Results from the Relativistic Heavy Ion Collider (RHIC) depict formation of a hot, dense system at an energy density greater than 5 GeV/fm3, well above that where hadrons exist. The measured relative particle abundances are reproduced by statistical-thermal models with a temperature and baryo-chemical potential at the quark-hadron phase boundary predicted by lattice QCD. A large amount of collective flow is observed in collisions at RHIC and provides evidence of strong pressure gradients very early in the collision, at a time when the system is dense and highly interacting. Hard-scattering, suppression of large transverse momentum hadrons, and quenching of di-jets are observed in central Au + Au collisions but not in d + Au collisions, providing evidence for extreme energy loss of partons attempting to propagate through the hot, dense matter at RHIC.
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