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Heavy Ion Physics Overview
Joakim Nystrand University of Bergen
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• Outline
• - Short Introduction to Relativistic Heavy Ion
  Collisions.
• Focus on the two phenomena that have attracted
  most attention at RHIC
• - Jet-quenching
• Suppression of particles with high pT RAA
• Modification of the jet structure
• - Collective flow
• In particular, elliptic flow v2

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Heavy-Ion Laboratories
Two main, high-energy laboratories Brookhaven
(Long Island, New York) ? AGS, RHIC CERN
(Geneve, Switzerland/France) ? SPS(fixed
target), LHC Other laboratories LBL
(Lawrence Berkeley Lab, California) ? Bevalac
(Betatron) GSI (Darmstadt, Germany) ?
FAIR JINR (Dubna, Russia) ?Synchrophasotron
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Heavy-Ion History
Birth of the field 1986 - 16O accelerated at
AGS and SPS Fixed target experiments. 1994 -
Truly heavy ions at AGS and SPS, Au and
Pb. SPS NAXX or WAXX (NA49, WA98, NA57, ) AGS
EXXX (E802, E872, ) 2000 - First collisions at
RHIC (AuAu) First Heavy-Ion Collider
Experiments STAR, PHENIX, PHOBOS, BRAHMS
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The Relativistic Heavy Ion Collider (RHIC)
Collider for heavy nuclei and (polarized)
protons at Brookhaven National Laboratory.
AuAu _at_ ?s 200 A GeV pp _at_ ?s 500 A GeV
(Physics _at_ 200 GeV so far)
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Large Hadron Collider (LHC) pp at 14 TeV
PbPb at 5.5 A TeV First Collisions pp at 0.9
TeV in Nov. 2007 (?)
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Jet-quenching and high-pT suppression
pT or p? Transverse momentum
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pT distribution in pp collisions at RHIC
PHENIX pp ? ?0 X
STAR pp ? h/- X
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From pp to AA The Nuclear Geometry
nucleusnucleus collision
where
is the nuclear overlap function
is the nuclear thickness function.
and
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pT distribution in AuAu collisions at RHIC
Central AuAu ??0 X compared with scaled pp
??0 X
Scaled with Ncoll from nuclear geometry,
Glauber Model.
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pT distribution in AuAu collisions at RHIC
Quantify the suppression using the RAA measure
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The experimental result
Charged hadrons (not identified)
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Centrality Dependence
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The high-pT suppression first observed at RHIC
not seen at the SPS vs 17 GeV (but the Croonin
enhancement is absent)
David dEnterria, Eur. Phys. J. C43 (2005) 295.
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Can AA collisions be understood from
partonparton or nucleon-nucleon interactions?
Prediction Bjorken (1982), Gyulassy Wang
(1992),
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Mechanism for energy loss Gluon bremsstrahlung.
A problem with this interpretation Suppression
of heavy quarks. mc 1.5 GeV mu/d 5-10 MeV ?
much less bremsstrahlung for charm/beauty

• describing the suppression is difficult for
  models
• radiative energy loss with typical gluon
  densities is not enough (Djordjevic et al., PLB
  632(2006)81)
• models involving a very opaque medium agree
  better (Armesto et al., PLB
  637(2006)362)
• collisional energy loss / resonant elastic
  scattering (Wicks
  et al., nucl-th/0512076,
  van Hees Rapp, PRC 73(2006)034913)
• heavy quark fragmentation and dissociation in the
  medium ? strong suppression for charm and
  bottom
  (Adil Vitev, hep-ph/0611109)

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Jet-quenching and mono-jets
pp Two jets, back-to-back AA Jet in only one
direction in central collisions
Azimuthal distribution of high pT particles
This was the initial observation (data from
Run-II 2001)
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Emergence of di-jets
STAR Collaboration nucl-ex/0604018 0-5 most
central collisions
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High-pT suppression jet modification ? Jet
Tomography, use jets to probe the medium
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Corresponding results from PHENIX
? Lot of interest in 3-particle correlations,
Mach cones etc. But interpretation difficult,
often depends on the ranges selected for pTtrig
and pTassoc.
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Elliptic Flow
A Nucleus-Nucleus Collision at intermediate
impact parameter
Reaction Plane Plane defined by beam axis and
b (impact parameter, 2-D vector)
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How are particles distributed in the transverse
plane?
No collective effects ? flat distribution in ?
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Definition of v2
For 180 symmetry
v2 measures the strength of the flow
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Experimentally, v2 is calculated as the average
of cos(2?) relative to the reaction plane.
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v2 vs collision energy
Low Energy The spectators block flow in reaction
plane, squeeze-out. High Energy
Hydrodynamic pressure leads to flow of
particles in the reaction plane.
RHIC
CERN SPS
Bevalac
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A few years ago
Predictions before RHIC
?The collective (elliptic) flow is stronger at
RHIC than anticipated (from hadronic models).
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An apparent paradox
v2 is calculated from the angular distributions
of pions, kaons, protons, emitted at the late
stages of the collision, yet v2 is sensitive to
the equation of state early in the collision.
Explanation Self-quenching
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Initial Configuration
Time evolution
P. Kolb, J. Sollfrank, and U. Heinz
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Time evolution
?x Spatial eccentricity ?p Momentum eccentricity
Kolb, Heinz 2003
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Another remarkable feature of v2 Scaling with
the number of valence quarks
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Indication of quark coalescence
?
? v2(m) 2 v2(q)
? v2(B) 3 v2(q)
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Summary
Focus in this talk on high-pT particle production
and collective flow. Many other interesting
results, for example J/? 10 years ago
NA50 observed a suppression in central PbPb
collisions at the SPS PHENIX data show a similar
suppression at RHIC (which is somewhat
surprising). Low mass dilepton spectrum
Interesting new results from NA60 at SPS (InIn).
Results expected from PHENIX (Hadron Blind
Detector). Initial state effects Nuclear
shadowing and the Color Glass Condensate(?).
Direct and (maybe) Thermal photons.
Global variables. Stopping. Limiting
fragmentation.
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References and Sources
White Papers (Summaries of First Three Years of
Operation of RHIC) I. Arsene et al (BRAHMS)
Nucl. Phys. A 757(2005)1-27 B.B.Back et al.
(PHOBOS) Nucl. Phys. A 757(2005)28-101 J. Adams
et a.(STAR) Nucl. Phys. A 757(2005)102-183 K.
Adcox et al. (PHENIX) Nucl. Phys. A
757(2005)184-283 The Quark Matter
Conferences QM2006 http//www.sinap.ac.cn/qm200
6/ QM2005 http//qm2005.kfki.hu/ QM2004
http//qm2004.lbl.gov/ Proc. J. Phys. G Vol. 30
The RHIC Experiments http//www.phenix.bnl.
gov/ http//www.star.bnl.gov/ http//phobos-srv.
chm.bnl.gov/ http//www4.rcf.bnl.gov/brahms/WWW/
The ALICE Experiment at the LHC http//aliceinfo
.cern.ch/
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From Thorsten Renk, talk at Quark Matter 2006