Matter in extremis: ultrarelativistic nuclear collisions at RHIC
Peter Jacobs, Xin-Nian Wang
TL;DR
The paper reviews the theoretical framework and experimental evidence for matter at extreme energy density produced in RHIC heavy-ion collisions, arguing for the creation of a Quark-Gluon Plasma that behaves as a near-ideal, strongly coupled fluid. It integrates bulk observables, hydrodynamic modeling, and hard probe measurements to establish rapid thermalization, high initial energy density, and strong partonic energy loss through jet quenching, while contrasting initial-state saturation scenarios. The work highlights the role of collective flow, hadron chemistry at chemical freezeout, and jet-related observables as complementary probes of the QGP, and outlines future directions including direct photon tagged jets and heavy-flavor measurements to further constrain the medium's properties and the QCD phase diagram.
Abstract
We review the physics of nuclear matter at high energy density and the experimental search for the Quark-Gluon Plasma at the Relativistic Heavy Ion Collider (RHIC). The data obtained in the first three years of the RHIC physics program provide several lines of evidence that a novel state of matter has been created in the most violent, head-on collisions of $Au$ nuclei at $\sqrt{s}=200$ GeV. Jet quenching and global measurements show that the initial energy density of the strongly interacting medium generated in the collision is about two orders of magnitude larger than that of cold nuclear matter, well above the critical density for the deconfinement phase transition predicted by lattice QCD. The observed collective flow patterns imply that the system thermalizes early in its evolution, with the dynamics of its expansion consistent with ideal hydrodynamic flow based on a Quark-Gluon Plasma equation of state.