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High energy heavy ion interactions and the search for the Quark-Gluon Plasma

High energy heavy ion interactions and the search for the Quark-Gluon Plasma. Alberta Marzari-Chiesa / Univ. TORINO Luciano Ramello / Univ. Piemonte Orientale. NURT 2003 - La Habana, Cuba, October 27-31, 2003. Plan of the presentation.

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High energy heavy ion interactions and the search for the Quark-Gluon Plasma

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  1. High energy heavy ion interactions and the search for the Quark-Gluon Plasma Alberta Marzari-Chiesa / Univ. TORINO Luciano Ramello / Univ. Piemonte Orientale NURT 2003 - La Habana, Cuba, October 27-31, 2003

  2. Plan of the presentation • Introduction to Quark Gluon Plasma physics and heavy ion collisions • Experimental observables to determine the centrality of each collision • Present and future experimental facilities at CERN and at Brookhaven National Lab. • Some results on the global features of the collisions • Specific QGP signatures: enhancement of strange particles, charmonium suppression • Why go to higher energy (experiment ALICE at the LHC) ? A. Marzari-Chiesa and L. Ramello - NURT 2003

  3. Nuclear matter and QGP • “Ordinary” Nuclear Matter is made of nucleons and electrons. Nucleons (and other hadrons) are made of quarks: a nucleon is made of 3 quarks, a meson (, K, ,…) is made of a quark and an antiquark. In ordinary matter the quarks are never free: they are confined inside the hadrons, and their mass is mumd300 MeV, ms 500 MeV. • The energy density of ordinary nuclear matter, for a nucleus of mass number A and radius R = r0A1/3, ro=1.2 fm is 0.14 GeV/fm3 • Quark Gluon Plasma (QGP) is a state of matter in which quarks and gluons are free, and their mass, being the “bare” mass, is smaller: mumd5 MeV, ms 150 MeV. QCD lattice calculations predict that this state occurs when the density is   10 o: when density is increased enough “interpenetration can occur and eventually each quark will find very many others in its immediate neighbourhood. [...] it has no way to remember which of these were the partners in the low-density nucleonic state”(H. Satz - Nature 324 (1986) 116) A. Marzari-Chiesa and L. Ramello - NURT 2003

  4. Phase transition to QGP • Such a high density can be obtained: • by compressing baryons • by heating a mesonic medium, increasing its density by particle production in collisions QCD lattice calculations predict that the phase transition from ordinary nuclear matter to QGP can occur for temperatures T~150-200 MeV and/or for energy densities ε > 2.5-3 GeV/fm3 A. Marzari-Chiesa and L. Ramello - NURT 2003

  5. QGP formation: where (and when) ? • In early Universe (<1ms after Big Bang) • possibly in the core of neutron stars • a transient state in Heavy Ion collisions A. Marzari-Chiesa and L. Ramello - NURT 2003

  6. Why HEAVY ION interactions ? • The system must be “large” (d>>1 fm) and of “long lifetime” ( > 1 fm/c = formation time) • Moreover it must be near equilibrium, and this can be realized only if the number of collisions per particle is > 1 The first condition is satisfied by heavy nuclei: RPb  1.2(208)1/3  7 fm. Also the second one is satisfied, since the mean free path for a hadron is much lower than the dimensions of a heavy nucleus (with  = 0.14 fm-3 and  = 40 mb = 4 fm2 lh=1/ = 1.6 fm << d ). For quarks and gluons lq 0.5 fm , lg 0.2 fm : the situation is therefore even better. Other conditions (temperature, energy density) cannot be “a priori” estimated: they must be determined experimentally. A. Marzari-Chiesa and L. Ramello - NURT 2003

  7. b Peripheral b  0 Very peripheral b = r1+ r2 Central b  0 Heavy nuclei are extended objects • the collision can be quite different, depending on the way in which the nuclei interact. The parameter that describes the collision is the impact parameter b, defined as the minimum distance between the centers of the two nuclei: A. Marzari-Chiesa and L. Ramello - NURT 2003

  8. Geometrical spectator/participant picture • The nucleons inside the interaction volume are called participants, and the other spectators. • Spectators proceed almost unperturbed with momentum close to the one of the beam • Participants interact, and many n-n collisions occur in the interaction volume, producing secondary particles A. Marzari-Chiesa and L. Ramello - NURT 2003

