Theory at Quark Matter ’ 02

The collisions of large nuclei may give us insight into the nature of QCD at high temperatures and/or densities. In this talk I review the status of theory for heavy ion collisions from Quark Matter 2002. I concentrate on the the results for the largest nuclei, with atomic number A ≈ 200. At the SPS at CERN, there are two notable results for AA collisions, for √ s/A : 5 → 17 GeV [1]: J/Ψ suppression: the number of J/Ψ pairs is smaller in the most central collisions, versus the extrapolation from peripheral collisions, or from collisions with smaller A [2]. The effect is most striking for the largest nuclei. Excess dileptons below the ρ: the rate of ee pairs exceeds that in conventional hadronic models [3,4], although a broadened ρ-meson explains the excess. The effect is more prominent at lower, and not high energies, suggesting a density dependent effect. This also supports interest in going to even lower energies, such as at the proposed GSI collider. At BNL, RHIC has run at energies of √ s/A = 55 GeV (briefly), at 130 GeV during Run I, and at 200 GeV during Run II. Results from Run I were first presented at Quark Matter 2001; those from Run II, at this Quark Matter, 2002. There is one notable change expected change between the SPS and RHIC. At the SPS, the particle multiplicity in AA collisions is a single peak about zero rapidity. By RHIC energies, a Central Plateau was expected to open up, in which physics is (approximately) boost invariant, independent of rapidity. Away from the incident nucleons of the fragmentation region, the Central Plateau is where a system at nonzero temperature, and almost zero quark density, might emerge. Perhaps even deconfined matter, as the Quark-Gluon Plasma. RHIC experiments find that the Central Plateau is rather narrow. In all, particles are spread out over ≈ ±5 units of rapidity. The multiplicites for identified particles are nearly constant over the central ≈ ±1 unit of rapidity [5,6,7]. However, the pion’s average