Quarks Matter in Budapest

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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 ▲

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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 collisions, 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. In addition to earlier only in the modification of hard processes by the medium, but also in high transverse-momentum (pT) measurements from CERES and the collective motion of soft particle production and its hadro- WA98, this year NA49 and NA57 showed new results that were chemical composition. One of the main discoveries of the first years extensively compared with the results of the experiments at RHIC. of running RHIC was the unprecedented large size of the collective The central topic of this Quark Matter conference was without doubt flow signals, measured in the asymmetries of particle production the full harvest of the high-luminosity gold–gold run at RHIC in 2004, with respect to the reaction plane. Remarkably, the measured mass- from which data analyses were shown for the first time. Equally impor- dependence of the transverse radial and elliptic flow supports the tant were results from the successful copper–copper run in the first assumption that different particle species emerge from a common half of 2005, which had been analysed in time for the conference in flow field. Flow measurements at intermediate transverse momenta a global effort by the participating institutions of the four RHIC exper- follow constituent-quark counting rules and are consistent with iments. With an integrated luminosity for 200 GeV gold–gold colli- quark coalescence as a medium-dependent hadronization scenario sions of almost 4 nb–1, this run increased statistics by more than a (figure 3). Moreover, to the surprise of many, the hydrodynamic

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QM ’05 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. 0.08 1 (A) LP Much of the discussion at QM ’05 focused on the emerging pic- φ ture of the matter produced in heavy-ion collisions at RHIC, which, c 0.04 φ ψ R far from being a weakly interacting gas of quarks and gluons, shows 13 φ features of a strongly coupled partonic system indicative of a perfect 0.00 12 3 (C) liquid. This liquid includes not only the light and strange quarks; the 0 first preliminary data on the elliptic flow of charmed hadrons from 40 2 (B) 0 80 φ the PHENIX collaboration indicates that even charmed quarks par- 13 40 ticipate in the collective expansion of this new form of matter. 80 120 The conference saw a lively theoretical discussion about the φ 120 160 12 160 dynamic mechanisms underlying a possible rapid thermalization. Emphasis was given in particular to the relationship to thermaliza- Fig. 4. Three-particle correlation functions are sensitive to the tion processes in Abelian plasmas, to formal analogies with the ther- appearance of a Mach cone in the direction opposite to the “near- mal properties of black holes, and to the possibility that plasma side” jet. These preliminary PHENIX data from gold–gold collisions instabilities accelerate equilibration. The intellectual richness of the at 200 GeV may thus give access to an essential property of a field was further illustrated by exciting reports from string theory, perfect liquid – the non-dissipative propagation of energy in shock where theorists have succeeded for the first time in calculating the waves through matter produced in heavy-ion collisions. viscosity to entropy density ratio in the physically relevant, strong- coupling limit of a certain class of thermal non-Abelian gauge theo- which will significantly enhance their abilities to characterize specific ries. The fact that this ratio is found to be very small indicates a properties of the matter created in heavy-ion collisions. Moreover, non-dissipative behaviour. It raises the exciting possibility that the Brookhaven envisages a luminosity upgrade of RHIC, which will non-dissipative character of an almost perfect liquid, which may be open yet another class of novel opportunities. Finally, the newly created in gold–gold collisions at RHIC, could be understood from approved Facility for Antiproton and Ion Research at the GSI first-principles calculations in QCD. Darmstadt is preparing for the start of a versatile heavy-ion pro- From the point of view of heavy-ion phenomenology, the central gramme in the next decade. Plenary talks provided overviews of the question of whether more direct signals of negligible viscosity can be status and possibilities of these three programmes. The field is now established led to another highlight of the conference. The widely dis- eagerly awaiting its future, the next slice of which will be served at cussed idea was that if dissipation is negligible, then energy, the 19th Quark Matter conference in Shanghai in November 2006. deposited by a jet in dense QCD matter, must propagate in a characteristic Mach cone, determined by the velocity of sound in the Further reading quark-gluon plasma. Reports about back-to-back particle correlations More details about the QM ’05 Conference, including an archive of from PHENIX, which may show such a Mach-cone-like structure, were the talks, videos and photos, can be found at the conference home hotly debated amongst theorists and experimentalists alike (see fig- page, http://qm2005.kfki.hu/. See also a press release by the ure 4). Most importantly, these discussions showed that heavy-ion Hungarian Academy of Sciences, www.mta.hu/index.php?id=858& physics at collider energies has a large set of novel tools available for backPid=856&begin_at=30&tt_news=1530&cHash=f55085886a. the controlled experimentation with hot and dense QCD matter, and that the field is moving towards characterizing specific properties of Résumé this matter, including its speed of sound, equation of state, and its “Quark Matter” à Budapest transport coefficients such as heat conductivity and viscosity. La 18e Conférence internationale sur les collisions noyau-noyau Past, present and future ultrarelativistes, “Quark Matter 2005”, a permis des échanges The Quark Matter conferences not only highlight the experimental animés sur les derniers résultats en physique des ions lourds. harvest of the recent past and the latest news from theory, they are Elle a porté essentiellement sur de nouvelles données recueillies also the arena for assessing perspectives for the future. The first au collisionneur d'ions lourds relativistes (RHIC) du Laboratoire heavy-ion beam at the Large Hadron Collider (LHC) at CERN is national de Brookhaven et sur une synthèse de l’interprétation expected in 2008, and heavy-ion researchers are now well prepared des données sur les ions lourds produites par des expériences for the jump in centre-of-mass energy by a factor of 30 above RHIC. menées au Supersynchrotron à protons du CERN (notamment Most importantly, the fact that dramatic medium-sensitive effects NA60). Figuraient aussi au programme divers grands thèmes persist unweakened at RHIC up to the highest measured transverse théoriques de la phénoménologie des ions lourds, de la théorie momentum strongly supports the expectation that the new kine- des champs à température et/ou densité finies, d’astrophysique matic regime accessible at the LHC will provide many qualitatively et de physique du plasma. 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 Tamás Csörgö and Péter Lévai, KFKI Research Institute for physics in the next decade. At Brookhaven, the STAR and PHENIX Particle and Nuclear Physics, Budapest, Helmut Satz, University collaborations are lining up for several important detector upgrades, of Bielefeld, and Jurgen Schukraft and Urs Wiedemann, CERN.

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