Exploring QCD with Heavy Ion Physics

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Exploring QCD with Heavy Ion Physics
Mark D. Baker
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The plan of attack - where are we?

• Collide gold nuclei at high energy
• Is it strongly interacting bulk matter
• Collective motion
• Temperature, density
• Probe the strong interaction
• What weve learned
• Some puzzles
• Confinement, Chiral Symmetry

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Can we probe the EARLY stages?
Crude estimate using t00.2-1 fm/c
Quark Gluon Plasma?
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Lattice QCD predictions
e can be measured using jet dE/dx T can be
measured using direct gs
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High-pT Particles _at_ RHIC Jet Tomography
Jets are produced early in parton-parton
scatterings with large Q2. Sensitive to
hot/dense medium parton energy loss (jet
quenching). Compare leading (in pT)
hadron yields to scaled pp yields (binary
scaling) Production yields calculable via pQCD
Glauber geometry
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Charged Hadron Spectra
QM2002 summary talk (Peitzmann)
C. Jorgensen, BRAHMS Parallel Saturday
200 GeV results from all experiments
J. Jia, PHENIX Parallel Saturday
J. Klay, STAR Parallel Saturday
C. Roland, PHOBOS Parallel Saturday
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Comparing AuAu to pp
LA1/3
A
of NN collisions A4/3 (formally Glauber
TAA(b)) of participating nucleons 2A
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Measuring Hadron Suppression
1. Compare AuAu to nucleon-nucleon cross
sections 2. Compare AuAu central/peripheral
T. Ullrich
Nuclear Modification Factor
If no effects R lt 1 in regime of soft
physics R 1 at high-pT where hard
scattering dominates Suppression R lt 1 at
high-pT
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Leading Hadrons at CERN-SPS energies
Central PbPb(Au) collisions at SPS
From compilation of X.N. Wang
Any parton energy loss effects buried in
initial state mult. scatt., transverse radial
flow, etc.
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Caveat The Cronin effect
Multiple scattering in initial state
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Suppression of High pT Hadrons
PHENIX

• AuAu data
• central (0-10) and
• peripheral (60-80)
• compared to N-N reference
• peripheral collisions
• described at high pT
• central collision
• suppressed at high pT

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Hadron Suppression AuAu at 130 GeV
Phenix PRL 88 022301 (2002) p0 and charged
hadrons, central collisions
STAR nucl-ex/0206011 Charged hadrons, centrality
dependence
Clear evidence for high pT hadron suppression in
central nuclear collisions
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yield in AuAu vs. p-p collisions
D. dEnterria
Yield ratio ?s200/130 GeV Consistent at at high
pT with pQCD predictions (STAR)
PHENIX Preliminary
70-80 Peripheral
Ncoll 12.3 4.0
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Hadron Suppression AuAu at 200 GeV
Phenix p0 peripheral and central over measured
pp
STAR charged hadrons central/peripheral
PHENIX preliminary
200 GeV preliminary data suppression of factor
4-5 persists to pT12 GeV/c
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How much energy loss _at_ RHIC?
scaled pp
X.N.Wang
energy loss
ltdE/dxgt 0.25 GeV/fm naïve 7
GeV/fm when expansion included
Still under systematic study, but its a large
effect.
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Another view

• Normalize by ltNpartgt/2
• Divide by the value at Npart65

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Hadron production ?Npart!
PHOBOS Preliminary
PHENIX nucl-ex/0207009
AuAu 200 GeV
UA1 pp (200 GeV)
C. Roland, QM2002
Normalize by Npart/2. Divide by the value at
Npart65
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Approximate Npart scaling is MAXIMAL jet
quenching!
Ncoll S / V Npart4/3 / L Npart
Only jets produced on the surface survive!
PHENIX preliminary
Ncoll
Npart
Even for 8 GeV hadrons!
- not drawn by PHENIX
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Jets in Heavy Ion Collisions
ee- ? q q (OPAL_at_LEP)
pp ?jetjet (STAR_at_RHIC)
AuAu ???? (STAR_at_RHIC)
Hopeless task? No, but a bit tricky
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Peripheral AuAu data vs. ppflow
Hardtke, QM2002
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Central AuAu data vs. ppflow
Hardtke, QM2002
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Ratio Signal (AA-flow)/pp
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Jet suppression _at_ RHIC!
Hardtke, QM2002
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Is final state jet quenching the only
explanation?
Kharzeev, Levin, McLerran hep-ph/0210332
Particle Yield/ltNpart/2gt
Particle Yield (spectrum)
Initial state parton saturation works too ...
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Initial state parton saturation?
See Iancu, Leonidov, McLerran hep-ph/0202270

• Gluon density cannot grow indefinitely.
• Non-Abelian diagram gg?g kicks in at
  low x or low k?.

