Is hydrodynamics relevant to RHIC collisions?

Abstract

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.

19 pages, 17 figures

References in corpus (18)

• Viscosity Information from Relativistic Nuclear Collisions: How Perfect is the Fluid Observed at RHIC?
• Systematic Measurements of Identified Particle Spectra in pp, d+Au and Au+Au Collisions from STAR
• Dihadron azimuthal correlations in Au+Au collisions at sqrt(s_NN)=200 GeV
• Hydrodynamic Models for Heavy Ion Collisions
• Centrality dependence of charged hadron and strange hadron elliptic flow from sqrt(s_NN) = 200 GeV Au+Au collisions
• Core-Corona Separation in Ultra-Relativistic Heavy Ion Collisions
• Systematic Comparison of Jet Energy-Loss Schemes in a realistic hydrodynamic medium
• A theory of jet shapes and cross sections: from hadrons to nuclei
• Indications of Conical Emission of Charged Hadrons at the BNL Relativistic Heavy Ion Collider
• The multiplicity dependence of inclusive $p_t$ spectra from p-p collisions at $\sqrt{s}$ = 200 GeV
• Evolution of minimum-bias parton fragmentation in nuclear collisions
• Centrality evolution of $p_t$ and $y_t$ spectra from Au-Au collisions at $\sqrt{s_{NN}}=200$ GeV
• Universal Centrality and Collision Energy Trends for $v_2$ Measurements From 2D Angular Correlations
• Extrapolating parton fragmentation to low $Q^2$ in $e^{+}$-$e^{-}$ collisions
• ZYAM and ${\mathbf{v}}_2$: Underestimating jet yields from dihadron azimuth correlations
• Azimuth quadrupole component spectra on transverse rapidity ${\mathbf{y}}_{\mathbf{t}}$ for identified hadrons from Au-Au collisions at $\sqrt{s_{NN}}=$ 200 GeV
• The RHIC azimuth quadrupole: "perfect liquid" or gluonic radiation?
• The azimuth structure of nuclear collisions -- I

Cited by in corpus (18)

• Tsallis Fits to pT Spectra and Multiple Hard Scattering in pp Collisions at the LHC
• Parton fragment yields derived from minimum-bias jet angular correlations
• Momentum Kick Model Description of the Ridge in (Delta-phi)-(Delta eta) Correlation in pp Collisions at 7 TeV
• Directed and elliptic flow of charged particles in Cu+Cu collisions at $\sqrt{{\mathbf{s}}_{\mathbf{NN}}}=$ 22.4 GeV
• A critical review of RHIC experimental results
• CCharge-multiplicity dependence of single-particle transverse-rapidity ${\mathbf{y}}_{\mathbf{t}}$ and pseudorapidity $\eta$ densities and 2D angular correlations from 200 GeV $p$-$p$ collisions
• Comparing the same-side "ridge" in CMS p-p angular correlations to RHIC p-p data
• Phenomenological analysis of angular correlations in 7 TeV proton-proton collisions from the CMS experiment
• Ensemble mean ${\mathbf{p}}_{\mathbf{t}}$ vs charged-hadron multiplicities in high energy nuclear collisions
• Gluon correlations from a Glasma flux-tube model compared to measured hadron correlations on transverse momentum $({\mathbf{p}}_{\mathbf{t}},{\mathbf{p}}_{\mathbf{t}})$ and angular differences $({\eta }_{\mathbf{\Delta }},{\varphi }_{\mathbf{\Delta }})$
• Charge-multiplicity and collision-energy dependence of $p_t$ spectra from $p$-$p$ collisions at the relativistic heavy-ion collider and large hadron collider
• Bose-Einstein Interference in the Passage of a Jet in a Dense Medium
• Glasma flux tubes vs minimum-bias jets in 2D angular correlations on $\eta$ and $\varphi$
• Optimal modeling of 1D azimuth correlations in the context of Bayesian inference
• Feynman Amplitude Approach to study the Passage of a Jet in a Medium
• The "soft ridge" - is it initial-state geometry or modified jets?
• Rescuing the nonjet (NJ) azimuth quadrupole from the flow narrative
• $\mathbf{p}$-$\mathbf{p}$ minimum-bias dijets and nonjet quadrupole in relation to conjectured collectivity (flows) in high-energy nuclear collisions