Relativistic Heavy Ion Collisions: The Past Through the Future (and vice versa)

1. Relativistic Heavy Ion Collisions: The Past Through the Future(and vice versa) W.A. ZajcColumbia University • Thanks to: • R. Averbeck, B. Cole, A. Drees, T. Csorgo, M. Gyulassy, H. Hiejima, F. Muehlbacher, J. Nagle, S. Sorensen, X. Yang,

2. Outline Q. How to • review 15 years of heavy ion data from two programs • preview 4 new experiments at a new collider in 40 minutes ?!? Answer: • I won’t • I will provide a • prejudiced • partial • selective view of recent developments in the field.

3. Relevant Heavy Ion Physics b Q1: How to (re)-create this deconfined state? Q2: How to (re)-create energy densities 10-20 x normal nuclear density? A: Relativistic Heavy Ion Collisions(Collide “large” nuclei at “large” energies) Event characterization (geometry is destiny) • Impact parameter b is well-defined in heavy ion collisions • Event multiplicity predominantly determined by collision geometry • Characterize this by global measures of multiplicity and/or “transverse energy”

4. A Tale of Two Labs • BNL • AGS: ECM ~5 GeV (1986-1998) • RHIC: ECM ~200 GeV (beginning 2000) • CERN • SPS: ECM ~20 GeV (1986-1998?) • LHC: ECM ~5500 GeV (beginning 2005++) • Note: The program at each laboratory has benefited (and will continue to do so) from developments at and insights from the other lab.

5. CERN Announcement • Available at http://press.web.cern.ch/Press/Releases00/PR01.00EQuarkGluonMatter.html

6. The Key Statements • The evidence for this new state of matter is based on a multitude of different observations. • Many hadronic observables show a strong nonlinear dependence on the number of nucleons which participate in the collision. • Models based on hadronic interaction mechanisms have consistently failed to simultaneously explain the wealth of accumulated data. • On the other hand, the data exhibit many of the predicted signatures for a quark-gluon plasma. • Even if a full characterization of the initial collision stage is presently not yet possible, the data provide strong evidence that it consists of deconfined quarks and gluons.

7. Formation of Dense Matter at CERN A combined analysis of their momentum distributions and two-particle correlations shows that, at the point where they stop interacting and "freeze out", the fireball is in a state of tremendous explosion, with expansion velocities exceeding half the speed of light, and very close to local thermal equilibrium at a temperature of about 100-120 MeV. This characteristic feature gave rise to the name "Little Bang". NA49 NA44

8. Formation of Dense Matter at the AGS ~same analysis for AGS data gives T ~ 93 MeV vT ~ 0.5 (!) mB ~ 540 MeV (Dobler, Sollfrank, Heinz) E866

9. Summary (1)

10. Non-Linear Dependences? • There is no a priori reason to expect • “a strong non-nonlinear dependence on the number of nucleons which participate in the collision” • That is, a linear dependence on the number of participants is one of many physically plausible scaling behaviors: ~ Number of participants (W, Npart, NWOUNDED) ~ Number of binary collisions (Nbin ~ A*B ) ~ Number of constituent quark interactions ~ Number of absorbers ( ~ Aa * Ba )

11. Determining NPART ET EZDC ET Best approach: • Directly measure in a “zero degree calorimeter” • (for A+A collisions) • Strongly (anti)-correlated with produced transverse energy: NA50

12. Non-linear Dependences? • Zero-th step: • Study systematics of transverse energy production ds/dET • A basic measure of • Nuclear overlap • “Thermalization” of initial directed energy • Calculate “transverse energy per participant” • Non-linear? WA98 (Preliminary)

13. Lesson in Non-linearity WA98 (Preliminary) • Same data • (plot of dET/dh maximum versus number of participants) is either Non-linear or Linear • Surprising strong dependence on inclusion of errors in determining number of participants • NB: This is (presumably) not the non-linearity referred to in the press release.

14. Strangeness as a QGP Signal Quark MatterStrange Quark Matter u d u d s Energy Level Strange Quark Mass • An old prediction: J. Rafelski and B. Müller, Phys. Rev. Lett. 48, 1066 (1982). • Based on • High rate for ggss relative to hadronic processes Or • Reduced threshold effects from • reduced mass in deconfined state and/or • Fermi energy of u,d quarks

15. Strangeness Enhancement Seen at CERN • Clear evidence for increase in K/p ratio with • NPARTICIPANTS Collision centrality (As presented by R. Stock at 10-Feb-00 CERN Announcement)

16. Small “Problem” • Same (actually larger) enhancement also seen in heavy ion collisions at the AGS(at much lower energy) • QGP at the AGS? • QGP everywhere??