  9. Experimental observables for centrality NA50 experiment at CERN SPS Transverse EnergyET Multiplicity Nch Forward energyEF A. Marzari-Chiesa and L. Ramello - NURT 2003

  10. Experimental observables (cont’d) • Forward Energyis measured with a ZDC (Zero Degree Calorimeter). Spectatorsproceed in beam direction at very forward angles (at SPS: θ <0.5 mrad) and their energy per nucleon is the same of the beam. A calorimeter covering angles < 0.5 mrad measures the total energy of spectators (contribution of secondaries coming from the interactions is negligible at these angles). The number of spectators (Nspec) can be obtained dividing EZDC by the energy per nucleon of the beam, Ebeam (158 GeV for Pb at SPS). The number of participants Npart will be: Npart = A – Nspec = A -EZDC/Ebeam • Charged Multiplicityis measured by (e.g.) asilicon detector, and it scales with the number of participants as well, similarly to transverse energy. The number of charged particles is proportional to the total number of particles (e.g.: p+p0p- are produced in equal amounts). • Transverse energy (defined as ET=Eisinθi) depends only on energy deposited in the interaction volume. It is therefore proportional to the number of participants. ET is invariant under the boost from the C.M. system to the lab system. Some experiments measure the electromagnetic transverse energy ET0, i.e. the e.m. showers from the gamma rays which arise mainly from the neutral pion decays. ET0 is proportional to total transverse energy. A. Marzari-Chiesa and L. Ramello - NURT 2003

  11. Transverse energy distributions The agreement between the model (Venus 4.12) and the data is clearly visible. It is therefore possible, with a rather simple calculation, convert ET or Nch into the number of participants. A. Marzari-Chiesa and L. Ramello - NURT 2003

  12. Multiplicity distributions (1) NA57 experiment at SPS: charged multiplicity Nch in the pseudorapidity range 2<η<4 measured with silicon microstrip detectors Events have been classified in five centrality classes, corresponding to given fractions of the total inelastic Pb-Pb cross-section at 158 A GeV A. Marzari-Chiesa and L. Ramello - NURT 2003

  13. Multiplicity distributions (2) PHENIX, one of the four RHIC experiments, measures charged multiplicity with two different detectors A. Marzari-Chiesa and L. Ramello - NURT 2003

  14. Heavy ion facilities at CERN and BNL • Accelerators for fixed target experiments: • AGS (1986) Si, Au beams • E(max,lab) = 14.5-11.5 AGeV • SPS (1986) O, S, In, Pb beams • E(max,lab) = 200-160 AGeV • Colliders: • RHIC (2000) Au beams • 100+100 AGeV • LHC (2007) Pb beams • 2.7 + 2.7 ATeV @SPS : experiments extremely specialized in studying particular phenomena @ RHIC and LHC: multipurpose experiments A. Marzari-Chiesa and L. Ramello - NURT 2003

  15. CERN accelerators A. Marzari-Chiesa and L. Ramello - NURT 2003

  16. The SPS heavy ion physics program dimuons 2003 hadrons NA60 • 1986 - 1987 : Oxygen @ 60 & 200 GeV/nucleon • 1987 - 1992 : Sulphur @ 200 GeV/nucleon • 1994 - 2000 : Lead @ 40, 80 & 158 GeV/nucleon • 2002 - 2003 : Indium and Lead @ 158 GeV/nucleon dielectrons multistrange NA49 2000 NA57 NA45 Ceres NA50 photons hadrons hadrons strangelets Pb NA52 WA98 WA97 NA44 1994 dimuons hadrons 1992 WA80 NA34/3 Helios-3 NA38 WA94 NA35 NA36 S O NA34/2 Helios-2 WA85 1986 A. Marzari-Chiesa and L. Ramello - NURT 2003

  17. Brookhaven National Lab A. Marzari-Chiesa and L. Ramello - NURT 2003

  18. Complex events • The events are very complex, having a very high multiplicity ( 500 charged tracks a @ SPS, > 1000 tracks @ RHIC). • Nevertheless many measurements have been made and understood •  Formidable experimental challenge expecially for tracking NA49 at CERN SPS uses magnets to separate out charged particles and Time Projection Chambers to measure their trajectories A. Marzari-Chiesa and L. Ramello - NURT 2003

