TThe SPS ion program and the first LHC data
Marek Gazdzicki
Goethe University Frankfurt, Germany and Jan Kochanowski University, Kielce, Poland
Abstract.
For the first time in the CERN history two experimental programs devoted to studynucleus–nucleus collisions at high energies are performed in parallel. In the SPS ion program,carried out by NA61/SHINE, interactions of light and medium size ions in the energy range √ s NN = 5–20 GeV are investigated. The program aims to discover the critical point of stronglyinteracting matter as well as establish properties of the onset of deconfinement. In 2010 ALICE,ATLAS and CMS at LHC recorded first data on Pb+Pb collisions at the highest energy reached upto now, √ s NN = 2760 GeV. This opens a new exciting area in the field of heavy ion collisions.The relation between the two programs is discussed in this presentation. Surprisingly, the firstLHC results strongly support the NA49 discovery of the onset of deconfinement and thus furtherexperimental study of nucleus-nucleus collisions at the CERN SPS. Keywords: heavy ion collisions, onset of deconfinement, critical point
PACS:
INTRODUCTION
Experimental studies of nucleus-nucleus (A+A) collisions started at the Super ProtonSynchrotron (SPS) of the European Organization for Nuclear Research (CERN) in themid 1980s. They were motivated by the possibility to discover a new state of matter, thequark-gluon plasma (QGP). Firstly, beams of oxygen and sulfur at 60 A and 200 A GeV/cwere available. Then, starting from the mid 1990s the lead beam at 158 A GeV/c wasused. These pioneering studies suggested that matter of unusual properties is created atthe early stage of A+A collisions at the top SPS energy [1]. Unambiguous evidence ofthe QGP was, however, missing. This should be attributed to the difficulty of obtainingunique predictions of the QGP signals from the theory of strong interactions, the QCD.For this reason, at the end of 1990, the NA49 Collaboration at the CERN SPS startedsystematic search for the signals of the onset of QGP creation. This search was motivatedby a statistical model of the early stage of A+A collisions [2] predicting that the onsetof deconfinement should lead to rapid changes of the collision energy dependence ofbulk properties of produced hadrons, all appearing in a common energy domain. Dataon central Pb+Pb collisions at 20 A , 30 A , 40 A , 80 A and 158 A were recorded and thepredicted features were observed at low SPS energies [3, 4].The NA49 evidence for the onset of deconfinement motivates the ion program ofNA61/SHINE [5] at the CERN SPS, as well as the beam energy scan at BNL RHICand the construction of the NICA ion collider at JINR, Dubna. The basic goals of thisexperimental effort are the study of the properties of the onset of deconfinement andthe search for the critical point of strongly interacting matter [5]. Both relay on thecorrectness of the NA49 results and their interpretation.Up to recently, the evidence for the onset of deconfinement was based on data of a a r X i v : . [ nu c l - e x ] S e p ingle experiment, NA49 at the CERN SPS. Thus, an independent verification of therelevant NA49 measurements is important. Furthermore, it is very crucial to confirmthe interpretation of the NA49 results in terms of the onset of deconfinement, firstly,for confirming the discovery of the onset of deconfinement and, secondly, to strengthenarguments for the NA61/SHINE and other experimental programs with high energy ionbeams.This year rich data from the RHIC beam energy scan program were released [6].They agree with the NA49 measurements relevant for the onset of deconfinement.Furthermore, the first results on Pb+Pb collisions at the CERN LHC were presented [7].The latter strongly confirm the interpretation of the NA49 results as an observation ofthe onset of deconfinement. This contribution summarizes the status of the evidence forthe onset of deconfinement including the new LHC and RHIC results. STATUS OF THE EVIDENCE FOR THE ONSET OFDECONFINEMENT
The NA49 evidence for the onset of deconfinement [3, 4] is based on the observation thatnumerous hadron production properties measured in central Pb+Pb collisions changetheir energy dependence in a common energy domain (starting from √ s NN ≈ ≈ A GeV/c beam momentum)) and that these changes are consistent with the predictionsfor the onset of deconfinement [2]. The four representative plots with the structuresreferred to as horn , kink , step and dale [4] are shown in Fig. 1. They present theexperimental results available in the mid of 2010.The relation between the horn , kink , step and dale structures and the onset of de-confinement is briefly discussed below. More detailed explanation is given in Ref. [4],where a comparison with quantitative models is also presented. The horn.
