Jet medium interactions
Pawan Kumar Netrakanti
(For the STAR Collaboration)
Purdue University, USA
Outline
 Motivation
 Parton energy loss
 Medium response to energetic partons
 Summary
Workshop on Hot & Dense Matter in the RHIC-LHC Era
February 21-14, 2008 TIFR
1
Motivation
Medium properties Physical phenomenon
Experimental probes
Energy density
Parton Eloss in the medium
High pT particle production,
 and  correlations
Velocity of sound
Mach cones
3-particle correlations
Partonic interactions
Mechanism of Eloss
Non-Abelian features of QCD Color factor effects, path length
effects of Eloss
Jet-medium coupling
High pT particle production
 and  correlations,
correlations with respect to
reaction plane
Collectivity and
Thermalization
Partonic collectivity, viscosity
and interactions
Azimuthal anisotropy
Medium effect on
particle production
mechanism
Parton recombination,
modified/vacuum
fragmentation
Identified particle
correlations
Correlations play a significant role in understanding medium properties
2
Basic approach
Calibrated probe
Look for modification
Medium formed in
heavy-ion collisions
Near side
j
STAR Preliminary
New STAR high pT p+p results
Jet and high pT particle production
in pp understood in pQCD framework
Leading/trigger particle
Associated particles
Absence of
medium
Away
side
near
STAR : PRL 97 (2006) 252001
STAR : PLB 637 (2006) 161
Is there any modification in heavy ion collisions ?
away
3
Advantage of di-hadron correlations
Di-hadron
Single
y (fm)
y (fm)
x (fm)
Less surface bias
x (fm)
Limited sensitivity of RAA to P(E,E)
T. Renk, PRC 74 (2006) 034906
T. Renk and Eskola,hep-ph/0610059
2IAA
2RAA
Di-hadron correlations more robust
probes of initial density ~
qˆ ~ 2.8  0.3 GeV 2 fm
4
H. Zhong et al., PRL 97 (2006) 252001
Current observations in STAR
High pT suppression
Parton Eloss
Away side yield modification
pTlp : 4 - 6 GeV/c
STAR : PLB 655 (2007) 104
STAR : PRL 97 (2006) 152301
STAR : PRL 91 (2003) 072304
pTasoc : 2 GeV/c - pTlp
Away side shape modification
2.5 < pTtrig< 4 GeV/c
1< pTassoc < 2.5 GeV/c
Enhanced correlated yield
at large  on near side
d+Au
Au+Au
Medium response
STAR: PRL 95 (2005) 152301
J.G. Ulery, QM 2005
STAR : J. Putschke, QM2006
STAR : M. J. Horner, QM2006
pTtrig=3-6 GeV/c,
2 GeV/c <pTassoc< pTtrig
Reappearance of di-jets
How can we understand
these features ?
STAR : PRL 97 (2006) 162301
5
Do they give answers to …
Mechanism of energy loss in medium Few hard interactions or multiple soft interactions ?
What is the Path length dependence of energy loss ? - L2 or L
What is the probability distribution of parton energy loss?
Do partons loose energy continuously or discretely?
Where does the energy from the absorbed jets go or how are
they distributed in the medium?
Shock waves in recoil direction
Coupling of radiation to collective flow
6
Di-hadron fragmentation function (Away side)
zT=pTassoc/pTtrig
1/Ntrig dN/dzT
6< pT trig < 10 GeV
IAA
STAR Preliminary
STAR Preliminary
STAR Preliminary
Npart
IAA
 Inconsistent with PQM calculations
 Modified fragmentation model better
zT
 Denser medium in central Au+Au
collisions compared to central Cu+Cu
 zT distributions similar for Au+Au
and Cu+Cu for similar Npart
C. Loizides, Eur. Phys. J. C 49, 339-345 (2007)
H. Zhong et al., PRL 97 (2006) 252001
7
Di-hadron correlations w.r.t reaction plane
in-plane S=0
trigger
in-plane
3< pTtrig < 4 GeV/c,
out-of-plane S=90o
pTassoc : 1.0- 1.5 GeV/c
20-60%
STAR Preliminary
trigger
out-of-plane
Au+Au 200 GeV
top 5%
STAR Preliminary
d+Au
Observations :
: away-side : from single-peak (φS =0) to double-peak (φS =90o)
 Top 5% : double peak show up at a smaller φS
 At large φS, little difference between two centrality bins
 20-60%
8
Path Length Effects
STAR Preliminary
v2 sys. error
RMS =
i ( i - )2 yi
i yi
RMS
v2{RP}
In-plane:
v2{4}
similar to dAu in 20-60%.
broader than dAu in top 5%.
