In the following, we will concentrate on the unique physics aspects of the experimental programme on heavy-ion physics at LHC and the main characteristics of the experiments to cover it

Paic, G

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1Heavy-ion physics at LHCG. PaicaaCERN, EP Division, Geneva1. INTRODUCTIONHeavy-ion collisions are a part of the complete set of inv estigations that the LargeHadron Collider (LHC) | to enter into operation in 2006 | wants to explore, namely theconnection between phase transitions in v olvingelementary quantum elds, fundamentalsymmetries of nature and the origin of masses. This programme is distributed over thefour planned experiments in the follo wing way.1. CMS and ATLAS search for particles predicted b y the Standard Model resultingfrom breaking of gauge symmetry of the electroweak force [the Higgs particle(s)]related to the mass generation mechanism of the electroweak gauge bosons. Beyondthe Standard Model they will also search for supersymmetric particles which wouldbe manifestations of a broken in trinsic symmetrybetween fermions and bosons.2. LHCb performs precision tests of the Standard Model Lagrangian and its CP sym-metry violating terms b ystudying the physics of the heavy b-quark.3. ALICE [1,2] and CMS [3] study QCD collective phenomena at extreme energy andparton densities. The aim of the heavy-ion physics at LHC is to study equilibriumand non-equilibrium eects in strongly interacting matter at very large energy den-sities (up to 100 GeV=fm3), as well as the collective evolution of the created systemto wards hadronization and freezout. Such studies should giv eexperimental insightin to the structure of the QCD phase diagram which predicts that at energy densitiesof  0:6 GeV=fm3n uclear matter undergoes a phase transition to a deconned stateof quarks and gluons. This transition is connected with the restoration of the chiralsymmetry where the quark masses are reduced from their large eective values totheir small bare ones. It is in teresting to note that this phase transition in v olvingquantum elds occurs at energy densities available in the laboratory and as such isthe only one directly accessible.In the follo wingwe will concentrate on the unique physics aspects of the experimentalprogramme on heavy-ion physics at LHC and the main characteristics of the experimentsto cov er it.   ALICE-PUB-2001-36             Publication24 July 200122. SOME BASIC CONSIDERATIONS2.1. Lattice gauge calculationsOur best handle to explore theoretically the phase transition and some of its charac-teristics is the lattice gauge theory (LGT). In spite of its limitations (assumption of zerobaryon density, static character of the simulations) it is the best existing guideline. InFig. 1 we show the dependence of the energy density on temperature as currently pre-dicted [4]. The critical energy density as reported by the LGT is "c= (6  2)Tc4andthe critical temperature is around 170 MeV with an error of about 10%. Although theerror may seem small, the fourth power in the exponent entails a considerable error onthe predicted energy density!T (MeV)1701614121086420 1.0 1.5 2.0 2.5 3.0 3.5 4.0210250 340 510LHCRHIC680T / Tcε / T4εSB / T46.34.32.91.80.6 GeV / fm3 = εcFigure 1. Phase diagram as obtained from lattice gauge calculations.Experimentally the measurement of the achieved energy density is not straightforward.The expression that links the energy density with the experimental observable is" = 3=2(dNch=dy)hEi=V;where (dNch=dy) represents the number of charged particles per unit of rapidity, hEi is theaverage energy carried by the particles, and V is the volume of the system. In its originalform derived by Bjorken the volume is chosen to be the transverse area of the collidingnuclei multiplied by 1 fm: The longitudinal length chosen corresponds (taking into accountthat the nuclei are longitudinally contracted so as to form two colliding `pancakes') toan estimated time interval for the formation of an equilibrated system. The well-foundedassumption that the equilibration times at LHC may be considerably shorter brings animportant experimental uncertainty. So the correct procedure for reporting experimentalresults would be to speak about the energy per unit area rather than per unit volume.There is another important message from the phase diagram: the right-hand side indi-cates the energy density limit for a Stefan{Boltzmann gas. The fact that all the lattice3gauge calculations miss that limit by a signicant amount is usually interpreted as anindication that the system of quarks and gluons above the critical temperature does notbehave as a free gas but that residual interactions are still present. This prompts theclaim that we are faced with a `quark{gluon soup' rather than with ideal plasma.Finally on the upper part of the phase diagram are marked the ranges of temperaturesachievable at the Relativistic Heavy Ion