The most dramatic change of the energy dependence is seen for the ratioof particle yields of kaons and pions, Fig. 1 (top-left). The steep threshold rise of theratio characteristic for confined matter changes at high energy into a constant value atthe level expected for deconfined matter. In the transition region (at low SPS energies) asharp maximum is observed caused by the higher strangeness to entropy production ratioin confined matter than in deconfined matter. This feature is not observed for proton–proton reactions as shown by the open dots in Fig. 1 (top-left).
The kink.
The majority of all particles produced in high energy interactions are pions.Thus, pions carry basic information on the entropy created in the collisions. On theother hand, entropy production should depend on the form of matter present at the earlystage of collisions. Deconfined matter is expected to lead to a final state with higherentropy than that created by confined matter. Consequently, the entropy increase at theonset of deconfinement is expected to lead to a steeper increase of the collision energydependence of the pion yield per participating nucleon. This effect is observed for centralPb+Pb collisions as shown in Fig. 1 (top-right). When passing the low SPS energies theslope of the (cid:104) π (cid:105) / (cid:104) N P (cid:105) vs F ≈ (cid:112) √ s NN dependence increases by a factor of about 1.3.Within the statistical model of the early stage [2] this corresponds to an increase of theeffective number of degrees of freedom by a factor of about 3. (GeV) NN s1 10 æ + pÆ / æ + K Æ NA49AGSRHICp+p ) F (GeV0 5 10 15 〉 P N 〈 / 〉 π 〈 NA49AGSRHICFIT p+ppp+ (GeV) NN s1 10 T ( M e V ) + K AGSNA49RHICp+p (GeV) NN s1 ( L a nd a u ) y s ) / - p ( y s Pb+Pb Au+AuAGSSPS(NA49)RHIC
FIGURE 1.
Heating curves of strongly interacting matter, status at the mid of 2010. Hadron productionproperties (see Ref. [4] for details) are plotted as a function of collision energy ( √ s NN and F ≈ (cid:112) √ s NN )for central Pb+Pb (Au+Au) collisions and p+p interactions (open circles): top-left – the (cid:104) K + (cid:105) / (cid:104) π + (cid:105) ratio,top-right – the mean pion multiplicity per participant nucleon, bottom-left – the inverse slope parameterof the transverse mass spectra of K + mesons, bottom-right: the width of the π − rapidity spectra relativeto predictions of the Landau ideal hydrodynamics. The observed changes of the energy dependence forcentral Pb+Pb (Au+Au) collisions are related to: decrease of the mass of strangeness carriers and the ratioof strange to non-strange degrees of freedom ( horn : top-left plot), increase of entropy production ( kink :top-right plot), weakening of transverse ( step : bottom-left plot) and longitudinal ( dale : bottom-right plot)expansion at the onset of deconfinement. The step.
The experimental results on the energy dependence of the inverse slope pa-rameter, T , of K + and K − transverse mass spectra for central Pb+Pb (Au+Au) collisionsare shown in Fig. 1 (bottom-left). The striking features of the data can be summarizedand interpreted [8] as follows. The T parameter increases strongly with collision energyup to the low SPS energies, where the creation of confined matter at the early stage of thecollisions takes place. In a pure phase increasing collision energy leads to an increase ofthe early stage temperature and pressure. Consequently the transverse momenta of pro-uced hadrons, measured by the inverse slope parameter, increase with collision energy.This rise is followed by a region of approximately constant value of the T parameter inthe SPS energy range, where the transition between confined and deconfined matter withthe creation of mixed phases is located. The resulting softening of the equation of state,EoS, ‘suppresses’ the hydrodynamical transverse expansion and leads to the observedplateau structure in the energy dependence of the T parameter [8]. At higher energies(RHIC data), T again increases with the collision energy. The EoS at the early stage be-comes again stiff and the early stage pressure increases with collision energy, resultingin a resumed increase of T . The dale.