  19. June 12, 2000 at 9pm RHIC event This is one of the first Au-Au collisions recorded by the Time Projection Chamber (TPC) of STAR, one of the four RHIC experiments A. Marzari-Chiesa and L. Ramello - NURT 2003

  20. 2 = + 2 m p m T T Global Measurements (to check whether the phase transition is possible): (1) temperature If a system of particles is in thermal equilibrium at temperature T, the transverse mass distribution is:  measuring the inverse slope of the mT distribution, we can obtain T From the mT spectra (see next slide), it is evident that they are consistent with an exponential low. BUT it is also evident that the slope parameter increases with the particle mass. This was explained with a collective flow (expansion of the interaction volume): which introduces a term that depends on the particle’s mass. A. Marzari-Chiesa and L. Ramello - NURT 2003

  21. 158 AGeV pions and deuterons not included in fit Temperature and flow from mTspectra NA49: Blast wave fit indicates temperature of 122-127 MeV and average flow velocity of 0.48 Pions were not included in the blast wave fit due to significant resonance contribution at low mT V. Friese, NA49, Strange Quark Matter 2003 A. Marzari-Chiesa and L. Ramello - NURT 2003

  22. T~ 190 MeV mT spectra at RHIC • First mT spectra from STAR (for negatively charged pions) show higher temperature with respect to SPS experiments A. Marzari-Chiesa and L. Ramello - NURT 2003

  23. Global Measurements: (2) energy density It is estimated from transverse energy using Bjorken’s model: o= formation time = 1 fm/c R=1.12 A1/3 fm;  = rapidity It is evident that in central Pb-Pb interactions the energy density (3.2 GeV/fm3) is well above the value expected for the phase transition (crit  1 GeV/fm3) A. Marzari-Chiesa and L. Ramello - NURT 2003

  24. PHENIX PRELIMINARY PHENIX PRELIMINARY Number of Clusters in PC1 dET/dh per participant @ percentile NA49-WA98 @ SPS Au+Au 130A GeV Energy in EMCal (GeV) Energy density at RHIC • ET per Participant & per Charged Particle is even higher at RHIC: ET/participant is 50% larger than for SPS; dET/d is 40% higher than SPS, As a consequence, Energy density is higher: approximately, more than 40% larger at RHIC than at SPS since the parameters of the Bjorken formula were calculated for SPS. A. Marzari-Chiesa and L. Ramello - NURT 2003

  25. Quark Gluon Plasma signatures • A probe for deconfined matter (QGP) must: • (obviously) distinguish between confined and deconfined matter • be present in the initial stages of the interaction (the QGP phase) • preserve a memoryof the initial state during the evolution of the system • Several signatures were proposed, and most of them were searched for.The results must be carefully studied, taking into account that: • Signals compete with backgrounds emitted from “normal” nuclear sources • Signals are modified by final-state interactions: after the QGP phase, as soon as the temperature becomes lower, a hadronisation phase occurs, in which the quarks become bound • Here we will present only two signatures: • strangeness enhancement • J/Ψ suppression A. Marzari-Chiesa and L. Ramello - NURT 2003

  26. How to validate QGP signatures • The way of analyzing the results is common to all the signatures: • The effect is measured in light systems as p-p or p-nucleus, where no QGP can be present, and then it is “extrapolated”to heavier systems. • The extrapolation is made assuming that a nucleus-nucleus interaction is the superposition of many nucleon-nucleon interactions. • If the experimental results are different from this extrapolation, one concludes that something different happened. A. Marzari-Chiesa and L. Ramello - NURT 2003

  27. Strangeness enhancement • In hadron interactions, strange particle production occurs via associated production. • The reaction that requires the minimum energy is: for which: • Strange anti-baryon production requires more energy: In a QGP the energy threshold is lower, being the energy to produce an couple: A. Marzari-Chiesa and L. Ramello - NURT 2003

  28. Multi-strange hadrons • the K production enhancement can in fact be explained through rescattering: • K+K  • BUT • for multistrange baryons or multistrange antibaryons the situation is different: • The strangeness enhancement is not conclusive if limited to K/ ratio: can be produced via: with a very high threshold or via a long series of interactions: which take a long time (~100 fm/c, to be compared to 5-10 fm/c of a single N-N collision) • in a QGP with strangeness enhancement factor Es the hadrons containing N strange quarks are produced with a rate EsN times higher than in an environment with no strangeness enhancement. So in QGP: E>E>E A. Marzari-Chiesa and L. Ramello - NURT 2003