17. But CERN is Different (?) • See “extra” enhancement for the multiply-strange baryons: (Kudos to CERN for this unique measurement!) • Assertion: Yield doesn’t “scale” from p-p, p-A • New physics! WA97

18. The Precise Statement A particularly striking aspect of this apparent "chemical equilibrium" at the quark-hadron transition temperature is the observed enhancement, relative to proton-induced collisions, of hadrons containing strange quarks…Lead-lead collisions are thus qualitatively different from a superposition of independent nucleon-nucleon collisions.That the relative enhancement is found to increase with the strange quark content of the produced hadrons contradicts predictions from hadronic rescattering models where secondary production of multi-strange (anti)baryons is hindered by high mass thresholds and low cross sections.Since the hadron abundances appear to be frozen in at the point of hadron formation, this enhancement signals a new and faster strangeness-producing process before or during hadronization, involving intense rescattering among quarks and gluons. • It’s not describable by a “superposition of independent nucleon-nucleon collisions” • Therefore it must signal“a new process … involving intense rescattering among quarks and gluons.”

19. Naïve Question Q. When is a nucleus-nucleus collision describable as a “superposition of independent nucleon-nucleon collisions”? A1. ~Never. A2. Not even in proton-nucleus collisions: Q64K$: In a nucleus-nucleus collision, how to scale effect of + collisions?? A1SF: Scale as NPARTICIPANTS (number of “wounded” nucleons) (??) In this cartoon are there 5 N-N collisions  5 x Npp? OR 6 “wounded” N’s  3 x Npp?

20. Inspired Answer • Let’s measure proton-nucleus as completely as possible • Measure ~ all charged particles in final state • Infer n = number of N-N sub-collisions event-by-event • Characterize particle yields versus n • Done in E910 at BNL AGS • B. Cole, Spokesperson • Based on • TPC • Downstream detectors for • Particle ID • Further tracking

21. E910 Strangeness Production WA97 data CQM BC WN • Systematic study of L productionversus n indicates • Initial scaling intermediate between NBINARY and NPARTICIPANTS • Saturation for n > ~3 hits • Suggestive of “ Constituent Quark Model” • Applying CQM to CERN L production data • Gives good parameter-free description of data • Strong hints towards explaining S>1 data starts to saturate p-p data

22. Summary (2)

23. NA45 • Study physics in e+e- channel • After heroic efforts to • Suppress Dalitz pairs • Suppress conversions • Understand background • Then: • Form M(e+e-) spectrum • Divide by charged yield • Compare to known sources • Excess seen for 0.3 GeV < M(e+e-) < 0.7 GeV from pp annihilation? r collision-broadening? density dependent masses? Chiral symmetry restoration?

24. Mixing the Cocktail • Requires detailed understanding of • Resonance yields • PT spectra • Form factors • Decay kinematics • Detector Resolution • Two versions: • GENESISG. Agakichiev at al.: Eur.Phys.Jour. C4(98)231 • EXODUSR. Averbeck, A. Drees

25. Screening by the QGP • In pictures:

26. Screening by the QGP In first-order finger physics: • Follow usual derivation of Debye screening • Now put in QGP scales and assumptions: • Hadrons with radii greater than ~ lD will be dissolved • Study “onium” bound states

27. Di-Muon Measurements • Physics: • Look at J/Y via decay to m+m- • Experiment: • Absorb “all” hadrons before they make muons! • Analysis: • Form spectrum of • Extract J/Y and Drell-Yan yields byfitting and removing background and open charm • Plot J/Y to Drell-Yan ratio versus measured ET in calorimeter • Compare to theory calculations of same Calorimeter Absorber Spectrometer m+ Incident Beam Calorimeter m- Target(s)

28. Emphasis

29. It’s Anomalous • Suppression pattern vs. L is different for Pb-Pb • What the L is L? • “the mean length of the path of the (cc) system through nuclear matter of mean density r0” • A way to combine different beams, targets and energies “Anomalous J/Y suppression in Pb-Pb interactions at 158 GeV/c per nucleon”, Phys. Lett. B410, 337 (1997).

