1. Heavy-Ion Physics XXIII Physics in Collision Raimond Snellings
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. “Large” as compared to mean-free path of produced particles.
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 Phase diagram of nuclear matter
We normally think of 4 phases:
Plasma
Gas
Liquid
Solid
Phase diagram of nuclear matter
Phase diagram of water

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

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

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

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

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

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

17. Soft Physics
Particle Yields
Spectra shapes
Elliptic Flow

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

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

20. Particle spectra at RHIC
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/ )

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

25. What makes elliptic flow an unique probe?y x
coordinate space
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 py px
Momentum space

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

27. Integrated Elliptic Flow
Hydrodynamic limit
STAR
PHOBOS
Compilation and Figure from M. Kaneta
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. Differential Elliptic Flow
Hydro calculation: P. Huovinen et. al.
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. Thermally-shaped Soft Production
Hard probes
p+p->p0 + X
hep-ex/ S.S. Adler et al.
At RHIC energies different mechanisms are responsible for different regions of particle production.
Rare process (Hard Scattering or “Jets”), a calibrated probe
Hard Scattering
Thermally-shaped Soft Production
“Well Calibrated”

32. 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”
hadrons q
leading
particle
leading particle
schematic view of jet production

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

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

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

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

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

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

39. High-pt phenomena: Initial state or final state effect?
nucl-ex/
Final state
Initial state
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
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?
/2 
suppression?
 (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/

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

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