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Tau scaling in DIS
1000
stot gp (mb)
Statso et al., PRL 86 (2001) 596
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Fit l0.25
10
1
0.1
1
103
10-3
t ? Q2 xl
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Parton Saturation predicts AA
Kharzeev Levin, nucl-th/0108006

• Saturated initial state gives predictions about
  final state.
• Nh c x Ng

l0.25 from fits to HERA data xG(x)x-l
t scaling Describes dN/dh shape correctly!
PRL 87 (2001)
Fit PHOBOS data at 130 GeV to set c, Qs
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Saturation Works at 200 GeV
L. McLerran, DNP 2001
nucl-ex/0112001
h
l0.25 from fits to HERA data
xG(x)x-l t scaling Describes dN/dh
energy evolution correctly!
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J/Y
Energy/Momentum
Data consistent with Hadronic comover breakup
(Ramona Vogt) w/o QGP Limiting suppression via
surface emission (C.Y. Wong) Dissociation
thermal regeneration (R. Rapp)
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Charm Does Scale (doesnt quench)
Favors quenching interpretation for pions.
Gluon radiation suppressed for heavy quarks.
Dokshitzer,Kharzeev hep-ph/0106202
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High pT v2 reaches geometric surface limit
QM2002 (Voloshin)
QM2002 (Filimonov, STAR)
E. Shuryak, nucl-th/0112042
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System is black to very high pT
Adler et al., nucl-ex/0206006
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Summary so far

• Soft physics
• System appears to be hydrodynamic bulk matter
• System is opaque and expanding explosively.
• Naïve energy density much higher than
  theoretical transition
• Freezeout along a universal curve near
  theoretical transition.
• Hard physics
• Strong suppression of inclusive yields
  back-to-back pairs
• Azimuthal anisotropy at high pT
• Open charm remains unsuppressed
• ? large parton energy loss and surface emission?
• Caveats
• dAu needed to disentangle initial state effects
  in jet production
• The PHENIX PHOBOS Puzzles...
  

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p/p
Tatsuya Chujo (PHENIX)
p/p
Central
Peripheral

• Proton yield is comparable with pions _at_ 2-4 GeV
  in central collisions, less in peripheral

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Extrapolate soft component using hydrodynamics
J. Burward-Hoy
Calculate spectra using hydro. parameters h h
- ? p, K, p
Compare sum to measured Charged particle pT
spectrum

• Hydrodynamic flow modifies pt threshold where
  hard physics starts to dominate
• physics is soft (thermal) until pt ? 3 GeV/c

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The PHENIX Puzzle

• Important baryon and/or kaon contribution out to
  pT 8 GeV/c?

T. Sakaguchi, PHENIX Parallel Saturday
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Baryons at high pT
Jia, Sorenson
Yields scale with Ncoll near pT 2 3
GeV/c Then start to fall
Accident? Complex hard/soft interplay? Medium
modified jet fragmentation function? Gluons and
quarks quenching differently?
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The PHOBOS Puzzle
nucl-ex/0211002
pQCD ee- Calculation
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Comparison of áNchñ vs. Energy
Central AuAu
ee-
nucl-ex/0211002
pp (pp) data _at_ ?seff
Central AA

• Different systems converge
• at high energy.
• Universality of Nch?

1 10 102 103
?s (GeV)
From talk by P. Steinberg
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More detailed comparison of AA, ee
200 GeV
ee- measures dN/dyT(rapidity relative
tothrust axis)
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Universal hadronization/tranistion?
LEP!
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Works vs. energy centrality
QM2002 nucl-ex/0212009
Need dAu data to cleanly isolate initial final
state effects!
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Heavy ion collisions in target frame
UA5, Z.Phys.C33, 1 (1986)
p p inel.
dNch/dh /ltNpartgt
6 central
dN/dh
PHOBOS AuAu
19.6 GeV is preliminary
Systematic errors not shown
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Summary (really!)

• Very interesting state
• System appears to be hydrodynamic bulk matter
• Initial e, T appear to be above theoretical
  transition.
• Freezeout along a universal curve near
  theoretical transition.
• We are beginning to probe this state
• large parton energy loss and surface emission?
• Caveats and puzzles remain
• Addressable with more RHIC running detector
  upgrades!
• Is it deconfined matter (Quark Gluon Plasma)?
• We have not achieved a clear proof yet
• But Probably! Evidence is building.
• Stay tuned!!

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Extras
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Interlude Many ways to slice pz
Rapidity Generalized velocity
Feynman x scaled pz
Pseudorapidity y easier to measure
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What is h?
Away from mid-rapidity
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Two Large Detectors at RHIC
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Two Small Experiments at RHIC
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v2 vs pseudorapidity
PHOBOS No boost invariance !
QM2002 summary slide (Voloshin)
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Elliptic Flow
dN/d(f -YR ) N0 (1 2V1cos (f-YR) 2V2cos
(2(f-YR)) ... )
Particle asymmetry
V2
midrapidity h lt 1.0
Normalized Multiplicity
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v2 of mesons baryons
AuAu at ?sNN200GeV
1) High quality M.B. data!!! 2) Consistent
between PHENIX and STAR pT lt 2 GeV/c
v2(light) gt v2(heavy) pT gt 2.5 GeV/c
v2(light) lt v2(heavy) Model P.Huovinen, et
al., Phys. Lett. B503, 58 (2001)
v2
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What do the data say?
G. Roland
dNch/dh 640
Rises somewhat faster than Npart
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Excitation function of mB
Nucl. Phy. A697 902-912 (2002)
Extrapolation of fit
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Same analysis _at_ CERN-SPS