Theory of Heavy-Ion Collisions

In high energy nucleus-nucleus collisions, a transient state of thermalized, hot and dense matter governed by Quantum Chromodynamics is produced. Properties of this state are reflected in the bulk low transverse momentum (P_T) hadron production which represent the remnant of the collective medium as well as in modifications of so-called probes which are not part of the thermalized medium, i.e. jets generated in high P_T processes or leptons and photons which do not participate in the strong interaction. Theory effords aim at deducing the properties of QCD thermodynamics and collectivity from such observables.Comment: Talk given at the XXIV International Symposium on Lepton Photon Interactions at High Energies, Hamburg, Germany, 17-22 August 200

Jet production in two-photon collisions at LEP

Jet and di-jet production are studied in collisions of quasi-real photons collected during the LEP2 program at e+e- center-of-mass energies from 189 to 209 GeV. OPAL reports good agreement of NLO perturbative QCD with the measured differential di-jet cross sections, which reach a mean transverse energy of the di-jet system of 25 GeV. L3, on the other hand, finds drastic disagreement of the same calculation with single jet production for transverse jet momenta larger than about 25 GeV.Comment: 3 pages, 4 figures, Talk presented at the EPS'03 conference in Aachen, Germany, to be published in the proceedings (EPJ direct