Heavy-Ion Physics

1. Heavy-Ion Physics Raimond Snellings XXIII Physics in Collision Zeuthen, Germany June 26-28, 2003

2. Outline • Brief introduction to Heavy-Ion Physics • CERN SPS: a new state of matter • BNL Relativistic Heavy Ion Collider • BRAHMS, PHOBOS, PHENIX and STAR • (a few selected) RHIC results from year 1-3 • Summary

3. Collisions of “Large” nuclei convert beam energy to temperatures above 200 MeV or 1,500,000,000,000 K • ~100,000 times higher temperature than the center of our sun. • “Large” as compared to mean-free path of produced particles.

4. QCD Phase Diagram • We normally think of 4 phases: • Plasma • Gas • Liquid • Solid Phase diagram of nuclear matter Phase diagram of water

5. Z. Fodor and S.D. Katz, hep-lat/01060002 QCD on the Lattice F. Karsch, hep-lat/0106019

6. Schematic Space-Time Diagram of a Heavy Ion Collision

7. p L e p K time p jet J/Y g Freeze-out Hadronization QGP? Thermalization? Hard Scattering space Au Au Schematic Time Evolution f g e  Expansion -----------

8. NA50 CERN SPS: A New State of Matter? • J/Y suppression indication of deconfinement? • Strangeness enhancement • Melting of the r Are hadronic scenarios ruled out? Co-mover absorption? canonical suppression?

9. SPS, NA49: Indications of a Phase Transition at ≈ 30 GeV ?

10. 3.83 km circumference • Two independent rings • 120 bunches/ring • 106 ns crossing time • Capable of colliding ~any nuclear species on ~any other species • Energy: • 200 GeV for Au-Au(per N-N collision) • 500 GeV for p-p • Luminosity • Au-Au: 2 x 1026 cm-2 s-1 • p-p : 2 x 1032 cm-2 s-1(polarized) • ` A New Era for Heavy Ion Physics: The Relativistic Heavy Ion Collider at BNL

11. Hadron PID over broad rapidity acceptance • Two conventional beam line spectrometers • Magnets, Tracking Chambers, TOF, RICH

12. Charged Hadrons in Central Spectrometer • Nearly 4p coverage multiplicity counters • Silicon Multiplicity Rings • Magnetic field, Silicon Pad Detectors, TOF

13. Electrons, Muons, Photons and Hadrons Measurement Capabilities • Focus on Rare Probes: J/y, high-pT • Two central spectrometers with tracking and electron/photon PID • Two forward muon spectrometers

14. Hadronic Observables over a Large Acceptance • Event-by-Event Capabilities • Solenoidal magnetic field • Large coverage Time-Projection Chamber • Silicon Tracking, RICH, EMC, TOF Online Level 3 Trigger Display

15. Heavy-ion Physics at RHIC • RHIC different from previous (fixed target) heavy ion facilities • ECM increased by order-of-magnitude • Accessible x (parton momentum fraction) decreases by ~ same factor • Study pp, pA to AA • Comprehensive set of detectors • All final state particles measured with overlap between the detectors • Study QCD at high density with probes generated in the medium • If QGP produced at RHIC most likely to live longer than at the SPS and therefore easier to observe and study its properties

16. 5% Central Ncoll Npart STAR Impact Parameter (b) Event Characterization • Cannot directly measure the impact parameter b! • but we can distinguish peripheral collisions from central collisions! b

17. Soft Physics • Particle Yields • Spectra shapes • Elliptic Flow

18. 19.6 GeV 130 GeV 200 GeV PHOBOS Preliminary Central Peripheral dNch/dh h Particle distributions (PHOBOS) central collisions at 130 GeV: 4200 charged particles ! mid rapidity plateau

19. Bjorken Estimate to R 503±2 GeV(130 GeV) preliminary Energy Density (Bjorken estimate) PRL 87 (2001)

20. Superimposed on the thermal (~Boltzmann) distributions: Collective velocity fields from Momentum spectra ~ ‘Test’ by investigating description for different mass particles: Excellent description of particle production (P. Kolb and U. Heinz, hep-ph/0204061) Particle spectra at RHIC

21. Particle spectra at the SPS Rather well described by Hydro motivated fit

22. Particle ratios: chemical potentials and freeze-out temperature • Assume distributions described by one temperature T and one ( baryon) chemical potential m • One ratio (e.g.,p / p ) determines m / T • A second ratio (e.g., K / p ) provides T m • Then predict all other hadronic yields and ratios:

23. Where is RHIC on the phase diagram?

24. Only type of transverse flow in central collision (b=0) is transverse flow. • Integrates pressure history over complete expansion phase y y x x x z • Elliptic flow, caused by anisotropic initial overlap region (b > 0). • More weight towards early stage of expansion. • Directed flow, sensitive to earliest collision stage (pre-equilibrium, b > 0) Three Forms of Collective Motion