30. It’s Lumpy • More data more wiggles! “Observation of a threshold effect in the anomalous J/Y suppression”, Phys. Lett. B450, 456 (1999). The sudden change of behaviour observed in our data suggest that the observed abnormal suppression results from a discontinuity in the state of nuclear matter. … A clear onset of the anomaly is observed as a function of transverse energy. It excludes models based on hadronic scenarios since only smooth behaviours with monotonic derivatives can be inferred from such calculations.

31. Discontinuous? “A clear onset of the anomaly is observed. It excludes models based on hadronic scenarios since only smooth behavior with monotonic derivatives can be inferred from such calculations” Phys. Lett. B 450, 456 (1999). The second suppression is preliminary and contradicts the published results shown here in the above paper.

32. The Models Catch Up More sophisticated calculations than the simple “co-movers” ansatz describe the qualitative features of the (pre-98) data But…

33. The Data Recedes • New data set disagrees substantially with • All models • (previous data) Leading to …

34. Latest Theory • Recent work (Capella, Ferreiro and Kaidolov, hep-ph/0002300) has dramatically improved description of data • As before: Two dissociation mechanisms • Nuclear absorption sABS • Break-up by co-movers sCO • New: • Now account for fluctuations in b  ET mapping( b 0 while ET continues to increase) • Smaller value of sABS (as implied by E866 data nucl-exp/9909007)

35. Summary (3)

36. Lessons Learned • Beware of false dichotomies: • Failure of “all” conventional models Success of “any” QGP model • All QGP’s increase strangeness All strangeness increasea are QGP • Beware of inclusive averages: • Beware of simple models: • Models should be as simple as possible– but no simpler • Beware of wording: • Press is insensitive to “evidence for” vs. “discovery of” • Press is sensitive to anything combining • New state of matter • Early Universe • Big Bang

37. Life on the Edge • CERN has done an admirable job of extracting maximal information from phenomena on threshold of • Phase transition • Excitation function • Energy distribution • RHIC will transcend these “boundaries” by factors of 4-50 RHIC CERN

38. RHIC • RHIC = Relativistic Heavy Ion Collider • Located at Brookhaven National Laboratory • Schedule: • Commissioningmachine as we speak • Will run through end of August(?)

39. RHIC Specifications • 3.83 km circumference • Two independent rings • Capable of colliding ~any nuclear species on ~any other species • Energy: • 500 GeV for p-p • 200 GeV for Au-Au(per N-N collision) • Luminosity • Au-Au: 2 x 1026 cm-2 s-1 • p-p : 2 x 1032 cm-2 s-1(polarized) 6 3 5 1’ 4 1 2

40. How is RHIC Different? • It’s a collider • Detector systematics independent of ECM • (No thick targets!) • It’s dedicated • Heavy ions will run 20-30 weeks/year • It’s high energy • Access to non-perturbative phenomena • Jets • Non-linear dE/dx • Its detectors are comprehensive • ~All final state species measured with a suite of detectors that nonetheless have significant overlap for comparisons

41. Uniqueness of RHIC • Substantial increase in ECM • Access to high Q2 probes • Dominance of mini-jets • Highest physics priority should be development of sufficient luminosity to access this new regime at RHIC (Argument by V. Pantuev, see also K. Eskola, hep-ph/9610365) Njets pT > 2 GeV/c

42. In Pictures -4.8, 0.66, 2.86, 9.39, 18.48, 35.96

43. (PID) Acceptances BRAHMS Acceptance PHOBOS Acceptance STAR Acceptance

44. PHOBOS An experiment with a philosophy: • Global phenomena • large spatial sizes • small momenta • Minimize the number of technologies: • All Si-strip tracking • Si multiplicity detection • PMT-based TOF • Unbiased global look at very large number of collisions (~109)

45. PHOBOS “Results”

46. BRAHMS An experiment with an emphasis: • Quality PID spectra over a broad range of rapidity and pT • Special emphasis: • Where do the baryons go? • How is directed energy transferred to the reaction products? • Two magnetic dipole spectrometers in “classic” fixed-target configuration

47. BRAHMS “Results”

48. STAR Time Projection Chamber Magnet Coils Silicon Vertex Tracker TPC Endcap & MWPC FTPCs ZCal ZCal Endcap Calorimeter Vertex Position Detectors Barrel EM Calorimeter Central Trigger Barrel or TOF RICH • An experiment with a challenge: • Track ~ 2000 charged particles in |h| < 1

49. STAR “Results” Demonstrate large hadronic rates from: Large acceptance coupled with Large multiplicities (Assuming centraltriggers ) ~ 1 count per hour limit F yield from ~12 minutes of running

50. STAR Challenge