Collisions of heavy ions (nuclei) at ultra-relativistic energies (sqrt(s_NN)
>> 10 GeV per nucleon-nucleon collision in the centre of mass system) are
regarded as a unique tool to produce in the laboratory a high energy density
and high temperature state of strongly-interacting matter. In this short
review, we will discuss the expected features of this hot and dense state,
describe indications on its properties emerged from the experimental programs
at the CERN-SPS and BNL-RHIC accelerators, and finally outlook the perspectives
for the forthcoming heavy-ion runs at the CERN-LHC.Comment: 8 pages, 4 figures, proceedings of the conference IFAE 2010 (Incontri
  di Fisica delle Alte Energie), Roma, April 201

Dainese, A.

English

arXiv

Collisions of heavy ions (nuclei) at ultra-relativistic energies (√sNN >>� 10GeV per nucleon-nucleon collision in the centre of mass system) are regarded as a unique tool to produce in the laboratory a high-energy density and

high-temperature state of strongly-interacting matter. In this short review, we will discuss the expected features of this hot and dense state, describe indications on its

properties emerged from the experimental programs at the CERN-SPS and BNLRHIC accelerators, and finally outlook the perspectives for the forthcoming heavyion runs at the CERN-LHC

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10751-yColloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010Review on heavy-ion physicsA. DaineseINFN, Sezione di Padova - via Marzolo 8, 35131 Padova, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online il 28 Gennaio 2011)Summary. — Collisions of heavy ions (nuclei) at ultra-relativistic energies(√sNN  10GeV per nucleon-nucleon collision in the centre of mass system) areregarded as a unique tool to produce in the laboratory a high-energy density andhigh-temperature state of strongly-interacting matter. In this short review, we willdiscuss the expected features of this hot and dense state, describe indications on itsproperties emerged from the experimental programs at the CERN-SPS and BNL-RHIC accelerators, and finally outlook the perspectives for the forthcoming heavy-ion runs at the CERN-LHC.PACS 12.38.Mh – Quark-gluon plasma.PACS 25.75.Nq – Quark deconfinement, quark-gluon plasma production and phasetransitions.1. – Introduction: phenomenology of hot and dense QCD matterOn the basis of thermodinamical considerations and of QCD calculations, strongly in-teracting matter is expected to exist in different states. Its behaviour, as a function of thebaryonic chemical potential μB (a measure of the baryonic density) and of the tempera-ture T , is displayed in the phase diagram reported in fig. 1 (left). At low temperaturesand for μB  mp  940MeV, we have ordinary matter. When the energy density ε of thesystem is increased, by “compression” (towards the right) or by “heating” (upward), aphase transition is predicted: the density of partons (quarks and gluons) becomes so highthat the confinement of coloured quarks in colourless hadrons vanishes and a deconfinedQuark-Gluon Plasma (QGP) is formed.From a theoretical point of view, the phase diagram can be explored within theframework of lattice QCD [1,2], although calculations are limited to cases of null or smallμB. The phase transition is identified by a large increase of the energy density ε(T ) whenthe temperature T reaches the critical value Tc = (175± 15)MeV [2]. Considering thatthe energy density of an equilibrated ideal gas of particles with ndof degrees of freedomis ε = ndof π230 T4, the dramatic increase of ε/T 4 at Tc can be interpreted as due to thechange of ndof from 3 in the pion gas phase to 37 (with 2 quark flavours) in the deconfinedc© Societa` Italiana di Fisica 115116 A. DAINESEfree hadronspre-equilibriumquark-gluon plasmahadronic matter tzFig. 1. – Left: phase diagram of QCD matter [3]. Right: the expected evolution of a high-energynuclear collision.phase, where the additional colour and quark flavour degrees of freedom are available(1).The energy density for a QGP at Tc is εc ≈ 1GeV/fm3.In heavy-ion collisions, both temperature and density increase, possibly bringing thesystem to the phase transition. In the diagram in fig. 1 (left) the paths estimated for thefixed-target (SIS√sNN  1GeV, AGS√sNN  5GeV, SPS√sNN  17GeV) and collider(RHIC√sNN = 200GeV, LHC√sNN = 2.75–5.5TeV) experiments are shown. Theevolution of a high-energy nucleus-nucleus collision is usually pictured in the form shownin fig. 1 (right). After a rather short equilibration time τ0  0.1–1 fm/c, the presence of athermalized medium is assumed, and for sufficiently-high energy densities, this mediumwould be in the Quark-Gluon Plasma phase. Afterwards, as the expansion reduces theenergy density, the system goes through a hadron gas phase and finally reaches