Collider (RHIC) in operation at BrookhavenNational Laboratory since 2000) and at LHC.2.2. Parton distributions and the size of the colliding objectsThe fast moving hadrons or nuclei represent a system consisting of three valence quarks:sea gluons and quark{antiquark pairs; each of these species has a distinct momentumdistribution represented by the parton distribution function.The partons are conned to a longitudinal scale | l  1=p  1=(p0x) wherex = 2pt=ps by the uncertainty relation. The nucleus itself is Lorentz contracted to asize L given by L = 2Rm=p where m and p are the mass and momentum of nucleons inthe nucleus, respectively. In the case when l > L the partons will, depending on theirx-value, eectively overlap each other and cover a longitudinal space that is larger thanL:The contribution of the three parton categories to the overall momentum as a functionof the x-variable is shown in Fig. 2.10010—110—110—210—4 10—3 10—2 10—1 10010—110110110010010210—2 10—2(a) valenceLHCMRST LOGRV94 LO(b) sea(c) gluonx10—4 10—3 10—2 10—1 100x10—4 10—3 10—2 10—1 100xxg (x)xqS (x)xqV (x)Figure 2. Proton parton distributions at Q2= 4 GeV2for two dierent parametrizations.The shaded area shows the range of x-values of the partonic distributions available forinvestigation at LHC. The range of x-values at RHIC is 20 times larger. One can deducefrom Fig. 3 that the partonic densities will be much dierent at both accelerators.4Figure 3. x-dependence of the ratio of the gluon distribution function in a Pb nucleusand in a proton for two dierent momentum transfers. Adapted from Ref. [5].2.3. Shadowing phenomenaAnother unique aspect of the heavy-ion physics at LHC is the importance of shadowing.It is a well known fact from deep inelastic scattering that the parton distributions in nucleiare modied by multiple interactions along the parton path and by recombination of long-wavelength partons. This eect is very pronounced in the x-value regions at LHC andalmost completely absent at RHIC. This feature tells us that the parton distributions alsodepend on the nucleon content of the nucleus.3. THE COLLISION { A LABORATORY IN ITSELFEssentially the collision can be subdivided in space-time into several well identied anddistinct phases: early pre-equilibrium stage where hard interactions occur; thermalization and subsequent expansion of the system of quarks and gluons; hadronization; freezout.According to the predictions of LGT and the experimental observations made at SPS(see talk by P. Giubellino) we believe that in the second stage a state of dense matterconsisting of quarks and gluons is formed. Our interest is to study the characteristics ofthis state, its interaction with particles crossing it, its behaviour over time, etc.In that sense, the initial encounters that yield some very hard collisions giving rise toeither energetic jets or to heavy quarks | of which a part will combine into quarkonia5| will give us the probe to test the matter created simultaneously. Comparing similarobservables (ptspectra, yields of jets or quarkonia, particle multiplicities, fragmentationfunctions, etc.) with the same observables measured in pp collisions one may deduce whathas happened to the fast initial particles on their way to the hadronic world.In that sense we can speak about a complete laboratory where the collision preparesboth the `particles' and the `target' to be analysed. The other aspects of the thermalizedsystem will doubtless | owing to the duration of its existence | also yield interestingresults that will not escape experimental scrutiny; however, the possibility of having well-established probes to traverse the quark{gluon system is likely to be the most interestingpart.3.1. The jets and minijetsA large part of the initial interactions will be semi-hard and hard, in contrast with thesofter interactions at lower energies. This qualitatively completely new situation is bestillustrated by the cross-section dependence of pp scattering on the centre-of-mass energyshown in Fig. 4. From the gure we see that at energies of about 200 GeV (today's energyat RHIC ) the proton{proton cross-sections start being dominated by `minijets'. At LHCthe dominance of hard processes is obvious. That a well-understood feature like hardproduction becomes prominent opens interesting avenues of research. For instance, onemay ask about the energy loss the created jets will suer in the surrounding plasma ontheir way out into vacuum. The phenomenon is called `jet quenching' and is the subjectof numerous theoretical discussions today. In his paper from 1982 [6] Bjorken writes that`The high-ptpartons may elastically scatter from the quarks and gluons in the plasmathereby degrading its energy and heating the plasma.' Although the theoretical framehas evolved and experimental observation has been inferred from p{A data [7], one maysay that the magnitude and more detailed features of the eect in AA