Quarks matter in Budapest

The Quark Matter conferences have historically been the most important venues for showing new results in high-energy heavy-ion collisions. The 18th in the series, Quark Matter 2005, held in Budapest in August 2005, attracted more than 600 participants from 31 countries in five continents; more than a third were junior participants, reflecting the momentum of the field. The major focus of the conference was the presentation of the new data from the Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) together with the synthesis of an understanding of heavy-ion data from experiments at CERN’s Super Proton Synchrotron (SPS), including new data from the NA60 experiment. The meeting also covered a broad range of theoretical highlights in heavy-ion phenomenology, field theory at finite temperature and/or density, and related areas of astrophysics and plasma physics.

After an opening talk by Norbert Kroó, vice-president of the Hungarian Academy of Science, the scientific programme of the conference began with a talk by Roy Glauber, who was soon to share the 2005 Nobel prize in physics. Glauber’s calculations in the 1960s laid the foundation for the determination of centrality in high energy heavy-ion collisions – a measure of how close to head-on they are – which is now one of the most elementary and widely used tools of heavy-ion physics. In his talk “Diffraction theory, quantum optics and heavy ions”, he discussed the concept of coherence in quantum optics and heavy-ion collisions and presented a new generalization of the Glauber-Gribov model. Further talks in the introductory session were given by Luciano Maiani, former director-general of CERN, who reassessed the main conclusions of the SPS fixed-target programme, and by József Zimányi, of the KFKI Research Institute for Particle and Nuclear Physics in Budapest, who gave an account of the evolution of the concept of quark matter.

It has become a tradition of the Quark Matter conferences to follow the introductory session with summary talks of all experiments. Thus, the first day sets the scene for the discussions of the rest of the week. This short report cannot summarize all the interesting novel experimental and theoretical developments, but it aims at illustrating the richness of these discussions with a few of the many highlights.

One of the main discoveries of the fixed-target heavy-ion programme at the SPS five years ago was the strong suppression of the J/ψ yield with increasing centrality of the collision, which probed the deconfinement phase transition. Another discovery concerned the significant enhancement of low-mass dileptons, which indicates modification in the medium of vector mesons and possibly provides information about the restoration of chiral symmetry. These major discoveries by the NA50 and CERES experiments at the SPS also raised a significant set of more detailed questions, which were recognized as central to understanding the dynamical origins of the observed effects.

In particular, the dimuon invariant-mass spectrum of NA50 showed an enhancement below the J/ψ peak, which different theoretical groups ascribed either to a dramatic enhancement of the charm cross-section in the medium, or to significant thermal radiation. Having implemented a telescope of silicon pixel detectors with improved pointing resolution, NA60 was able to report in Budapest that data taken in the 2003 indium-indium run allow them to rule out conclusively an increased charm cross-section as the source for the dimuon excess. The data are, however, consistent with the exciting possibility of a significant thermal contribution. In addition, for more than a decade, there has been a theoretical debate on whether the embedding of ρ mesons in dense quantum chromodynamic (QCD) matter leads to a shift in the ρ mass, or to a density-dependent broadening, both scenarios being consistent with the original CERES dielectron data. NA60 now concludes, from data taken in the indium-indium run, that the shifting-mass scenario is not consistent with their data, which instead support a broadening induced in the medium (see figure 1). NA60 also presented their first indium-indium measurements of J/ψ suppression as a function of centrality. These confirm the strong anomalous suppression seen by NA50 in central lead-lead collisions at the SPS.

The SPS experiments NA49, CERES, NA50 and NA57 also showed new results from their continuing data analysis. In addition to earlier high transverse-momentum (p_T) measurements from CERES and WA98, this year NA49 and NA57 showed new results that were extensively compared with the results of the experiments at RHIC.

The central topic of this Quark Matter conference was without doubt the full harvest of the high-luminosity gold-gold run at RHIC in 2004, from which data analyses were shown for the first time. Equally important were results from the successful copper-copper run in the first half of 2005, which had been analysed in time for the conference in a global effort by the participating institutions of the four RHIC experiments. With an integrated luminosity for 200 GeV gold-gold collisions of almost 4 nb^-1, this run increased statistics by more than a factor of 10, and made much-wanted information accessible for the first time. One of the most important early discoveries of the heavy-ion experiments at RHIC was the strong suppression of hadronic spectra by up to a factor of five in the most central collisions. This so-called “jet-quenching effect” supports the picture that the matter created in heavy-ion collisions is of extreme density and thus very opaque to hard partons.

Results from the PHENIX experiment at RHIC now indicate that even neutral pions of p_T = 20 GeV show this dramatic energy degradation (figure 2). Moreover, the increased luminosity allowed the STAR experiment to study the recoil of hadron trigger particles up to 15 GeV, and for sufficiently high transverse momenta, this recoil is for the first time observed to punch through the soft background. However, compared with reference data from proton-proton collisions, the particle yield of the recoil is strongly reduced, consistent again with the picture of a medium that is dense and very opaque to partonic projectiles. In further support, PHENIX also reported that high-p_T photons are not suppressed (figure 2), and that photons at intermediate transverse momenta show an excess, which may be attributed to thermal radiation from the hot and dense matter.

