Transcript Slide 1

Recent Results on Angular Correlations
in Probing the Heavy-Ion Medium
Fuqiang Wang
Purdue University
Outline
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Why measure angular correlations?
The “cone” and the “ridge”
Recent new measurements
Recent theoretical efforts
What further works?
Summary
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STAR detector
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High pt in heavy-ion collisions
Jet event in e+e- collision
Fuqiang Wang
STAR Au+Au collision
Nucleus-Nucleus 2009, August 2009
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Why measure angular correlations?
• High-pt hadron suppression – jet quenching. Partons
interact with medium, thus providing information of
the medium.
• However surface bias due to energy loss. Single
hadron suppression has limited sensitivity.
Use di-jets to probe the
interior of the medium.
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Penetrating probe
well calibrated: can be
calculated
Au+Au by pQCD.
hadrons
p+p
leading
particle
hadrons
q
q
q
q q
q
hadrons
leading particle
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di-jet, back-to-back
well calibrated – pQCD
• embed di-jet in AA
• interact with the medium
• probe medium properties
Angular correlation shape and yields provide rich information.
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Dihadron correlation results
STAR, arXiv:1004.2377
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Angular correlations are suppressed at high-pt  limited sensitivity.
Intermediate and low-pT angular correlations provide richer
information, however, large background.
• Away-side is doublepeaked.
• Near-side ridge
contribution.
• Away-side is double-peaked.
• Near-side ridge contribution.
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Correlations evolve with pT
1/Ntrig dN/dDf
• Dihadron correlations evolve with trigger and associated pT.
• Away-side double-peak structure is the strongest at low trigger pT
(~3 GeV/c) and low-intermediate assoc. pT (~1-2 GeV/c).
• Jet-like correlations (after removing the ridge) for the trigger
pT ~ 3 GeV/c are invariant from pp, d+Au, peripheral Au+Au to
central Au+Au, suggesting hard-scattering origin.
• At higher trigger pT, jet-fragmentation (punch-thru) plays a more
significant role, consistent with a weaker than linear E dependence
of dE/dx.
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3-particle correl: Evidence of conical emission
Two-component model: event = correlated + uncorrelated (flow-bkgd)
STAR, PRL 102, 052302 (2009)
pTtrig = 3-4 GeV/c
pTassoc = 1-2 GeV/c
Au+Au 0-12% central
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STAR Preliminary
pTtrig = 4-6 GeV/c
pTassoc = 1-2.5 GeV/c
pTtrig = 6-10 GeV/c
pTassoc = 1-2.5 GeV/c
0.5
0
pTassoc = 1-2 GeV/c
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pTassoc = 1-2 GeV/c
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Evidence of conical emission
STAR, PRL 102, 052302 (2009)
pTtrig = 3-4 GeV/c
pTassoc = 1-2 GeV/c
Two-component
model:
event = correlated
+ uncorrelated
(flow-bkgd)
Au+Au 0-12% central
trigger
q
Away-side
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q = 1.37
± 0.02 (stat.)
± 0.06 (syst.)
Constant cone angle vs pT
suggests Mach Cone
shock waves may be the
underlying mechanism.
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The “ridge”
dAu
AuAu
0-12%
pTTrig > 4 GeV/c
2<pTAssoc<pTTrig GeV/c
Near-side “ridge” (long range Dh correlations):
• Present in central Au+Au collisions; absent in d+Au.
• Extends even up to higher rapidities.
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Ridge is Bulk-Like
• Jet pT-spectra harder and increasing with
pTtrig, as expected from jet fragmentation.
• Ridge pT-spectra are “bulk-like” and approx.
independent of pTtrig.
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Ridge correlation evolves with pT
Cu+Cu 200 GeV 0–10%, Chanaka De Silva, April APS Meeting 2010, STAR Preliminary
>150 MeV/c
Δφ
>300 MeV/c
>500 MeV/c
Δη
>700 MeV/c
>900 MeV/c
>1100 MeV/c
Au+Au 0-10% √sNN = 200GeV
>1300 MeV/c
>1500 MeV/c
3<pTtrig<4 GeV
pT,ssoc.>2 GeV
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Ridge present in untriggered correlations
M. Daugherity (STAR), QM’08, J.Phys.G35:104090,2008
Ridge may be a property of the underlying event.
