CCEJan/FebQUARK25-27 24/1/06 8:22 Page 25 QM ’05 Quarks matter in Budapest Quark Matter 2005, the 18th International 3000 In–In SemiCentral Conference on Ultra-Relativistic Nucleus– all pT 2500 Nucleus Collisions, provided a lively forum 2000 for new results in heavy-ion physics. 1500 The Quark Matter conferences have historically been the most 1000 important venues for showing new results in high-energy heavy-ion dN/dM per 20 MeV collisions. The 18th in the series, Quark Matter 2005, held in Budapest in August 2005, attracted more than 600 participants 500 from 31 countries in five continents; more than a third were junior participants, reflecting the momentum of the field. The major focus 0 of the conference was the presentation of the new data from the 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Brookhaven National Laboratory’s Relativistic Heavy Ion Collider M (GeV) (RHIC) together with the synthesis of an understanding of heavy-ion Fig.1. First measurement of modification in the medium of the ρ data from experiments at CERN’s Super Proton Synchrotron (SPS), spectral function in indium–indium collisions at CERN’s SPS, as including new data from the NA60 experiment. The meeting also presented by the NA60 experiment. The dashed line shows the covered a broad range of theoretical highlights in heavy-ion phe- unmodified spectral weight of the ρ; other curves are theoretical nomenology, field theory at finite temperature and/or density, and predictions for a ρ broadening and a falling mass scenario. related areas of astrophysics and plasma physics. After an opening talk by Norbert Kroó, vice-president of the 10 PHENIX Au+Au (central collisions) Hungarian Academy of Science, the scientific programme of the con- direct γ ␲0 preliminary ference began with a talk by Roy Glauber, who was soon to share the ␩ 2005 Nobel prize in physics (CERN Courier November 2005 p8). GLV parton energy loss (dNg/dy=1100) Glauber’s calculations in the 1960s laid the foundation for the deter- 1 AA mination of centrality in high energy heavy-ion collisions – a measure R of how close to head-on they are – which is now one of the most ele- mentary and widely used tools of heavy-ion physics. In his talk –1 “Diffraction theory, quantum optics and heavy ions”, he discussed 10 the concept of coherence in quantum optics and heavy-ion collisions and presented a new generalization of the Glauber–Gribov model. 02468101214 16 18 20 p (GeV/c) Further talks in the introductory session were given by Luciano Maiani, T former director-general of CERN, who reassessed the main conclu- Fig.2. The nuclear modification factor, RAA, as a function of π η sions of the SPS fixed-target programme, and by József Zimányi, of the transverse momentum, pT, for neutral pions ( ), etas ( ) and KFKI Research Institute for Particle and Nuclear Physics in Budapest, photons (γ) in gold–gold collisions at RHIC, as presented by the who gave an account of the evolution of the concept of quark matter. PHENIX collaboration. Compared with proton–proton collisions, It has become a tradition of the Quark Matter conferences to fol- pions and etas are suppressed by up to a factor of five. This “jet- low the introductory session with summary talks of all experiments. quenching” effect is well reproduced in models (yellow line), Thus, the first day sets the scene for the discussions of the rest of the which account for strong medium-induced parton energy loss in week. This short report cannot summarize all the interesting novel dense QCD matter created in heavy-ion collisions. By contrast, experimental and theoretical developments, but it aims at illustrating photons can escape the system without further interaction. the richness of these discussions with a few of the many highlights. One of the main discoveries of the fixed-target heavy-ion pro- coveries by the NA50 and CERES experiments at the SPS also raised gramme at the SPS five years ago was the strong suppression of the a significant set of more detailed questions, which were recognized as J/Ψ yield with increasing centrality of the collision, which probed the central to understanding the dynamical origins of the observed effects. deconfinement phase transition. Another discovery concerned the sig- In particular, the dimuon invariant-mass spectrum of NA50 showed nificant enhancement of low-mass dileptons, which indicates modifi- an enhancement below the J/Ψ peak, which different theoretical cation in the medium of vector mesons and possibly provides groups ascribed either to a dramatic enhancement of the charm cross- information about the restoration of chiral symmetry. These major dis- section in the medium, or to significant thermal radiation. Having ▲ CERN Courier January/February 2006 25 CCEJan/FebQUARK25-27 24/1/06 8:23 Page 26 QM ’05 factor