Quasiparticle degrees of freedom versus the perfect fluid as descriptors of the quark-gluon plasma

Abstract

The hot nuclear matter created at the Relativistic Heavy Ion Collider (RHIC) has been characterized by near-perfect fluid behavior. We demonstrate that this stands in contradiction to the identification of QCD quasi-particles with the thermodynamic degrees of freedom in the early (fluid) stage of heavy ion collisions. The empirical observation of constituent quark "nq" scaling of elliptic flow [1] is juxtaposed with the lack of such scaling behavior in hydrodynamic fluid calculations followed by Cooper-Frye freeze-out to hadrons. A "quasi-particle transport" time stage after viscous effects break down the hydrodynamic fluid stage, but prior to hadronization, is proposed to reconcile these apparent contradictions. However, without a detailed understanding of the transitions between these stages, the "nq" scaling is not a necessary consequence of this prescription. Also, if the duration of this stage is too short, it may not support well defined quasi-particles. By comparing and contrasting the coalescence of quarks into hadrons with the similar process of producing light nuclei from nucleons, it is shown that the observation of "nq" scaling in the final state does not necessarily imply that the constituent degrees of freedom were the relevant ones in the initial state.

Centrality dependence of charged hadron and strange hadron elliptic flow fromsNN=200GeV Au+Au collisions

“…It is interesting to note that the ideal hydrodynamic results scale at neither low nor intermediate p T . Therefore the observed scaling cannot be a general characteristic of hydrodynamic model calculations [17], although such calculations do show the observed mass ordering in the low p T region. Figures 10 and 11 show the centrality dependence of the scaling properties and the ratios, respectively.…”

“…Further detailed analyses of the hadron mass dependence of v 2 suggest that the system has been in the deconfined state with constituent quark degrees of freedom prior to hadronization [11][12][13]. Furthermore, results of multistrange hadron transverse momentum distributions and v 2 indicate that the system reached thermalization at the partonic stage [14][15][16][17].…”

Scaling of elliptic flow, recombination, and sequential freeze-out of hadrons in heavy-ion collisions

“…The influence of resonance decays on pion elliptic flow was also studied in Ref. [14], where the authors assumed that direct pions follow the KE T -scaling and then found the scaling still holds at the 10% level when resonances are included.…”

“…This hampers attempts to extend the description into the low-p T regime where thermalization is believed to prevail. This has become a more pressing issue once it was realized that CQNS can be generalized to encompass both low and intermediate p T if v 2 is plotted versus transverse kinetic energy, KE T ≡ m T − m, where m T = (p 2 T + m 2 ) 1/2 is the hadron's transverse mass [12][13][14][15][16][17]. In this representation, elliptic-flow data for all ob-served hadrons fall onto one universal curve for v 2 /n q versus KE T /n q [12,13].…”

Gluon energy loss in the gauge-string duality

“…It has recently been emphasized in [27] that a quasi-particle description of the QGP may not be valid. A quasi-particle picture is even less likely to capture the physics of strongly coupled N = 4 super-Yang-Mills, where the weakly coupled degrees of freedom are manifest only in the dual gravitational description.…”