  29. Experimental measurement of strangeness Strangeness production was measured by experiments WA97 and NA57 at CERN SPS, with Pb beam at 158 GeV/nucleon A. Marzari-Chiesa and L. Ramello - NURT 2003

  30. Multi-strange hyperon enhancement WA97 has seen a clear enhancement: for  and anti- there is a factor 17 with respect to extrapolation of p-Be and p-Pb results. NA57 later confirmed the result. A. Marzari-Chiesa and L. Ramello - NURT 2003

  31. where  not published : K/p ratio vs. center of mass energy Data at 30 AGeV support phase transition scenario (Statistical Model of the Early Stage) Volker Friese (NA49), Strange Quark Matter 2003, Atlantic City, March 2003 A. Marzari-Chiesa and L. Ramello - NURT 2003

  32. K/p ratio vs. energy BRAHMS results at y=0 seem to indicate saturation of K+/π+ reached at top SPS energy A. Marzari-Chiesa and L. Ramello - NURT 2003

  33. Charmonium suppression • Quark binding can be dissolved in quark matter. The mechanism is similar to the Debye screening observed in atomic physics: • “The force between the charged partners of a bound state is considerably modified, if this bound state is placed in an environment of many other such objects. The Coulomb potential between two electric charges e, separated by a distance r, in vacuum is proportional to e2/r. In the presence of many other charges it becomes subject to Debye screening: where the screening radius rDis inversely proportional to the overall charge density of the system. • If in atomic matter the Debye radius becomes less than the atomic radius rA , then the binding force between electron and nucleus is effectively screened, and the electron becomes “free”. For atomic systems, an increase in density thus results in an insulator-conductor transition” (H. Satz, Nature, 1986). • Something similar can happen in a deconfined medium for the colour charge between a quark and an antiquark A. Marzari-Chiesa and L. Ramello - NURT 2003

  34. Charmonium suppression (cont’d) For the colour charge, in a “normal” nuclear medium: where r is the term responsible of the quark confinement and /r is the Coulomb-like term In a QGP where quarks are deconfined and many colour charges are present: If rC is smaller than the distance at which a quark and an antiquark become bound to form a particle, the bound state cannot be formed. rC is inversely proportional to the charge density. Since the quark density is proportional to the temperature, we expect that rC is decreasing with temperature. A. Marzari-Chiesa and L. Ramello - NURT 2003

  35. Charmonium suppression (cont’d) J/Ψ, Ψ’ and χc are different bound states of the charm-anticharm system (charmonium) Each of them has a different bound state radius ri: when temperature T is high enough so that rD(T) < ri, then the i-th charmonium state is dissolved by the QGP. This means that as soon as 1.1 TC (TC is the phase transition critical temperature) is reached, c (and’) cannot be formed, while 1.3 TC is needed to dissolve alsoJ/. A. Marzari-Chiesa and L. Ramello - NURT 2003

  36. Experimental study of charmonium • EXPERIMENTS NA38 & NA50 at CERNstudied the charmonium suppression measuring the J/ production as a function of the number of participants.NA50 is the upgrade of NA38, having three centrality detectors instead of one, and a higher rate capability. J/’s were detected through their decay in +-:the experimental apparata consisted therefore essentially of a dimuon spectrometer + centrality detector(s). Characteristic of these experiments is the high beam intensity ( 107 Pb/s), due to the low J/ production cross section and to the low branching ratio in two muons: A. Marzari-Chiesa and L. Ramello - NURT 2003

  37. The NA50 experiment J/Ψ Drell-Yan The absorber stops all the hadrons, and only muons can reach the last chambers. Measuring the emission angle and the curvature of both muons, it is possible to reconstruct the +- invariant mass A. Marzari-Chiesa and L. Ramello - NURT 2003

