Systematic Study of Di-jet Correlation in Heavy Ion Collisions
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Transcript of Systematic Study of Di-jet Correlation in Heavy Ion Collisions
N. GrauOral Defense 11/21/05
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Systematic Study of Di-jet Correlation in Heavy Ion
Collisions
OutlineQCD and the phase transitionEvidence for new matter at RHIC Jet studies in d+Au compared to p+p
• Limit on initial state effects/cold nuclear matter
Jet studies in Au+Au compared to p+p and d+Au• Hot,dense medium modification
What do we learn about the matter at RHIC?
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QCD – Theory of Strong Interactions (I)
Quantum Chromodynamics• Describes forces between
hadrons: protons, neutrons
• ~ 5% of the universe Asymptotic Freedom –
s0 as Q2 increases.• Allows for perturbative
calculations (Feynman Calculus)
• Hard processes, high Q2, high outgoing energy (Jets) Q (GeV)
S(Q
)
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QCD – Theory of Strong Interactions (II)
QCD potential energy between quark pairs
Linear at large separations• Force is constant!
Confinement of partons• Not free at large
separations
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Deconfining the Partons
Free energy of heavy quark pairs as a function of separation
At high temperature Tc ~ 170 MeV ~1012K
Potential not linear but flat at large separation• F = 0! Deconfinement!
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Quark-Gluon Plasma (QGP)
Above Tc partons contribute to degrees of freedom
Increase energy density, entropy
Energy density required ~ 1 GeV/fm3
Dilute, non-interacting gas
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QCD Phase Diagram
AGS root-s ~ 5 GeV/nucleon
SPS root-s ~ 20 GeV/nucleon
RHIC root-s ~ 200 GeV/nucleon
LHS root-s ~ 7000 GeV/nucleon
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Relativistic Heavy Ion Collider (RHIC)
Began Operation Summer 2000
Can collide most any system (p+p, d+Au, Au+Au, Cu+Cu, Cu+Au, etc.)
Maximum Beam energy for nuclei= 100 GeV/nucleon
RHIC from Space
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RHIC Runs to Date
Run Year Species s1/2 [GeV ] Ldt NTot p-p Equivalent
01 2000 Au+Au 130 1 b-1 10M 0.04 pb-1
02 2001/2002 Au+Au 200 24 b-1 170M 1.0 pb-1
p+p 200 0.15 pb-1 3.7G 0.15 pb-1
03 2002/2003 d+Au 200 2.74 nb-1 5.5G 1.1 pb-1
p+p 200 0.35 pb-1 6.6G 0.35 pb-1
04 2003/2004 Au+Au 200 241 b-1 1.5G 10.0 pb-1 Au+Au 62 9 b-1 58M 0.36 pb-1
05 2004/2005 Cu+Cu 200 3 nb-1 8.6G 11.9 pb-1 Cu+Cu 62 0.19 nb-1 0.4G 0.8 pb-1 Cu+Cu 22.5 2.7 b-1 9M 0.01 pb-1 p+p 200 3.8 pb-1 85B 3.8 pb-1
Vacuum and cold nuclear medium effects
Hot, dense nuclear medium effects
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PHENIX Detector
Each central arm 90o in azimuth, 70-100o in Charged particle tracking outside of the fieldPhotons with highly segmented EmCal
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Evidence for New Matter:Energy Density
time for system to thermalize (0 ~ 1 fm/c)~6.5 fm
dy
dE
RT
Bj0
2
11
PRL 87, 052301 (2001)
dydz 0
Bjorken Energy Density
Bj >> 15 GeV/fm3 >> necessary
R2
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Evidence for New Matter:Thermal Radiation
Potentially first evidence of radiation from a hot source
Red is PHENIX data on photons not from decay
Dashed blue pQCD expectation
Black line prediction from pQCD and photons from a source at 500 MeV >> 170 MeV!
Need p+p data to compare data with data.
