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

Through the blackness -- high p_T hadrons probing the central region of 200 AGeV Au-Au collisions

The energy loss of high p_T partons propagating through a hot and dense medium is regarded as a valuable tool to probe the medium created in ultrarelativistic heavy-ion collisions. The angular correlation pattern of hadrons associated with a hard trigger in the region of p_T ~1-2 GeV which exhibits a dip in the expected position of the away side jet has given rise to the idea that energy is lost predominantly to propagating collective modes ('Mach cones'). Recent measurements by the STAR collaboration have shown that for a high p_T > 8 GeV trigger the angular pattern of associate hadrons for p_T > 4 GeV shows the emergence of the expected away side peak. These di-jet events suggest that the away side parton may emerge occasionally without substantial energy loss. Since in such a back-to-back configuration one of the partons may travel through the central region of the fireball, the average in-medium pathlength is substantial and the expected energy loss is not only sensitive to the initial geometry of matter but also to the change of geometry due to expansion. We show that radiative energy loss is able to explain the dijet events provided that the expansion of the medium is taken into account.Comment: 6 pages, 2 figures, expanded version with update of experimental dat