thefreeze-out, when the final state hadrons do not interact with each other anymore.2. – Evidence for deconfinement in heavy-ion collisions: the SPS programmeThe desire to test this fascinating phase structure of strongly-interacting matter led,in the 1990s, to the fixed-target experiments at the AGS in Brookhaven (Au-Au√sNN 5GeV) and at the CERN-SPS (Pb-Pb√sNN  17GeV).The energy density reached in central Pb-Pb collisions at the SPS was estimated (fromthe measured transverse energy of the produced particles [4, 5]) to be ε ≈ 3GeV/fm3,i.e. above the critical value εc. The two clearest indications for the production of adeconfined medium in Pb-Pb collisions at the SPS are:– the enhancement of the production of strange and multi-strange baryons (hyperons)with respect to the rates extrapolated from pp data (predicted by J. Rafelski andB. Mu¨ller in 1982 [6]);(1) In a pion gas the degrees of freedom are only the 3 values of the isospin for π+, π0, π−. Ina QGP with 2 quark flavours the degrees of freedom are ng + 7/8 (nq + nq¯) = Ng(8)Npol(2) +7/8 × 2 × Nflav(2)Ncol(3)Nspin(2) = 37. The factor 7/8 accounts for the difference betweenBose-Einstein (gluons) and Fermi-Dirac (quarks) statistics.REVIEW ON HEAVY-ION PHYSICS 1171101 10 102 10 3pT > 0,   |y-ycm| < 0.5< Nwound >Particle / event / wound. nucl. relative to pBeΛΞ-pBe pPb PbPb1101 10 10 2 10 3pT > 0,   |y-ycm| < 0.5< Nwound >Particle / event / wound. nucl. relative to pBeΛ⎯Ξ⎯+Ω-+Ω⎯+pBe pPb PbPbFig. 2. – Strange baryon production in Pb-Pb per participant nucleon, normalized to the ratiofrom p-Be, as a function of the number of participant nucleons, as measured by NA57 at theSPS [7].– the suppression of the production of the J/ψ meson (the lowest cc bound state),always with respect to the rates extrapolated from pp (predicted by T. Matsui andH. Satz in 1986 [8]).In the QGP, the chiral symmetry restoration decreases the threshold for the produc-tion of a ss pair from twice the constituent mass of the s quark, ≈ 600MeV, to twice thebare mass of the s quark, ≈ 300MeV, which is less than half of the energy required toproduce strange particles in hadronic interactions. In the QGP, multi-strange baryonscan be produced by statistical combination of strange (and non-strange) quarks, while ina hadronic gas they have to be produced through a chain of interactions that increase thestrangeness content in steps of one unit. For this reason a hyperon enhancement growingwith the strangeness content was indicated as a signal for QGP formation. This effectwas, indeed, observed by the WA97/NA57 experiment [7]: in fig. 2 one can see that theproduction of strange and multi-strange baryons increases by 10 times and more (up to20 times for the Ω) in central Pb-Pb collisions in comparison to p-Be, where the QGPis not expected. As predicted, the enhancement E is increasing with the strangenesscontent: E(Λ) < E(Ξ) < E(Ω).The other historic predicted signal of deconfinement was observed by the NA50 andNA60 experiments (in Pb-Pb and In-In collisions, respectively): the suppression of theJ/ψ particle yield, with respect to the reference Drell-Yan process [9,10]. The suppressionis interpreted as due to the fact that, in the high colour-charge density environment ofa QGP, the strong interaction between the two quarks of the cc pair is screened and theformation of their bound state is consequently prevented.3. – RHIC: focus on new observablesThe Relativistic Heavy Ion Collider (RHIC) in Brookhaven began operation duringsummer 2000. With a factor 10 increase in the centre-of-mass energy with respect tothe SPS,√sNN up to 200GeV, the produced collisions are expected to be well abovethe phase transition threshold. Moreover, in this energy regime, the so-called “hard pro-cesses” —production of energetic partons (E > 3–5GeV) out of the inelastic scattering118 A. DAINESE (GeV/c) Tp0 2 4 6 8 10 12 14 16 18 20 22      AA Rη   or     AA R0π00.20.40.60.811.2 , PRL 101, 232301(2008)0πη = 200 GeVNNsAu+Au Minimum Bias  PHENIX0 2 (GeV/c)Tp4 6 8 1000.511.52 d+Au FTPC-Au 0-20%d+Au Minimum Bias T /c)Au+Au CentralR AB(p T)Fig. 3. – The ratio of transverse-momentum distributions of hadrons in Au-Au collisions andpp collisions, scaled by the number of binary nucleon-nucleon collisions at√sNN = 200GeV.Left: PHENIX data for π0 and η [18]. Right: STAR data for charged hadrons [19]; also theratio d-Au to pp is shown.of two partons from the colliding nuclei— have a significantly large cross section andthey become experimentally accessible.In this scenario, beyond the “traditional” observables we have already introduced, theinteresting phenomenon of in-medium parton energy loss [11-13], predicted for the firsttime by J. D. Bjorken in 1982 [11], can be addressed.Hard partons are produced at the early stage of the collision and they propagatethrough the medium formed in the