As discussed above, the weakening of the transverse expansion is expecteddue to the onset of deconfinement because of the softening of the EoS at the early stage.Clearly the latter should also weaken the longitudinal expansion. This expectation ischecked in Fig. 1 (bottom-right), where the width of the π − rapidity spectra in centralPb+Pb collisions relative to predictions of the Landau ideal hydrodynamics is plottedas a function of the collision energy. In fact, the ratio has a clear minimum at low SPSenergies.In 2011 new results on central Pb+Pb collisions at the LHC and data on centralAu+Au collisions from the RHIC beam energy scan program were released. The updatedplots [9] are shown in Fig. 2. The RHIC results [6] agree with the NA49 measurementsat the onset energies.The LHC data [7] demonstrate that the energy dependence of hadron production prop-erties shows rapid changes only at low SPS energies. A smooth evolution is observedbetween the top SPS (17.2 GeV) and the current LHC (2.76 TeV) energies. This stronglysupports the interpretation of the NA49 structures as due to the onset of deconfinement.Above the onset energy only a smooth change of the quark-gluon plasma properties withincreasing collision energy is expected. Consequently, in agreement with the first LHCdata, one expects: • an approximate independence of the K + / π + ratio of energy above the the top SPSenergy, Fig. 2 (top-left), • a linear increase of the pion yield per participant with F with the slope defined bythe top SPS data, Fig. 2 (top-right), • a monotonic increase of the kaon inverse slope parameter with energy above thetop SPS energy, Fig. 2 (bottom).The width of the π − rapidity spectra in central Pb+Pb collisions relative to predictionsof the Landau ideal hydrodynamics should continuously increase from the top SPS toLHC energies. The LHC data on rapidity spectra are needed to verify this expectation.The confirmation of the relevant NA49 measurements and their interpretation in termsof the onset of deconfinement by the new LHC and RHIC data strengthen the argumentsfor the planned [5] NA61/SHINE measurements with secondary Be and primary Ar aswell as Xe beams in the SPS beam momentum range (13 A -158 A GeV/c). (GeV) NN s1 ) » ( y + p / + K Pb+Pb Au+Au
AGSSPS(NA49)RHICLHC(ALICE) ) F (GeV0 10 20 30 40 50 æ w N Æ / æpÆ Pb+Pb Au+Au
AGSSPS(NA49)RHICLHC(ALICE)FIT (3.2 < F < 15)FIT (F < 1.85) (GeV) NN s1 T ( M e V ) Pb+Pb Au+Au
AGSSPS(NA49)RHICLHC(ALICE) + K (GeV) NN s1 T ( M e V ) AGSSPS(NA49)RHICLHC(ALICE) - K Pb+Pb Au+Au
FIGURE 2.
Heating curves of strongly interacting matter: status at the mid of 2011. For more detailssee see Ref. [4] and the caption of Fig. 1. The new LHC and RHIC data are included in the horn (top-left), kink (top-right) and step (bottom) plots. The K + / π + ratio is measured by ALICE [7] and STAR [6] atmid-rapidity only and thus the horn plot is shown here for the mid-rapidity data. There are no new resultsfor the dale plot. ACKNOWLEDGMENTS
I would like to thank the organizers of the workshop on Early Physics with heavy–Ionsat LHC in Bari for their kind invitation to this stimulating and pleasant event. This workwas supported by the German Research Foundation under grant GA 1480/2-1.
REFERENCES
1. U. W. Heinz, M. Jacob, [nucl-th/0002042].2. M. Gazdzicki, M. I. Gorenstein, Acta Phys. Polon.
B30 , 2705 (1999). [hep-ph/9803462].3. C. Alt et al. [NA49 Collaboration], Phys. Rev. C , 024903 (2008) [arXiv:0710.0118 [nucl-ex]].4. M. Gazdzicki, M. Gorenstein, P. Seyboth, Acta Phys. Polon. B42 , 307 (2011) [arXiv:1006.1765 [hep-ph]].5. N. Antoniou et al. [NA61/SHINE Collaboration], CERN-SPSC-2006-034.. L. Kumar [ for the STAR Collaboration ], [arXiv:1106.6071 [nucl-ex]],B. Mohanty [ STAR Collaboration ], [arXiv:1106.5902 [nucl-ex]].7. J. Schukraft et al. [ for the ALICE Collaboration ], [arXiv:1106.5620 [hep-ex]],A. Toia et al. [ for the ALICE Collaboration ], [arXiv:1107.1973 [nucl-ex]].8. M. I. Gorenstein, M. Gazdzicki and K. A. Bugaev, Phys. Lett. B , 175 (2003) [arXiv:hep-ph/0303041].9. A. Rustamov,, 175 (2003) [arXiv:hep-ph/0303041].9. A. Rustamov,