PHENIX preliminary
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Evidence for New Matter:Pressure
Non-overlapping (peripheral) collisions: almond shape overlap region
Results in differential pressure gradient in-plane vs. out-of-plane
v2 sensitive to the pressure
x
yz
2cosv212 2N
d
dN
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Evidence for New Matter: Thermal Equilibrium
All particles flow!Flow reproduced by
hydrodynamic (fluid) model assuming an EOS and thermal equilibrium
First time ever!• Not at AGS or at SPS
At pT > 2 GeV/c, not just bulk properties but hard scattering (jets)
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Evidence for New Matter: Strongly-Interacting QGP
Hydrodynamical models confirm• Large pressures• Substantial collective
motion• Viscocity = 0
Heavy quarks flow• Expect much less flow
since heavy particles rescatter less
Strongly interacting, perfect fluid!
Greco,Ko,Rapp: PLB595(2004)202
Non-photonic electrons, dominantly decays of heavy quarks: c and b.
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Single Particles in Peripheral Collision
0 production in p+p
Compared with peripheral Au+Au on right
# n+n collisions in peripheral Au+Au
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Single Particlesin Central Collisions
p+p production scaled to central, head-on collisions
Suppression seen in Au+Au compared to the scaled p+p
New phenomenon at RHIC!
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Nuclear Modification Factor
Single 0 and are suppressed similarly• Partonic
phenomenon
Direct-photons not suppressed• Strong interaction
phenomenon
ppColl
AATAA YieldN
Yield)(pR
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Parton Energy Loss
Gluon bremsstrahlung• Like QED
bremsstrahlung• Radiated gluons
interact with the medium as well
Single particle suppression related to the gluon density
“Jet Quenching”
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Jets in Heavy Ion Collisions
Jet highly collimated spray of hadrons associated with fragmentation of a fast parton
LO diagram in QCD is 22
Easily seen in p+p at root-s 200 GeV 2 jet event in STAR
Find the jet in
Beam View
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Statistical Determination of Jets:Correlation Functions
Exploit property that jets are highly collimated, correlated in azimuth
Correlate the hadrons• ~ 0 intrajet pairs• ~ interjet pairs
X.-N. Wang Phys. Rev. D 47, 2754 (1993)
CP
P P( )
( , )
( ) ( )
1 2
1 2
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Away-side (Interjet) Suppression
Hadron pair (jet) correlations in Au+Au and p+p collisions
Near-side unmodified
Far-side suppressed
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Broadening of the Away-side Distribution
Incoherent multiple scattering increases the acoplanarity of di-jet pairs
Not measured in data because of the suppression of the distribution
QGP rest frame
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Suppression Summary as of 2002
Single particles are strongly suppressedAway-side correlations are suppressedExplained in terms of parton energy loss
• Color (gluon) density dN/dy ~ 1000Expected broadening of the away side
• Not measured due to the strong suppression
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Contribution of this Thesis
First systematic study of jet correlations in nuclear collisions
Study the cold nuclear medium modification in d+Au compared to p+p
Study the hot, dense nuclear medium modification• Extend the pT reach of previous analyses
(more jet-like)• Extract other distributions
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Importance of a Systematic Study
d+Au
= cold medium
Initial State Effects Only
Au+Au
= hot and dense medium
Initial + Final
State Effects
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Single particle Spectrum in d+Au
Single particle spectrum is enhanced and not suppressed!
Contrast with the final state suppression of single particles from Au+Au
PRL 91, 072303 (2003)
ppColl
AudTdA YieldN
Yield)(pR
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Cronin Effect: Parton Transverse Momentum
Multiple elastic scattering increase parton transverse momentum, kT.
Smears single particle spectrum
Measure kT from final state pair pT
• Drell-Yan • Pair pT of leptons is
pair pT of quarks• Same for di-jets
But the jets fragment
kT due to• Confinement• Hard gluon radiation,
i.e. 23 diagrams
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Direct Measure of kT Broadening
in Nuclear Collisions
Factor of 2 increase from p+p to p+Pb Increases as A1/3 – length of the medium Test to see if this increase is as dramatic at RHIC
+A and +A offset due to an effect from jet reconstruction
After offset all data agree point-to-point
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Correlations in p+p and d+Au
Trigger 0 from 5-10 GeV/c
Increase in di-jet distribution should be seen in away-side
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Jet Acoplanarity Difference Between p+p and d+Au
J. Qiu, I. Vitev PLB 570 161 (2003)
Multiple scattering model not inconsistent with the data.