collision. During this propagation they undergointeractions (both inelastic and elastic) with the gluons present in the medium and theylose energy. Such energy loss is not peculiar of a deconfined medium, but, quantitatively,it is strongly dependent on the nature and on the properties of the medium, beingpredicted to be much larger in the case of deconfinement.The measurement of high-pt (projection of the momentum on the plane transverseto the beam line) particle production has been addressed at RHIC mainly by thePHENIX [14] and STAR [15] experiments, but has been observed also by the two smallerexperiments BRAHMS [16] and PHOBOS [17]. The results are striking. Figure 3 reportsthe yield of neutral and charged hadrons measured by PHENIX [18] and STAR [19] incentral Au-Au collisions at√sNN = 200GeV, divided by the yield in pp collisions atthe same energy and by the estimated number of binary nucleon-nucleon collisions. Thisratio, RAA, should be one at high pt if no medium effects are present. In central colli-sions the yield is reduced of a factor 4–5 with respect to what expected for incoherentproduction in nucleon-nucleon collisions.Another impressive result, obtained by STAR and PHENIX, is the disappearance ofthe back-to-back azimuthal correlations of high-pt particles in central Au-Au collisions.In fig. 4 the azimuthal correlations of charged particles with respect to a high-pt triggerparticle (star markers) in central Au-Au are shown and compared with reference datafrom pp collisions (histogram): in central collisions the opposite-side (Δφ = ±π) corre-lation is strongly suppressed with respect to the pp and peripheral Au-Au cases. Thiseffect suggests the absorption of one of the two jets (usually produced as back-to-backpairs) in the hot matter formed in central collisions.The effects of leading particle and jet suppression are not observed in d-Au (deuteron-gold) collisions at√sNN = 200GeV (circle and triangle markes in figs. 3-right and 4),where the formation of a dense medium is not expected.REVIEW ON HEAVY-ION PHYSICS 11900.10.2 d+Au FTPC-Au 0-20%d+Au min. bias00.10.2 p+p min. biasAu+Au central(1/N trigger) dN/d(Δφ  )Δφ  (radians)0 π /2 π (a)(b) h++h-Fig. 4. – Azimuthal correlations of charged particles relative to a high-pt trigger particle forcentral Au-Au, compared to d-Au and pp collisions at√sNN = 200GeV [19].In summary, experiments at RHIC have shown that a very dense QCD medium isformed in high-energy heavy-ion collisions. Other measurements, namely elliptic flow andbaryon-to-meson ratios, indicate that this medium is characterized by partonic degrees offreedom and that its expansion and cooling is well described by hydrodynamical modelswith high viscosity. Thus, this medium is more similar to a liquid than to a gas of gluonsand quarks.4. – LHC: study of “deeply deconfined” matterThe Large Hadron Collider (LHC) started operation with pp collisons at√s =900GeV (2009) and 7TeV (since March 2010). The first Pb-Pb run, at√sNN = 2.75TeV,is scheduled for November 2010. The maximum Pb-Pb energy of 5.5TeV should bereached in 2013. LHC will provide nuclear collisions at a centre-of-mass energy 15–30times higher than at RHIC, opening a new era for the field, in which particle produc-tion will be dominated by hard processes, and the energy densities will possibly be highenough to treat the generated Quark-Gluon Plasma as an ideal gas. These qualitativelynew features will allow to address the task of the LHC heavy ion program: a systematicstudy of the properties of the Quark-Gluon Plasma state.Table I presents a comparison of expected values for the most relevant parameters forSPS, RHIC and LHC energies [20]. At the LHC, the high energy in the collision centre ofmass is expected to determine a large energy density and an initial temperature at leasta factor 2 larger than at RHIC. This high initial temperature extends also the lifetimeand the volume of the deconfined medium, since it has to expand while cooling down tothe freeze-out temperature, which is ≈ 170MeV (it is independent of √s, above the SPSenergy). In addition, the large expected number of gluons favours energy and momentumexchanges, thus considerably reducing the time needed for the thermal equilibration of themedium. To summarize, the LHC will produce hotter, larger and longer-living “drops”of QCD plasma than the previous heavy-ion facilities.120 A. DAINESETable I. – Comparison of the parameters characterizing central nucleus-nucleus collisions atdifferent energy regimes [20].Parameter SPS RHIC LHC√sNN [GeV] 17 200 2750–5500dNgluons/dy  450  1200  5000dNch/dy 400 650  2000–3000Initial temperature [MeV] 200 350 > 600Energy density [GeV/fm3] 3 25 120Freeze-out volume [fm3] few 103 few 104 few 105Lifetime [fm/c] < 2 2–4 > 10The key advantage in this new “deep deconfinement” scenario is that the Quark-GluonPlasma studied by the LHC experiments will be much more similar to the Quark-GluonPlasma that can be investigated from a theoretical point of view by means of latticeQCD.Heavy-ion collisions at the LHC access not only a quantitatively different regime ofmuch higher energy density but also a qualitatively new regime, mainly because:– High-density parton distributions are expected to dominate particle production.The number of low-energy partons (mainly gluons) in the two colliding nuclei is,therefore, expected to be so large as to produce a significant initial-state saturationthat would be the dominant effect governing the bulk particle production.