25. y coordinate space x Momentum space py px What makes elliptic flow an unique probe? • Non central collisions coordinate space configuration anisotropic (almond shape). However, initial momentum distribution isotropic (spherically symmetric). • Only interactions among constituents (mean free path small) generate a pressure gradient which transforms the initial coordinate space anisotropy into the observed momentum space anisotropy • Multiple interactions lead to thermalization -> limiting behavior hydrodynamic flow

26. Elliptic Flow at the SPS (NA49 and CERES) Clearly deviates from ideal hydrodynamic model calculations

27. Hydrodynamic limit STAR PHOBOS Compilation and Figure from M. Kaneta Integrated Elliptic Flow First time in Heavy-Ion Collisions a system created which at low pt is in quantitative agreement with hydrodynamic model predictions for v2 up to mid-central collisions

28. Hydro calculation: P. Huovinen et. al. Differential Elliptic Flow • Typical pt dependence • Heavy particles more sensitive to velocity distribution (less effected by thermal smearing) therefore put better constrained on EOS

29. Soft Physics • Energy density estimate well above critical Lattice values • Particle yields are well described in a thermal model • Spectra shapes are consistent with thermal boosted distributions • Elliptic flow reaches hydrodynamical model predictions • First time in heavy-ion collisions • Observables consistent with strong early partonic interactions and approaching early local equilibrium • However, size measurements (HBT) are not completely understood yet

30. Hard probes and the produced medium

31. p+p->p0 + X Hard Scattering Thermally-shaped Soft Production “Well Calibrated” hep-ex/0305013 S.S. Adler et al. Hard probes • At RHIC energies different mechanisms are responsible for different regions of particle production. • Rare process (Hard Scattering or “Jets”), a calibrated probe

32. schematic view of jet production leading particle hadrons q q hadrons leading particle Hard Probes and the Produced Medium • Hard scatterings in nucleon collisions produce jets of particles. • In the presence of a color-deconfined medium, the partons strongly interact losing much of their energy “Jet Quenching”

33. p+p jet+jet (STAR@RHIC) Au+Au X (STAR@RHIC) find this in this Jets at RHIC

34. Energy loss  softening of fragmentation  suppression of leading hadron yield Binary collision scaling p+p reference Find partonic energy loss with leading hadrons

35. BRAHMS preliminary Relative to UA1 p+p nucl-ex/0304022 Binary scaling Participant scaling nucl-ex/0305015 Measurements of jet suppression

36. M. Gyulassy, I. Vitev and X.N. Wang STAR preliminary Elliptic Flow at higher-pt R.S, A.M. Poskanzer, S.A. Voloshin, STAR note, nucl-ex/9904003

37. Centrality 24-30% Centrality 11-15% Cronin Effect: Multiple Collisions broaden high PT spectrum Back to back “jets” at the SPS (CERES) SPS. CERES: Away side jet broadening, no disappearance

38. near side away side central peripheral Disappearance of back to back “jets” PRL 90, 082302 (2003) In central Au+Au collisions the away-side “jet” disappears !!

39. nucl-ex/0305015 Final state Initial state High-pt phenomena: Initial state or final state effect? pT>5 GeV/c: well described by KLM saturation model (up to 60% central) and pQCD+jet quenching

40. Theory expectations for d+Au Inclusive spectra RAB If Au+Au suppression is final state 1.1-1.5 1 If Au+Au suppression is initial state (KLM saturation: 0.75) ~2-4 GeV/c pT High pT hadron pairs broadening? pQCD: no suppression, small broadening due to Cronin effect saturation models: suppression due to mono-jet contribution? 0 suppression? /2  0  (radians) All effects strongest in central d+Au collisions

41. Comparison of Au+Au to d+Au (PHOBOS and BRAHMS) central Au+Au PHOBOS d+Au: nucl-ex/0306025

42. Comparison of Au+Au to d+Au (PHENIX and STAR) • Dramatically different behavior of Au+Au observables compared to d+Au observables. • Jet Suppression is clearly a final state effect.

43. ? “PHENIX Preliminary” results, consistent with STAR data in submitted paper Back to back “jets” in d+Au Central Au+Au d+Au

44. Summary • High-pt probes are a new unique tool at RHIC to understand heavy-ion collisions • New phenomena have been found: • Suppression of the inclusive yields (“jet quenching”) • Large elliptic flow • Disappearance of the away-side “jet” • Pointing at very dense (≈ 30x nuclear densities) and strongly interacting matter • Low-pt (bulk) and high-pt observables consistent with expectations from a QGP (but not as proof, still more work to be done. RHIC program just started)

45. Thanks • Many figures on the slides are “borrowed” from: • W. Zajc, P. Steinberg, N. Xu, P. Jacobs, F. Laue, P. Kolb, U. Heinz, T. Hemmick, G. Roland, I. Bearden, M. van Leeuwen and many others

46. Time Evolution in a Hydro Calculation Calculation: P. Kolb, J. Sollfrank and U.Heinz • Elliptic Flow reduces spatial anisotropy -> shuts itself off

47. Structure Functions