Another important piece in the puzzle of reconstructing the properties of the produced matter came from the first measurements of high-pT single-electron spectra. These spectra are thought to be dominated by the semi-leptonic decays of D- and B-mesons, thus giving for the first time experimental access to the propagation of heavy quarks in dense QCD matter. Data from STAR and PHENIX reveal a medium-induced suppression of electrons, which is of similar size to that of light-flavoured hadrons. There were many parallel talks, by both experimentalists and theorists, which contrasted these data with the theoretical expectation that massive quarks should lose less energy in the medium than massless quarks or gluons due to the so-called “dead-cone effect” in QCD. While a final assessment is still awaited, there was widespread agreement that these data will help significantly in refining our understanding of the interaction between hard probes and the medium, which is much needed for a better characterization of the dense QCD matter produced in nucleus-nucleus collisions.

Another much awaited result that gave rise to a great deal of discussion was the first statistically significant J/ψ measurement at RHIC. This was presented by the PHENIX collaboration and showed a similar pattern and strength to that observed in lead-lead and indium-indium collisions at the SPS. This result was of particular interest also to lattice QCD theorists, who now find that the dissociation of the directly produced J/ψ in a deconfined medium sets in at much higher energy densities than previously expected.

The bulk properties of dense QCD matter reveal themselves not only in the modification of hard processes by the medium, but also in the collective motion of soft particle production and its hadro-chemical composition. One of the main discoveries of the first years of running RHIC was the unprecedented large size of the collective flow signals, measured in the asymmetries of particle production with respect to the reaction plane. Remarkably, the measured mass-dependence of the transverse radial and elliptic flow supports the assumption that different particle species emerge from a common flow field. Flow measurements at intermediate transverse momenta follow constituent-quark counting rules and are consistent with quark coalescence as a medium-dependent hadronization scenario (figure 3). Moreover, to the surprise of many, the hydrodynamic description of the collision in terms of an adiabatically expanding, perfect fluid of vanishing viscosity and heat conductivity appears, at RHIC energies, to be satisfactory for the first time.

Much of the discussion at QM ’05 focused on the emerging picture of the matter produced in heavy-ion collisions at RHIC, which, far from being a weakly interacting gas of quarks and gluons, shows features of a strongly coupled partonic system indicative of a perfect liquid. This liquid includes not only the light and strange quarks; the first preliminary data on the elliptic flow of charmed hadrons from the PHENIX collaboration indicates that even charmed quarks participate in the collective expansion of this new form of matter.

The conference saw a lively theoretical discussion about the dynamic mechanisms underlying a possible rapid thermalization. Emphasis was given in particular to the relationship to thermalization processes in Abelian plasmas, to formal analogies with the thermal properties of black holes, and to the possibility that plasma instabilities accelerate equilibration. The intellectual richness of the field was further illustrated by exciting reports from string theory, where theorists have succeeded for the first time in calculating the viscosity to entropy density ratio in the physically relevant, strong-coupling limit of a certain class of thermal non-Abelian gauge theories. The fact that this ratio is found to be very small indicates a non-dissipative behaviour. It raises the exciting possibility that the non-dissipative character of an almost perfect liquid, which may be created in gold-gold collisions at RHIC, could be understood from first-principles calculations in QCD.

From the point of view of heavy-ion phenomenology, the central question of whether more direct signals of negligible viscosity can be established led to another highlight of the conference. The widely discussed idea was that if dissipation is negligible, then energy, deposited by a jet in dense QCD matter, must propagate in a characteristic Mach cone, determined by the velocity of sound in the quark-gluon plasma. Reports about back-to-back particle correlations from PHENIX, which may show such a Mach-cone-like structure, were hotly debated amongst theorists and experimentalists alike (see figure 4). Most importantly, these discussions showed that heavy-ion physics at collider energies has a large set of novel tools available for the controlled experimentation with hot and dense QCD matter, and that the field is moving towards characterizing specific properties of this matter, including its speed of sound, equation of state, and its transport coefficients such as heat conductivity and viscosity.

Past, present and future

The Quark Matter conferences not only highlight the experimental harvest of the recent past and the latest news from theory, they are also the arena for assessing perspectives for the future. The first heavy-ion beam at the Large Hadron Collider (LHC) at CERN is expected in 2008, and heavy-ion researchers are now well prepared for the jump in centre-of-mass energy by a factor of 30 above RHIC. Most importantly, the fact that dramatic medium-sensitive effects persist unweakened at RHIC up to the highest measured transverse momentum strongly supports the expectation that the new kinematic regime accessible at the LHC will provide many qualitatively novel tools for the study of ultra-dense QCD matter.

The LHC will not be the only big player in the field of heavy-ion physics in the next decade. At Brookhaven, the STAR and PHENIX collaborations are lining up for several important detector upgrades, which will significantly enhance their abilities to characterize specific properties of the matter created in heavy-ion collisions. Moreover, Brookhaven envisages a luminosity upgrade of RHIC, which will open yet another class of novel opportunities. Finally, the newly approved Facility for Antiproton and Ion Research at the GSI Darmstadt is preparing for the start of a versatile heavy-ion programme in the next decade. Plenary talks provided overviews of the status and possibilities of these three programmes. The field is now eagerly awaiting its future, the next slice of which will be served at the 19th Quark Matter conference in Shanghai in November 2006.