– Hard processes should contribute significantly to the total nucleus-nucleus cross-section. The hard probes are at the LHC an ideal experimental tool for a detailedcharacterization of the QGP medium.Three experiments will study Pb-Pb collisions at the LHC: ALICE (the dedicatedheavy-ion experiment), ATLAS, and CMS. Their excellent detector capabilities, withmany complementary features, will allow to pursue an exciting heavy-ion physics pro-gram. As mentioned before, the goal of this program is a systematic study of the prop-erties of the QGP state. In detail:– Energy density of the hot and dense system produced in the collision, via measure-ment of the charged-particle multiplicity and charged-particle rapidity density.– Temperature and baryon density of the system at the chemical freeze-out andtemperature at the kinetic freeze-out, via measurement of identified-particle yields(probe chemical freeze-out) and momentum spectra (probe kinetic freeze-out).– Hadronization mechanism of the system: interplay between quark recombinationfrom a partonic medium and parton fragmentation outside the medium, via mea-surement of the baryon-to-meson ratios as a function of momentum and rapidity.– Size of the hot particle-emitting source, via Bose-Einstein interferometry with iden-tical bosons.REVIEW ON HEAVY-ION PHYSICS 121– Pressure-driven expansion of the system, to be compared to hydrodynamical mod-els, via measurement of radial flow and elliptic flow (azimuthal particle productionanisotropy in non-central collisions).– Fluctuations induced by the QCD phase transition, via measurement of event-by-event particle spectra– Effect of parton energy loss on heavy quarks (charm and beauty): dependence ofenergy loss on the parton colour charge (c quarks vs. gluons) and on the partonmass (b quarks vs. c quarks), via measurement of transverse momentum spectra ofD and B mesons.– Effect of parton energy loss on jet structure and jet fragmentation function, viameasurement of reconstructed jets and photon-tagged jets (provide initial jet energycalibration).– Quarkonium states (J/ψ, ψ′, Υ, Υ′, Υ′′) suppression pattern by temperature-dependent dissociation due to colour screening in a deconfined medium, via mea-surement of charmonium and bottomonium yields.– Initial temperature of the QGP (which would radiate thermal photons), via mea-surement of the transverse momentum spectrum of single photons.5. – ConclusionsHigh-energy heavy-ion physics addresses the limits of colour confinement and theproperties of QCD in extended high-density systems.Experiments at the SPS and RHIC accelerators have provided indications on theproperties of the medium formed in the collisions: the medium is deconfined, partonic,dense, and hydrodynamic.The LHC machine and experiments will bring a big step forward in the explorationof these new territories of QCD, because the medium will be hotter and denser, becausethe probes for its characterization will be abundantly available, and because the newgeneration detectors will provide unprecedented experimental performance.∗ ∗ ∗The author thanks the Organizers of the IFAE2010 Conference for the kind invitation.REFERENCES[1] Wilson K. G., Phys. Rev. D, 10 (1974) 2445.[2] Karsch F. et al., Phys. Lett. B, 478 (2000) 447.[3] Heinz U., Presentation given for CERN Press Release “A New State of Matter”, Feb.2000: http://cern.web.cern.ch/CERN/Announcements/2000/NewStateMatter/Slides/.[4] Bjorken J. D., Phys. Rev. D, 27 (1983) 140.[5] Abreu M. C. et al. (NA50 Collaboration), Phys. Lett. B, 477 (2000) 28.[6] Mu¨ller B. and Rafelski J., Phys. Rev. Lett., 48 (1982) 1066.[7] Antinori F. et al. (NA57 Collaboration), J. Phys. G, 32 (2006) 427.[8] Matsui T. and Satz H., Phys. Lett. B, 178 (1986) 416.[9] Alessandro B. et al. (NA50 Collaboration), Eur. Phys. J. C, 39 (2005) 335.[10] Arnaldi R. et al. (NA60 Collaboration), Phys. Rev. Lett., 99 (2007) 132302.122 A. DAINESE[11] Bjorken J. D., FERMILAB-PUB-82-59-THY.[12] Gyulassy M. and Wang X. N., Nucl. Phys. B, 420 (1994) 583, arXiv:nucl-th/9306003.[13] Baier R., Dokshitzer Y. L., Mueller A. H., Peigne´ S. and Schiff D., Nucl. 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