Evidence for a quark-gluon plasma at RHIC

Collisions of heavy nuclei at RHIC form a hot, dense system at energy densities greater than 5 GeV/fm(3), where normal hadrons cannot exist. The system cools rapidly to a temperature T similar to 175 MeV and a vanishingly small baryo-chemical potential at the quark-hadron phase boundary predicted by lattice QCD calculations, where hadrons coalesce from quarks. A large amount of collective (elliptic) flow at the quark level provides evidence for strong pressure gradients very early in the collision when the system is dense and highly interacting prior to coalescence into hadrons. The suppression of both light (u, d, s) and heavy (c, b) hadrons at large transverse momenta, that form from fragmentation of hard-scattered partons, and the quenching of di-jets provide evidence for extremely large energy loss of par-tons as they attempt to propagate through the dense, strongly-coupled, colored medium created at RHIC.