Pacs Numbersmentioning

confidence: 99%

“…It should be noted here that in our calculations we use both the effective number of baryons, in Eq. (8), and pions, in Eq. (6).…”

Section: Pacs Numbersmentioning

confidence: 99%

Fast Equilibration of Hadrons in an Expanding Fireball

2008

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Due to long chemical equilibration times within standard hadronic reactions during the hadron gas phase in relativistic heavy ion collisions it has been suggested that the hadrons are "born" into equilibrium after the quark gluon plasma phase. Here we develop a dynamical scheme in which possible Hagedorn states contribute to fast chemical equilibration times of baryon anti-baryon pairs (as well as kaon anti-kaon pairs) inside a hadron gas and just below the critical temperature. Within this scheme, we use master equations and derive various analytical estimates for the chemical equilibration times. Applying a Bjorken picture to the expanding fireball, the kaons and baryons as well as the bath of pions and Hagedorn resonances can indeed quickly chemically equilibrate for both an initial overpopulation or underpopulation of Hagedorn resonances. Moreover, a comparison of our results to (B +B)/π + and K/π + ratios at RHIC, indeed, shows a close match. PACS numbers:(Anti-)strangeness enhancement was first observed at CERN-SPS energies by comparing anti-hyperons, multistrange baryons, and kaons to pp-data. It was considered a signature for quark gluon plasma (QGP) because, using binary strangeness production and exchange reactions, chemical equilibrium could not be reached within the hadron gas phase [1]. It was then proposed that there exists a strong hint for QGP at SPS because strange quarks can be produced more abundantly by gluon fusion, which would account for strangeness enhancement following hadronization and rescattering of strange quarks. Later, multi-mesonic reactions were used to explain secondary production of anti-protons and antihyperons [2,3]. At SPS they give a typical chemical equilibration time τȲ ≈ 1 − 3 fm c using an annihilation cross section of σ ρȲ ≈ σ ρp ≈ 50mb and a baryon density of ρ B ≈ ρ 0 to 2ρ 0 , which is typical for SPS. Therefore, the time scale is short enough to account for chemical equilibration within a cooling hadronic fireball at SPS.A problem arises when the same multi-mesonic reactions were employed in the hadron gas phase at RHIC temperatures where experiments show that the particle abundances reach chemical equilibration close to the phase transition [4]. At RHIC at T = 170 MeV, where σ ≈ 30mb and ρ eq B ≈ ρ eq B ≈ 0.04fm −3 , the equilibrium rate for (anti-)baryon production is τ ≈ 10 fm c , which is considerably longer than the fireball's lifetime in the hadronic stage of τ < 5 fm c . Moreover, τ ≈ 10 fm c was also obtained in Ref.[5] using a fluctuation-dissipation theorem and a significant deviation was found in the population number of various (anti-)baryons from experimental data in the 5% most central Au-Au collisions [6]. These discrepancies suggest that hadrons are "born" into equilibrium, i.e., the system is already in a chemically frozen out state at the end of the phase transition [7,8].In order to circumvent such long time scales it was suggested that near T c there exists an extra large particle density overpopulated with pions and kaons, which drive the baryo...

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“…Indeed, there are some signals -such as charmonium suppression, relative strangeness enhancement, etc. -of the (phase) transition to the deconfined phase have been observed in heavy ion collisions (HICs) at SPS energies [1,2,3,4]. Additional information about the space-time structure of the particle emission source (the region of homogeneity) can be extracted by Femtoscopy [5] or namely Hanbury-Brown-Twiss interferometry (HBT) [6].…”

Section: Introductionmentioning

confidence: 99%

Pion freeze-out as seen through HBT correlations in HICs from FAIR/AGS to RHIC energies

Li¹

2008

Proceedings of Critical Point and Onset of Deconfinement - 4th International Workshop — PoS(CPOD07)

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In this talk we present the results of two-pion HBT correlation at freeze-out in heavy ion collisions (HICs) from AGS to RHIC energies. The UrQMD hadron-string transport model as well as the CRAB analyzing program are adopted. Based on the cascade mode, in general, the calculations are satisfying and well in line with the experimental data although discrepancies are not negligible. Such as: I), the HBT time-related puzzle exists at all energies. II), at low AGS energies, the calculated volume as well as the mean free path of pion source at freeze-out are lower than data. It implies that a better description of interactions of particles at early stage of HICs is required.

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Universal chemical freeze-out as a signature of a phase transition

Cited by 28 publications

References 45 publications

Examination of the relevance of hydrodynamics for data measured at the BNL relativistic heavy ion collider

Examination of the relevance of hydrodynamics for data measured at the BNL relativistic heavy ion collider

Fast Equilibration of Hadrons in an Expanding Fireball

Pion freeze-out as seen through HBT correlations in HICs from FAIR/AGS to RHIC energies

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