The quark-gluon plasma at RHIC

I present a theory-guided review of RHIC data, arguing that they provide strong evidence for formation of a thermalized quark-gluon plasma at RHIC. Strong radial flow reflects high thermal pressure in the reaction zone. Large elliptic flow proves that the pressure builds up quickly and the system thermalizes on a very short time scale of < 1 fm/c. The observed hadron yields are consistent with statistical hadron formation from a quark-gluon plasma, followed by immediate chemical decoupling due to strong radial expansion. The observed suppression of jets appears to confirm the predicted large energy loss suffered by hard partons moving through a quark-gluon plasma; more work is required to quantitatively understand this effect. Source size measurements using twoparticle correlations do not seem to fit into this picture; the origin of this discrepancy (“HBT puzzle”) is presently not understood.