Focus on Quark Gluon Plasma Searches in Heavy Ion Collisions

Abstract

These special focus articles in New Journal of Physics include selected articles focusing on the recently claimed discovery of the QCD (quantum chromo dynamics) phase transition in CERN-SPS heavy ion experiments, which continues to be investigated at higher energies at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and at the Large Hadron Collider (LHC) at CERN. RHIC, which started in June 2000, is the world's highest energy heavy ion collider with a maximum energy of 100+100 GeV per nucleon. Since then, four experiments have started data collection, and the first results have already been published. In 2005, the LHC will be commissioned and heavy ion collisions in the energy range 2.76+2.76 TeV per nucleon are expected to be mainly studied with the ALICE experiment.The experimental program on heavy ion collision has the outstanding goal of searching for the QCD phase transition. This transition from deconfined quark and gluon matter, the so-called quark-gluon plasma (QGP), to colourless hadrons is believed to have happened in the early universe, a few microseconds after the Big Bang. Among the many phase transitions which occurred in the early universe, the QCD phase transition is the only one which is experimentally reproducible today. This is because the temperatures and energy densities needed for the transition can be reached in ultrarelativistic heavy ion collisions.In collisions of heavy nuclei, for example lead or gold, one expects to create a multiparticle system at high enough initial energy density, temperature and volume, which undergoes a transition to a quark and gluon phase and subsequently hadronizes back to particles, which can be detected by experiments.In recent years, compelling evidence for a QCD phase transition arose from experiments at the CERN SPS accelerator. They are summarized in the CERN press release (nucl-th/0002042) of 10 March 2000 on the CERN SPS results (experiments NA44, NA45, NA49, NA50, NA52, WA97/NA57 and WA98), where it is stated that 'a common assessment of the collected data leads us to conclude that we now have compelling evidence that a new state of matter has indeed been created ... in heavy ion collisions at the CERN SPS ... which features many of the characteristics of the theoretically predicted quark-gluon plasma'.Clearly, much more experimental and theoretical work is needed in order to substantiate these findings. For example, no experimental data exist on open charm production, or on measurements near the expected critical energy density of 1 GeV fm-3, or above the largest energy density reached at CERN of 3 GeV fm-3. These topics will be addressed at low energies at the CERN SPS and at high energies at RHIC and LHC.These focus papers contribute to the discussion and interpretation of the experimental data in view of the QCD phase transition. A wide spectrum of different points of view gives a 'dynamical picture' of the ongoing QGP searches today and their theoretical understanding. Focus on Quark Gluon Plasma searches in heavy ion collisions contents Hyperon enhancement in the dual parton model A Capella and C A Salgado On the role of energy conservation in high-energy nuclear scattering H J Drescher, M Hladik, S Ostapchenko, T Pierog and K Werner Simple predictions from ALCORc for rehadronization of charmed quark matter P Lévai, T S Biró, T Csörgo and J Zimányi Mapping out the QCD phase transition in multiparticle production Sonja Kabana and Peter Minkowski Microscopic coloured quark dynamics in the soft non-perturbative regime - description of hadron formation in relativistic S + Au collisions at CERN S Scherer, M Hofmann, M Bleicher, L Neise, H Stöcker and W Greiner Search for QGP and thermal freeze-out of strange hadrons Giorgio Torrieri and Johann Rafelski Charmand strangeness in nuclear reactions at s1/2⩽19 GeV Sonja Kabana Klaus Pretzl and Sonja Kabana Laboratory for High Energy Physics, University of Bern, Switzerland