  38. The Drell-Yan reference process Drell-Yan is a rare, “hard” collision process and its cross-section scales with the number of nucleon-nucleon collisions. A nucleons B nucleons This is in effect what is observed: had an absorption been present, it would change the scaling to (AB) , with  < 1. Drell Yan reactions are therefore taken as a reference and many of the J/ results were presented as a ratio J//DY. A. Marzari-Chiesa and L. Ramello - NURT 2003

  39. Nuclear absorption J/ absorption with respect to Drell-Yan was already observed by the NA38 experiment. Unfortunately, it was not possible to conclude that the QGP had been observed since the suppression, observed in Oxygen and Sulphur interactions, is already present in p-nucleus interactions. The plot B vs AB, that for Drell Yan events is flat, here is consistent with a continous decreasing pattern from p-p to S-U interactions: B(J/)  (AB)0.920.015 This behaviour can be accounted for by nuclear absorption. A. Marzari-Chiesa and L. Ramello - NURT 2003

  40. Anomalous charmonium suppression The observations in p-A, A-B collisions can be fitted by the empirical law: where r0 = nuclear density, L = length of nuclear matter crossed by the charm quark-antiquark pair after its formation L can be calculated using a simple geometrical model (hard spheres) or with more refined models of the nuclei. Going to heavier systems the situation changes: for Pb-Pb the “normal” nuclear absorption does not justify the results and an “anomalous” additional suppression is clearly present. A. Marzari-Chiesa and L. Ramello - NURT 2003

  41. Anomalous suppression (cont’d) • In this figure the ratio J//D.Y. is divided by the same ratio expected under the hypothesis of “normal” nuclear absorption. The number of participants is obtained from the measured transverse energy. A. Marzari-Chiesa and L. Ramello - NURT 2003

  42. Anomalous suppression (cont’d) The same analysis is possible with EZDC and Nch as centrality variables. Here the ZDC analysis is reported. It is clear that the suppression pattern is compatible with a double step in EZDC. The first could be due to the  absorption, the second to the J/ one. All the models, based on “normal” nuclear effects, are ruled out. A. Marzari-Chiesa and L. Ramello - NURT 2003

  43. Why go to higher energies ? Significant quantitative improvements in the experimental conditions are expected when going from SPS energy to RHIC (already running since June 2000) and later to LHC (startup foreseen in 2007) Energy density, volume and lifetime of the plasma are very much improved by going to RHIC, and even more by going to LHC A. Marzari-Chiesa and L. Ramello - NURT 2003

  44. More extended baryon free region A net-baryon free region (no excess of protons over antiprotons) allows easier comparison with theory Net protons distributions indicate high degree of stopping at AGS energies, less stopping at top SPS energy and almost full transparency at RHIC A. Marzari-Chiesa and L. Ramello - NURT 2003

  45. “Onium” suppression revisited The main advantage of LHC for “onium” physics will be the access to Y (beauty-antibeauty) states: this will allow unambiguous confirmation of the results already obtained from charmonium studies at lower energies. RHIC is presently accumulating data on charmonium, which should allow access to a higher transverse momentum region than the one previously explored. A. Marzari-Chiesa and L. Ramello - NURT 2003

  46. The ALICE experiment A. Marzari-Chiesa and L. Ramello - NURT 2003

  47. The ALICE Internal Tracking System 6 cylindrical layers of silicon detectors: • pixel detectors • drift detectors • double sided microstrip detectors A. Marzari-Chiesa and L. Ramello - NURT 2003

  48. Drift Drift segmented 2 x 256 anodes MOS charge injectors for drift velocity monitoring Wafer: 5”, NTD, 3 k.cm, 300 m Active area: 7.02  7.53 cm2 guard region implanted HV voltage dividers 256 anodes (294 mm pitch) A. Marzari-Chiesa and L. Ramello - NURT 2003

  49. The Internal Tracking System mechanical support A. Marzari-Chiesa and L. Ramello - NURT 2003

  50. Challenges: rates and data volumes ALICE will push the previous limits of high energy physics experiments in the direction of very large data volumes, while other LHC experiments will be demanding very high trigger rates and Data Acquisition bandwidth To insure that ALICE data will be analyzed in a timely manner, the offline software is being prepared well in advance of the beginning of data taking in 2007, and is being tested with M.C. events. The GRID software technology is being developed in order to be able to process data stored in several regional centers in an efficient way, moving around programs rather than data. A. Marzari-Chiesa and L. Ramello - NURT 2003

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