Ambivalent signals

14th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions Torino, 10. - 15. May, 1999

The 14th Quark Matter Conference took place in Torino, Italy (Fig.1). About 500 physicists from many countries discussed new theoretical and experimental implications of nucleus-nucleus collisions at ultra-relativistic energies. The focus was on fixed-target experiments, but also the upcoming physics with colliding beams was debated: The Relativistic Heavy-Ion Collider (RHIC) in Brookhaven will start operating already this year, and experiments at the Large Hadron Collider (LHC) in Geneva will begin in the middle of the next decade at even higher energies. With this last conference before the start of the collider experiments, the field has reached a plateau. In about 30 invited lectures, 100 presentations in up to three parallel sessions and about as many posters the conference took the opportunity to review the present achievements of the field, and to take a look into its future.

The main aim is still the search for signals of quark-gluon plasma (QGP) formation. Lattice gauge theory predicts on the basis of quantum chromodynamics (QCD) that hadronic matter undergoes a phase transition to this new phase when the energy density is increased by about one order of magnitude above the normal value. These very elaborate first-principle numerical calculations still have unsolved technical problems at finite baryon densities. However, there is general confidence that the quark-gluon plasma not only existed in thermodynamic equilibrium at the beginning of the evolution of the universe, and still exists in the interior of neutron stars, but can also artificially be created for a very short time interval in the nonequilibrium collision of heavy nuclei at relativistic energies. The hope that it can be detected is the main cornerstone of the field. An unambiguous detection of the QGP would be of basic importance for the progress of knowledge in science. In the past 20 years, the methods to provide evidence for QGP- formation have constantly been improved, and the results were discussed on the Quark-Matter conferences.

Essential conditions for plasma formation are indeed fulfilled in present experiments with heavy projectiles and fixed targets. In the - now terminated - experiments at the Alternating Gradient Synchrotron (AGS) in Brookhaven with a gold beam of 11 GeV per nucleon incident energy one has almost reached the critical value of the energy density of about 1.5 GeV/fm^3. At the SPS in Geneva, central Pb + Pb - reactions at 156 GeV per nucleon incident energy produce an energy density of about 3 GeV/fm^3, which is safely above the critical value. Various observations indicate that in the dense hot zone - the "fireball" - created in central collisions of heavy nuclei at relativistic energies, quarks may be deconfined from hadrons into the quark-gluon plasma, and that the so-called chiral symmetry of QCD is restored. When leaving the reaction zone, quarks and gluons hadronize: they form strongly interacting hadrons, which are then - together with the weakly interacting leptons that are created during the collision - detected with their momentum distributions and cross sections.

Even though AGS- and SPS- experiments have meanwhile delivered the precise data they were built for, it remains extremely difficult to draw conclusions regarding a possible deconfinement from the large variety of hadronic and leptonic observables. Most promising are presently three groups of signals, which were discussed in Torino by the AGS- and SPS- collaborations as well as by theorists in all details:

In spite of such hints at deconfinement, all the possible signals found so far are necessary, but not yet sufficient conditions for quark-gluon plasma formation in relativistic nucleus-nucleus-collisions. The signals were discussed in great detail in the various experimental and theoretical sessions. For example, the strangeness-enhancement that had been predicted theoretically in 1982 can be investigated in terms of the K^+/p^+ - ratio. Whereas the K^-/p^- - ratio is approximately equal to the result from proton-proton- collisions, a considerable enhancement for the positively charged particles had been found already in 1987 at the AGS (Si+Au) and the SPS (S+S) due to the high baryon density, which was later carefully investigated and discussed again in Torino. Comparing to transport calculations there are difficulties to interpret AGS- and SPS- data simultaneously in a consistent fashion. The AGS-results appear to support a hadronic scenario, which is not the case for the SPS data. Strangeness-enhancement increases with the strangeness-content of the produced particles and hence, is most prominent for Xi-hyperons (two strange quarks) and Omega-hyperons (three strange quarks). It is often assumed that strangeness- production freezes out in the early stages of the reaction when the energy density is at its maximum value (J. Rafelski). Hence, chemical equilibrium would not be achieved, the available time is not sufficient for equilibration.