collisions willbe only understood following experimental data at RHIC and at larger energy at LHC.The experimental observables linked to jet quenching are many: overall multiplicity perevent, particle spectra, dijet vs monojet ratio at high Et; azimuthal correlations, rapiditydistributions, etc.3.2. Interplay of nuclear shadowing and jet quenchingThe experimental observables will have to cope with a constant and pernicious interplayof eects due to shadowing and those due to jet quenching and/or saturation.Jet quenching brings with it an increase in the number of particles: The energy lossin the medium manifests itself as a loss of momentum of the leading partons in the jetsand a subsequent increase of the low-momentum partons, giving rise to low-momentumhadrons in the end. On the other hand, the shadowing phenomenon suppresses the numberof gluons and hence decreases the nal multiplicity. Another eect that inuences themultiplicity is `saturation'. Partons may be attributed an elementary transverse areagiven by =psat2; where psat2corresponds to the saturation momentum. When the sumof these elementary areas becomes larger than the transverse area of the nucleus, namelyRA2, the number of partons saturates and is not allowed to grow any further [8,9]. Thiseect is clearly also a limiting factor for the rise of the multiplicity.6LHCRHICFermilab ISRUA4UA5UA1 σTOTUA1 σTRIGUA1 σJET1201008060402020 102 103 1040σ (mb)s (GeV)IFigure 4. Total, inelastic non-diractive and minijet cross-sections (full lines) from topto bottom, respectively. The dashed line for minijets represents the input cross-section(which violates unitarity) before unitarity regularization. Dashed-dotted and dashed linesfor total and inelastic cross-sections show the behaviour without minijet contribution; fromRef. [10].3.3. Quarkonia production and Debye screeningCharmonium and bottomium states called quarkonia are produced, like jets, at earlytimes. While J= has been extensively studied at SPS and has been the determiningfactor to conrm the evidence for the creation of a quark{gluon plasma, (bb) will becomeaccessible as a probe of the system | because of its large mass | only at RHIC and LHC.The main parameter that is involved is the measurement of the survival probabilities ofthe bound quarkonia states. Through the mechanism of colour screening, the Coulomb-like colour charge potential between the two heavy quarks will be `screened', i.e. modiedby the presence of other colour charges in the medium in the following way:V (r) = exp [ r=D(T )]r;where r is the distance between the heavy quarks and D(T ) is the Debye screeninglength which is a function of the temperature of the system, hence of the energy density.(For an extended account of the screening consult Ref. [11].) In Table 1 we show theradii, binding energies and screening masses for a sample of quarkonia. Measuring thesurvival probability for the quarkonia coming out of the dense environment should give usinformation on the temperature reached in the plasma. Figure 5 shows the dependence ofthe screening potential on the temperature. We see that the LHC range of temperatures(see Fig. 1) should reach the temperature necessary to achieve melting of the :71210100 140 180 220 260 3008642ET1 (GeV)Rmanojet (ET > ET) / Rdijet (ET   > ET)<∆ET> = 0<∆ET> = 9 GeV<∆ET> = 18 GeV11.2Figure 5. Ratio of monojets to dijets as simulated for the CMS barrel calorimeters as afunction of the jet transverse energy [3].Table 1Binding energies, radii, and screening masses for a sample of quarkonia.J=  0a(1P)   0b(1P)M [GeV] 3:07 3:698 3:5 9:445 10:004 9:897r [fm] 0:453 0:875 0:696 0:226 0:509 0:408D[GeV] 0:699 0:357 0:342 1:565 0:671 0:558Figure 6. Screening mass as a function of temperature. The curve is taken from Ref. [11]for the high initial temperature limit.8Table 2 presents the expected performances in the CMS and ALICE detectors for themeasurement of J= and :Table 2Expected performances in the CMS and ALICE detectors for the measurement of J= and :ALICE CMS2:5 <  < 4 jj < 1:3 barrelJ= jj < 2:4 full det.all ptpt> 5 GeV=c2:5 <  < 4 jj < 1:3 barrel jj < 2:4 full det.all ptall pt4. EXPERIMENTS AT LHC AND THE OBSERVABLESAs shown above the experimental context in the study of heavy-ion collisions at LHCis fundamentally dierent from the one we are familiar with at the low-energy machineslike AGS and SPS. Already the rst results from RHIC demonstrate the importance ofwell thought out and multiparameter experiments | experiments where the observablesare measured in the widest range. The underlying physics predicts features that translateinto dierent trends in the results, as pointed to earlier. Hence the experiments will haveto provide suÆcient numbers of measured parameters to possibly disentangle the variouseects. Concretely we will want to measure the identied spectra of particles and compare these with the ones measured in ppand p{A collisions. Measuring