of 10, and made much-wanted information accessible for the polynomial fit first time. One of the most important early discoveries of the heavy-ion STAR preliminary 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 0.05 2 “jet-quenching effect” supports the picture that the matter created in v /n π+ + π– p+p heavy-ion collisions is of extreme density and thus very opaque to 0 Λ Λ KS + hard partons (CERN Courier September 2003 p18). K++K– Ξ –+Ξ+ Results from the PHENIX experiment at RHIC now indicate that 0 even neutral pions of pT =20GeV show this dramatic energy degra- dation (figure 2, p25). Moreover, the increased luminosity allowed 1.5 the STAR experiment to study the recoil of hadron trigger particles up 1.0 to 15 GeV, and for sufficiently high transverse momenta, this recoil is data/fit 0.5 for the first time observed to punch through the soft background. However, compared with reference data from proton–proton colli- sions, the particle yield of the recoil is strongly reduced, consistent 0123again with the picture of a medium that is dense and very opaque to pT/n (GeV/c) partonic projectiles. In further support, PHENIX also reported that Fig. 3. In nucleus–nucleus collisions, due to collective motion, high-pT photons are not suppressed (figure 2, p25), and that photons soft-particle production is strongly preferred to lie within the at intermediate transverse momenta show an excess, which may be reaction plane, as measured by the elliptic flow v2. At attributed to thermal radiation from the hot and dense matter. intermediate transverse momenta, the elliptic flow of identified Another important piece in the puzzle of reconstructing the proper- hadron species shows a characteristic scaling with valence ties of the produced matter came from the first measurements of high- quark number n, as presented by the STAR collaboration. This pT single-electron spectra. These spectra are thought to be dominated supports a quark-coalescence picture of hadronization. by the semi-leptonic decays of D- and B-mesons, thus giving for the Deviations from quark-number scaling at small transverse first time experimental access to the propagation of heavy quarks in momenta are accounted for in a hydrodynamic description of the dense QCD matter. Data from STAR and PHENIX reveal a medium- produced matter as a perfect liquid. induced suppression of electrons, which is of similar size to that of light-flavoured hadrons. There were many parallel talks, by both exper- implemented a telescope of silicon pixel detectors with improved point- imentalists and theorists, which contrasted these data with the theo- ing resolution, NA60 was able to report in Budapest that data taken in retical expectation that massive quarks should lose less energy in the the 2003 indium–indium run allow them to rule out conclusively an medium than massless quarks or gluons due to the so-called “dead- increased charm cross-section as the source for the dimuon excess. cone effect” in QCD. While a final assessment is still awaited, there The data are, however, consistent with the exciting possibility of a sig- was widespread agreement that these data will help significantly in nificant thermal contribution. In addition, for more than a decade, there refining our understanding of the interaction between hard probes has been a theoretical debate on whether the embedding of ρ mesons and the medium, which is much needed for a better characterization in dense quantum chromodynamic (QCD) matter leads to a shift in of the dense QCD matter produced in nucleus–nucleus collisions. the ρ mass, or to a density-dependent broadening, both scenarios Another much awaited result that gave rise to a great deal of dis- being consistent with the original CERES dielectron data. NA60 now cussion was the first statistically significant J/Ψ measurement at concludes, from data taken in the indium–indium run, that the shifting- RHIC. This was presented by the PHENIX collaboration and showed mass scenario is not consistent with their data, which instead support a similar pattern and strength to that observed in lead–lead and a broadening induced in the medium (see figure 1, p25). NA60 also indium–indium collisions at the SPS. This result was of particular presented their first indium–indium measurements of J/Ψ suppres- interest also to lattice QCD theorists, who now find that the dissoci- sion as a function of centrality. These confirm the strong anomalous ation of the directly produced J/Ψ in a deconfined medium sets in suppression seen by NA50 in central lead–lead collisions at the SPS. at much higher energy densities than previously expected. The SPS experiments NA49, CERES, NA50 and NA57 also showed The bulk properties of dense QCD matter reveal themselves not new results from their continuing data analysis.