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Recent new measurements
• Dihadron correlation w.r.t. the event plane.
• Three-particle Dh-Dh correlations to probe the ridge.
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RP-dependent dihadron correlations
• Jet = (|∆η|<0.7) – Accept.*(|∆η|>0.7)
• |∆η|>0.7 = near-side Ridge + away-side
Flow-background subtracted by ZYAM.
v2 = (v2{4}+v2{2,away-side}) / 2
flow syst. = v2{4} : v2{2,away-side}
STAR Preliminary Feng, QM’08; Konzer, QM’09; FW, SQM’09.
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Ridge depends on reaction plane
Ridge yield
STAR Preliminary Feng, QM’08; Konzer, QM’09; FW, SQM’09.
|fs|= ftrig – ψRP
Ridge is mainly in-plane
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A model prediction motivated by data
in-plane
jet flow aligned
more ridge
out-of-plane
jet flow misaligned
less ridge
Chiu,Hwa, arXiv:0809.3018
Correlated Emission Model (CEM)
Alignment of jet propagation and
medium flow produces the ridge.
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Prediction:
Asym. peak
Analyze
trigger
particles at
the two sides
separately.
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RP-dependent dihadron correlations
Separate 1st and
4th quadrants •
trigger particles •
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Jet = (|∆η|<0.7) – Accept.*(|∆η|>0.7)
|∆η|>0.7 = ridge + away-side
Flow-background subtracted by ZYAM.
2-Gaus fit to away-side and subtract
Ridge
STAR Preliminary Konzer, QM’09; FW, SQM’09.
Away 1
RP
Ridge
Jet
Trig.
Away 2
Au+Au 20-60%, pTtrig=3-4 GeV/c, pTassoc=1-1.5 GeV/c
|∆η|>0.7
φs = 0o to -15o
Ridge
-15o to -30o
-30o to -45o
-45o to -60o
-60o to -75o
-75o to -90o
0.05
Jet
0
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-1 0 1
π
∆f = fassoc – ftrig
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Azimuthal Correlation Asymmetry
STAR Preliminary Konzer, QM’09.
pTtrig=3-4 GeV/c
A
v2 syst.
CEM model
N0f 1 - N-1f 0
N0f 1 + N-1f 0
Ridge
• Away-side is
Asymmetric (not shown
in plot).
• Jet is symmetric.
Ridge: assoc pt=1-1.5 GeV/c
Ridge: assoc pt=1.5-2 GeV/c
Jet: assoc pt=1.5-2 GeV/c
in-plane
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|fs|= ftrig – ψRP
Jet
out-ofplane
• Ridge is Asymmetric!
• Ridge may be due to
jet-flow alignment.
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RP-dependent Large-Dh dihadron Correlations
Separate 1st and
4th quadrants
trigger particles
Azimuthal correlation for large |Dh|>0.7.
|∆η|>0.7 = near-side Ridge + away-side.
Flow-background subtracted by ZYAM.
STAR Preliminary Konzer, QM’09; FW, SQM’09.
Au+Au 20-60%, 3<pTtrig<4, 1<pTassoc<2 GeV/c
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(1/Ntrig)dN/dDf
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5
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STAR Preliminary
Df (rad.)
Fit: Back-to-back ridge + Away conical emission peaks
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Away-side asymmetric cone positions
STAR Preliminary FW, SQM’09.
1st cone peak
peak
∆φ
RP
2nd cone peak
trigger
φs
Indication of flow effect on conical emission.
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Medium absorption?
STAR Preliminary FW, SQM’09.
Peak area
Peak width
Jia et al, PRL 103 (2009) 022301
φs
φs
• Conical emission peaks are broadened, but signal strengths appear symmetric.
• No evidence of medium absorption.