the particle ratios as a function of momentum willallow us to get information on the fragmentation function of emerging jets and tocompare them with the corresponding values measured in pp collisions. This willrequire a high quality particle identication | for instance, the ALICE experiment(see below) will use in its set-up all the particle identication methods currentlyavailable (dE=dx; time of ight, Cherenkov light, transition radiation, secondaryvertex reconstruction, kinematical constraints and calorimeters)! The direct ob-servation of jets is also possible although the identication of jets above 100 GeVtransverse energy may be diÆcult because of the large number of low-energy jets.CMS will approach the jet quenching mainly by measuring the dijet vs monojetratio. the azimuthal anisotropy of various particles with respect to the event plane, whichshould give us the possibility to understand the dependence of the yields and/ormomentum spectra on the geometry of the collision zone. Peripheral collisions willpresent a very non-symmetric shape with respect to the reaction plane. Thereforeone expects a correlation of some observables (jet quenching for instance) with thepath length of particles through the medium. (Since the particles from jets are9emitted at early times the emission zone for noncentral events keeps a distinct geo-metrical almond shape and the emission of particles should have dierent patterns inand out of the event plane. This will require changing the centrality of the collisions,i.e. the impact parameter. the behaviour as a function of the initial number of nucleons, calling for the use ofbeams with dierent nuclei masses. Currently at least three dierent systems areconsidered: Pb{Pb, Sn{Sn, and Ar{Ar. In addition, measurements of the proton{proton collisions and p{A collision are necessary to provide comparison data.The two experiments planned for heavy-ion physics are ALICE and CMS. The main dier-ence between the two detectors is that ALICE is devoted to the measurement of heavy-ioncollisions and as such has been specically designed to cover most of the observables inan experiment, while CMS has been optimized for hard pp physics | supersymmetricparticles, jets and Higgs searches. Therefore with a considerable calorimetry they cancover mainly the jet quenching physics and the quarkonia production [3].The ALICE experiment (Fig. 7) has been conceived around a powerful tracking providedby six layers of silicon trackers, a time projection chamber and a transition radiationchamber with tracking capabilities (devoted to measurements of quarkonia, and opencharm and beauty). The tracking detectors and the accompanying time-of-ight detectorsfor particle identication have been built in a rapidity acceptance of jj  0:9:This rapidity acceptance has been deemed suÆcient to be able to measure the globalcharacteristics of the events on an event-by-event basis. A photon spectrometer for themeasurement of the photon energies up to 50 GeV and a high-momentum particle iden-tication (HMPID) cover only partially the central region since their task is inclusivemeasurements. At higher rapidities we have the muon spectrometer for the measurementof quarkonia, the forward multiplicity detectors, the photon multiplicity detector and theT0 detectors. At very high rapidities ( = 6) we have the Zero Degree calorimeters andthe CASTOR calorimeter for the detection of exotic products (Centauros, Strangelets)Figure 7. View of the ALICE experiment.105. CONCLUSIONThe heavy-ion beams at LHC will provide the experimentalists and the theoreticalcommunity investigating the QCD vacuum state with a tool with qualitatively new fea-tures which have been reviewed. In addition, the complete spectrum of currently avail-able oberservables of heavy-ion physics | connected with the thermal expansion andhadronization can also be explored. The detectors ALICE and CMS, with their detectingcapabilities and complementarity, are well set to cover this eld.REFERENCES1. ALICE Collaboration, Technical proposal, CERN/LHC 95{71 (1995).2. ALICE Collaboration, Addendum to the ALICE technical proposal, CERN/LHC 96{32 (1996).3. Heavy ion physics programme in CMS, CMS note 2000/060.4. U. Heinz, hep-ph/0009170; CERN{TH{2000{276.5. K. Eskola, V.J. Kolhinen, R. Vogt, hep-ph/0104124; JYFL{2001{6.6. J.D. Bjorken, Energy loss energetic partons in quark{gluon plasma: possible extinc-tion of high pt jets in hadron-hadron collisions, Fermilab-Pub-82/59-THY.7. M.B. Johnson et al., Phys. Rev. Lett. 86 (2001) 4483.8. J.P. Blaizot and A.H. Mueller, Nucl. Phys. B 289 (1987) 847.9. K. Eskola et al., Multiplicities and transverse energies in central AA collisions atRHIC and LHC from pQCD, saturation and hydrodynamics, hep-ph/0104010.10. A. Capella et al., Phys. Rev. Lett. 58 (1987) 2015.11. R. Vogt, Phys. Rep. 310 (1999) 197.

Heavy-ion physics at LHC

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