Summary of Heavy Ion Theory

Can we study hot QCD using nuclear collisions? Can we learn about metallic hydrogen from the impact of comet Shoemaker-Levy 9 on Jupiter? The answer to both questions may surprise you! I summarize progress in relativistic heavy ion theory reported at DPF ‘94 in the parallel sessions. Lattice simulations of QCD demonstrate that matter at temperatures exceeding Tc ∼ 150 MeV is very different from matter composed of hadrons. Simulations commonly display dramatic changes in thermodynamic quantities, such as the energy density, in a narrow interval |T − Tc| < ∼ 5 MeV, indiciting an abrupt transformation from hadronic to quark-gluon degrees of freedom. The underlying aim of the theoretical speakers in the heavy ion sessions has been to understand how properties of high temperature matter can be deduced from collisions of nuclei at RHIC and LHC at √ s = 200 and 5500 GeV per nucleon, respectively. Is the high temperature state deconfined? Is chiral symmetry restored? Is the expected abrupt transformation a true phase transition? Physics demands experimental answers to these questions. Talks in this session addressed the global dynamics of heavy ion collisions as well as specific probes of the high temperature state. The significant progress in understanding the collision dynamics at the Brookhaven AGS, √ s ∼ 5 AGeV, and the CERN SPS, √ s ∼ 20 AGeV was surveyed by Schlagel and Vogt. Sarcevic and Shuryak discussed two important probes of the dynamics that will be more important at higher energies: open charm and direct photon production. The suppression of J/ψ production in ion-ion collisions probes the deconfinement of the high temperature state. This topic was presented by Satz and Thews. Ayala and Petrides discussed the modification of parton distributions in nuclei, a related topic. Schäfer and Shuryak discussed the nature of the chiral transition. Disoriented Chiral Condensates, a possible probe of the dynamics of chiral symmetry breaking at RHIC and LHC, was discussed by Kluger. What can we learn about high temperature QCD from nuclear collisions? An analogous, similarly-complex question is, what can the impact of a comet with Jupiter teach us about the equation of state of hydrogen? We have all seen exciting images of the collisions of the fragments of the comet Shoemaker-Levy 9 with Jupiter that took place ∗This manuscript has been authored under contract number DE-AC02-76CH00016 with the U.S. Department of Energy. from 16 to 22 July, 1994. Observations of the impacts are providing new information on comet structure and the stratification and composition of Jupiter’s atmosphere. Indeed, the speed of sound in metallic hydrogen can be measured if reflections of downward-launched accoustic waves from Jupiter’s core can be observed. 1. Collision Dynamics A very important issue in the AGS and SPS fixed-target experiments has been the stopping power. The extent to which the projectile ion is stopped (‘slowed’ is more accurate) as it crashes through the target nucleus depends on how the constituents interact. Should one treat the constituents as nucleons or quarks on the time scales of the collision? Is resonance formation important in the nucleon rescattering? Is there a formation time for secondary particle production? Data on stopping comes primarily from the rapidity distribution of protons. Schlagel showed that the AGS proton data for projectiles as large as Au can be described by a purely hadronic rescattering model that incorporates resonance formation. He also showed that the omission of resonance formation does not describe the data. We expect formation time effects to become more important at the higher SPS energy. Vogt showed that SPS data for light projectiles can be described by string models that incorporate these effects. She argued that the Pb beam runs commencing this fall will be useful in deciding between string and hydrodynamic models. In the case of Shoemaker-Levy 9, one is also interested in how the comet is stopped by Jupiter’s atmosphere. The comet’s structure determines the depth to which the comet penetrates. Stopping therefore provides information on comet structure. High temperature H2O emission lines that are likely from the comet remnants have been observed. In the following table, I list aspects of the dynamics in nuclear collisions discussed at this meeting together with their analogs in the comet-Jupiter impact. Ion + Ion Comet + Jupiter stopping baryon distribution comet remnants proton dN/dy H2O thermalization γ, ee, μμ; emission lines π, K, . . . , D H2S, CH4 flow directed flow plume, ejecta density J/ψ, ψ,Υ absorption lines EOS DCC seismic waves Another central question in AGS/SPS nuclear collisions is thermalization: how effectively is the momentum of the projectile distributed among the participant nucleons and produced hadrons? Do these hadrons reach local thermal equilibrium and undergo collective flow? The abundance of produced particles such as pions, kaons and antiprotons indicates the extent to which particles interact and thermalize. Shuryak observed that photon and dilepton production can be used to measure the temperatures that the system achieves, although backgrounds can be formidable in practice. In the comet-Jupiter collision, the excitation of high temperature emission lines provide information on energy deposition. The observation of hot H2S indicates that the -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 ycm -0.10 -0.05 0.00 0.05 0.10 < P x > ( G eV /c ) ARC, b<13 D. Kahana, Y. Pang, T. J. Schlagel Protons Pions Anti-Nucleons Kaons