0 – crosses +/- – circles
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Why No Significant Difference Between p+p and d+Au?
“Intrinsic” kT larger than the kT due to multiple scattering.• More hard gluon radiation at higher root-s
Could it be recombination (Hwa and Yang)?• Shower (fragmented) partons recombining
with the soft (radiated) background• Initially predicted factor of 2 increase in the
near angle yield (R. Hwa, C.B. Yang: PRC 70,054902 (2004))
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Associated Particle YieldsR
a ti o
to
p+
p
Hwa and Tan, nucl-th/0503060
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Conclusions About Jet Correlations in d+Au Collisions
Two-particle correlations: d+Au like p+p• Lack of acoplanarity increase
Yields and acoplanarity are consistent with multiple scattering picture
And consistent with recombination picture.
Conclusion: Need different observables
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Away-Side Jet at High pT in Au+Au
Away-side jet seen in correlations with high trigger and associated pT
Distribution seen to measure broadening!
STAR Preliminary
D. Magestro QM2005
Trigger h 8-15 Associated h > 2
0-20%
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Extracted Away-Side Jet Widths at High pT
Far angle widths unchanged from p+p through central Au+Au!
8 < pT(trig) < 15 GeV/c
5-7 GeV/c(open), 7-10 GeV/c
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pout: Alternative Observable for Broadening
pout,F directly related to kTy
From gluon radiation from energy loss/suppression expect kT broadening
Correlation widths reveal lack of broadening…
trigg.
assoc.
Pout.F
sin,assocTout pp
Component of associated pT transverse to trigger
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pout Distributions
pout Distribution from jet only• Removed flow and
background
Broadening in kT would be seen in broadening of the pout distribution
Distribution is approximately Gaussian: dominated by intrinsic kT
5-7 GeV/c 0 1-10 GeV/c h
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Broadening of pout
Fit distribution with Gaussian
RMS of Gaussian is unchanged with centrality
Lack of broadening is not seen in pout
Bands are error due to assumed v2 value
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No broadening but Suppression
At lower pT suppression of yields in away-side measured
Suppression is also evident from STAR data
8 < pT(trig) < 15 GeV/c
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Measurement of Yields: dN/dxE
Measure the yield of particles per trigger as a function of xE
Compare central to peripheral distribution, ICP
If trigger emitted at the surface ICP ~ RCP
If trigger near center ICP ~ 1
Sensitive to differential energy loss between the particles assoc.
trigg.
trig
assoc
trigT
assocTtrigTE z
z
p
ppx
2,
,,
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xE Distributions
xE distribution due to jets only5-7 GeV/c 0 1-10 GeV/c h
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Comparison of Central to Peripheral
Ratio of central 0-20% to peripheral 40-92%
Weighted average of the data shows suppression!
11% normalization error band at 1
Point-to-point band error due to v2
5-7 GeV/c 0 1-10 GeV/c h
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Centrality and pT Dependence of ICP
Trigger pT dependence of average ICP
20-40%/40-92% (open) 0-20%/40-92% (closed) Suppression increases
with centrality, independent of pTtrig.
Possible evidence for differential energy loss• ICP between RCP and 1
~RCP
~RCP
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Conclusions from Au+Au Di-jets Correlations
Away-side jet is seen at high pT in central Au+Au
Away-side distribution is not broadened• p+p, d+Au, and Au+Au widths are similar
Away-side yield is suppressed when comparing central and peripheral
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Implications for Energy Loss Models in Au+Au
Energy loss picture before Aug 2005• Yield of single particles and away-side pairs from
gluon radiation• Broadening is the natural consequence of
incoherent multiple scattering Data shows
• Single particles and away-side pairs are suppressed• Distribution is not broadened
What does this imply for energy loss?• Coherent gluon radiation? A cone of radiation?• What property of the medium are the correlations
sensitive to? Color density, something else???
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Constructing Correlation Functions
ddN
ddNC
d
dN
N mix
realpairs
trig /
/1
Real events: events with two particle that are correlated
Mixed events: two particles from different events
Mixed events represent the pair efficiency and acceptance in .
Trigger: 0
Associated: h+/-