This is in contrast to other studies of hadron production rates which have been used to gain information about the system at freeze-out. They show that the relative abundancies of produced hadrons at AGS- and SPS-energies are essentially determined by the freeze-out temperature and the baryo-chemical potential, as is expected for thermal and chemical equilibrium. As already stated by Rolf Hagedorn in 1965, the available phase space dominates the hadron production. It obeys the principle of maximum entropy at freeze-out. The agreement of this picture with the data is good, J. Stachel even showed an extrapolation to RHIC energies. The chemical freeze-out parameters at SIS-, AGS- and SPS- energies correspond to a single value of 1 GeV for the mean energy per hadron (K. Redlich et al.). Transverse momentum distributions and other data may be interpreted with the assumption of thermal equilibrium and collective expansion. Numerous flow-analyses and hydrodynamic calculations were also performed and presented.

However, the good agreement of the phase-space model with many data is not a sufficient condition for the attainment of thermal and baryo-chemical equilibrium in the system. Due to the short interaction times of about 10^-23 s even in central collisions, equilibrium is generally not achieved. Transport calculations of the BUU-type, event generators and also analytical models (such as a relativistic diffusion model) show in comparison with data that heavy systems at AGS- and, in particular, SPS-energies do not equilibrate dynamically. As a consequence, the scenario of a phase transition into an equilibrated quark-gluon phase, as well as the direct applicability of latttice gauge calculations to nucleus-nucleus collisions appears questionable. This view is supported by the fact that so far no indications for critical phenomena expected in the presence of a phase transition have been found. An example is the analysis of event-by-event fluctuations that becomes possible due to the high multiplicities at SPS- energies, as performed by the NA49 collaboration. Here, the fluctuations of the transverse momentum of produced pions and other observables behave statistically and give no hints at any critical behaviour that may be indicative of a phase transition.

The SPS-physics is by no means completed, and will continue to generate interesting and relevant results, for example, with the forthcoming experiments at lower beam energy of 40 (and probably 80) GeV per nucleon, which will produce better stopping and higher baryon density. Nevertheless, in Torino much attention was caught by the upcoming collider-physics with heavy ions. The detector systems PHENIX, PHOBOS and STAR at RHIC, and ALICE (and CMS) at LHC were introduced, and a whole day was reserved for theoretical predictions of the various models and event-generators at RHIC energies. Significant differences in the predictions for important observables such as rapidity distributions for protons or negative hadrons could be noticed, such that the measurements will surely not be replaced by event-generation in the computer. Even stronger J/Psi- suppression will be expected at RHIC. M. Gyulassy gave a summary of RHIC- predictions and showed a rosette of the most important observables, with the possibility to detect the quark-gluon plasma in the center. P. Braun-Munzinger presented a visionary talk about the future physics at LHC, where center-of-mass energies of 1150 TeV - corresponding to a macroscopically relevant energy of 0.18 mJ - are produced, energy densities of 1 TeV/ fm^3 are expected and 17 units of rapidity are covered. His remark that at collider energies J/Psi- mesons are also produced indirectly from B-decays which must be detected separately is likely to become relevant for RHIC, too. In addition, Upsilon-mesons (bottom-antibottom systems) should become a sensible signal for plasma formation in the region of low baryon content at collider energies.

In summary, even though there are signs for deconfinement, there is still a long way to go until the possible transient existence of a quark-gluon plasma in relativistic heavy-ion collisions may be proven without doubt. The aim of the field remains relevant, and attracts many young scientists. It will continue to expand on the basis of the future collider experiments in Brookhaven - where the next conference of this series will take place - and Geneva.

G. Wolschin


Fig.1: The 14th Quark-Matter Conference in May 1999 took place in Torino. The conference logo includes the symbol of the city, Mole Antonelliana. This building was conceived at the end of the 19th century as a Synagogue. Today it hosts the National Museum of Cinema.

(Source: QM99, Torino)

Fig.2: The anomalous suppression of the J/Psi-production found by the NA 50-collaboration in central Pb + Pb collisions (high E_T values) is interpreted as the hitherto most significant hint at quark-gluon plasma formation in collisions of heavy nuclei at SPS-energies. Results of the 1998 run with a 3mm target are shown here together with the 1996 results, and a typical calculation for hadronic suppression.There are, however, also purely hadronic scenarios which agree with the data and hence, offer no indication for anomalous suppression, or plasma formation.

(Source: NA50/ L. Kluberg).

(Source: NA50/ L. Kluberg).

(Source: NA50/ L. Kluberg).