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Using charge property to separate
jet and ridge
Jet has charge ordering
Ridge does not
jet
ridge
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3-particle Dh-Dh correlations
STAR, PRL 105, 022301 (2010)
Same-sign triplets
(AAT)
Ridge : 4* AAT
3<pTtrig<10 GeV/c 1<pTassoc<3 GeV/c |Df|<0.7
Like-sign triplets:
Dominated by ridge
Same-sign associated
pair and opposite sign
trigger particle
(AAT )
Jet-like: Total - Ridge

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Average pair densities
Au+Au 0-12%, 3<pTtrig<10 GeV/c, 1<pTassoc<3 GeV/c, near-side |Df|<0.7
STAR, PRL 105, 022301 (2010)
Jet-like + cross-pairs
Ridge
<Pjr>
<Prr>
<Pjr>
<Prr>
<Prr> : 0.114  0.039
<Pjj>
<Pjr>
<Pjr>
<Pjj> : 0.077  0.026
Jet-ridge cross pairs
<Pjr> : -0.004  0.025
Ridge production appears to be uncorrelated with the presence of jet.
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Radial and Angular dependence
STAR, PRL 105, 022301 (2010)
+/2
R
-/2
Ridge is broad.
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No prominent substructures in ridge.
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Data vs Ridge Models
1) In medium radiation +
longitudinal flow push
Model : Diagonal excess
Data: Uniform
3) Recombination Model
Model: jet-ridge cross pairs.
Data : <Pjr> ~ 0
5) Transverse flow boost
Model: Uniform, Jet-ridge cross pairs
2) Turbulent color fields
Model: uniform ridge, jet-ridge cross pairs.
Data:Broad Ridge, no jet-ridge cross pairs
4) Momentum Kick
Model : Jet-ridge cross
pairs
Data: <Pjr> ~ 0
6) Glasma Flux tube
Model: Uniform. Jet-ridge cross pairs ????
??
??
??
??
Data: Uniform.
No jet-ridge cross pairs
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Data: Uniform.
No jet-ridge cross term
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New ideas and models
• Initial energy density fluctuation: the NeXSPheRIO
model
Takahashi et al, PRL 103, 242301 (2009)
Hama et al, arXiv:0911.0811
• Initial overlap geometry fluctuation: triangular flow
Alver, Roland, PRC 81, 054905 (2010)
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NeXSPheRIO
Hama et al, arXiv:0911.0811
Takahashi et al, PRL 103, 242301 (2009)
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Initial density fluctuation (hot spots); no explicit jet.
Two peaks in single particle distribution.
Near-side ridge and away-side double-peak in two-particle correlation.
Ridge and cone are of the same origin; ridge amplitude is x2 of cone’s due
to topology.
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Confront NeXSPheRIO with data
Slice 6
STAR Preliminary
Away-side cone
Near-side
ridge
Df
• NeXSPheRIO: Near-side ridge is always larger (x2) than awayside cone.
• Not observed in data.
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Triangular flow
Alver, Roland, PRC 81, 054905 (2010)
• AMPT model:
V32/v22 ~ 10%
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Confront Triangular Flow with data
Au+Au 20-60%, 3<pTtrig<4, 1<pTassoc<2 GeV/c
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STAR Preliminary
Separated trigger particles
in quandrants
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3
STAR Preliminary
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5
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• Triangular flow gives identical tri-peaks,
independent of reaction plane.
• Not a major contribution to measured
correlation data.
• Data correlation signal/bkbg ~ v22.
Data may allow ~10% triangular flow
contribution, in-line with AMPT model.
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Further experimental works
• g-triggered correlation at low-intermediate associate
pT:
– Absent flow complications
– No near-side jet-medium interactions.
Is there still a ridge?
• Heavy-flavor triggered correlation: Mach cone?
• PID (both trigger & assoc.) correlations
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Summary
• Wealth of data; Extensive systematic studies vs
centrality, pt, reaction plane, etc.
• Near-side ridge; Away-side conical emission. Physics
mechanisms still open question.
• Active theoretical work; phenomenological models.
Quantitative predictions are key.
• More differential measurements falsifying models.
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