New state of nuclear matter: Nearly perfect fluid of quarks and gluons in heavy-ion collisions at RHIC energies

摘要

This article reviews several important results from RHIC experiments and discusses their implications. They were obtained in a unique environment for studying QCD matter at temperatures and densities that exceed the limits wherein hadrons can exist as individual entities and raises to prominence the quark-gluon degrees of freedom. These findings are supported by major experimental observations via measuring of the bulk properties of particle production, particle ratios and chemical freeze-out conditions, and elliptic flow; followed by hard probe measurements: high-pT hadron suppression, dijet fragment azimuthal correlations, and heavy-flavor probes. These measurements are presented for particles of different species as a function of system sizes, collision centrality, and energy carried out in RHIC experiments. The results reveal that a dense, strongly interacting medium is created in central Au + Au collisions at root(NN)-N-S = 200 GeV at RHIC. This revelation of a new state of nuclear matter has also been observed in measurements at the LHC. Further, the IP-Glasma model coupled with viscous hydrodynamic models, which assumes the formation of a QGP, reproduces well the experimental flow results from Au + Au at root(NN)-N-S = 200 GeV. This implies that the fluctuations in the initial geometry state are important and the created medium behaves as a nearly perfect liquid of nuclear matter because it has an extraordinarily low ratio of shear viscosity to entropy density, eta/s approximate to 0.12. However, these discoveries are far from being fully understood. Furthermore, recent experimental results from RHIC and LHC in small p + A, d + Au and He-3 + Au collision systems provide brand new insight into the role of initial and final state effects. These have proven to be interesting and more surprising than originally anticipated; and could conceivably shed new light in our understanding of collective behavior in heavy-ion physics. Accordingly, the focus of the experiments at both facilities RHIC and the LHC is on detailed exploration of the properties of this new state of nuclear matter, the QGP.