Au+Au 200 GeV
3< pTtrig < 4 GeV/c
1.0 < pTasso < 1.5 GeV/c
Out-of-plane:
not much difference between the
two centrality bins.
Away-side features reveal path length effects
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Conical Emission
near
STAR Preliminary
dAu
STAR Preliminary
Medium
away
deflected jets
(1-2)/2
Conical emission or
deflected jets ?
Experimental evidence of
T-trig < 4 GeV/c
Conical emission 13<<ppT-assoc
< 2 GeV/c Two component approach
near
-Correlated to trigger (jets..)
- Uncorrelated to trigger
(except via anisotropic flow)
Bkg normalization 3-particle
ZYAM
Medium
away
Conical
Emission
(1-
2)/2
Central Au+Au 0-12%
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Cone angle (radians)
Mach Cone or Cerenkov Gluons
C3

STAR Preliminary
pT (GeV/c)
 Mach-cone:
j 12
Angle independent of associated
pT
 Cerenkov gluon radiation:
Decreasing angle with associated pT
STAR Preliminary
Subtraction of v2v2v4 terms
using on v2 = 0.06

Subtraction of v2v2v4 term
using v2 = 0.12
Strength and shape of away side structures
observed depends on assumedmagnitude
of flow
j 12
coefficients
In cumulant approach: no conclusive evidence
for conical emission so far
Claude Pruneau : STAR : QM2008(Poster),
PRC 74 (2006) 064910
Naively the observed cone angle ~ 1.36 radians leads to
very small (time averaged) velocity of sound in the
medium
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Ridge in Heavy Ion Collisions
Au+Au
d+Au
d+Au, 40-100%
Au+Au, 0-5%
3 < pT(trig) < 6 GeV
2 < pT(assoc) < pT(trig)
What does these features reveal about the medium ?
Can we get an idea about the energy lost by partons in the medium?
12
Features of the Ridge (at QM2006)
STAR Preliminary
J. Putschke (QM06)
Yield at large  independent on 
STAR : J. Putschke, QM2006
STAR Preliminary
Ridge persists up to high pT-trig
TRidge ~ Tinclusive < Tjet
Indication of two contributions
Jet contribution + contribution arising due to jet
propagating in the medium
13
Jet and Ridge : Observations
Near-side jet yield independent of colliding system, Npart
and trigger particle type
High pT-trig leads to higher jet yields
Supports : Parton fragmentation after parton Eloss in the medium
Ridge yield increases with Npart
14
Particle Ratios: Jet & Ridge
Jet Cone vs. Inclusive
Ridge vs. Inclusive
STAR Preliminary
STAR Preliminary
Jet
Jet : /K0s ~ 0.5 < inclusive
Ridge : /K0s ~ 1 ~ inclusive
 Ratios in cone smaller than inclusive
 Ratios in ridge similar to inclusive
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Theoretical model interpretations
1)In medium radiation +
longitudinal flow push
N.Armesto et.al Phys.Rev.Lett.
93(2004) 242301
2)Transverse flow boost
S.A.Voloshin, Phys.Lett.B. 632(2006)490
E.Shuryak, hep-ph:0706.3531
3)Turbulent color fields
4)Momentum Kick
A.Majumder et.al
Phys. Rev. Lett.99(2004)042301
C.Y. Wong hep-ph:0712.3282
5)Recombination
between thermal and
shower partons
R.C. Hwa & C.B. Chiu
Phys. Rev. C 72 (2005) 034903
Can we discriminate between
these physics interpretations?
 3-particle Correlation in 
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Motivation for 3-particle correlation in 
2
1
T : Trigger particle
A1: First Associated particle
A2: Second Associated particle
STAR TPC acceptance for
3-particle correlation in 
(||<1 and full azimuth)
1 = A1-T
2 = A2-T
1) Jet fragmentation
in vacuum
2) In medium radiated
gluons diffused
in 
3) In medium radiated
gluons still collimated
4) Combination between
jet fragmentation and
diffused gluons
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Analysis techniques
Au+Au and d+Au at sNN = 200 GeV
Trigger
: 3<pT<10 GeV/c, ||<1
Associated : 1< pT<3 GeV/c, ||<1
Select both associated particles
Near Side: || <0.7
STAR Preliminary
Away Side: | - |<0.7
Mixed events to obtain background :
(a) Min-bias events with same centrality
(b) (primary vertex z) < 1 cm
(c) Same magnetic field configuration
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3-particle correlation background
correlated
-
-
 Raw  Raw Raw signal
 Raw  Bkg Hard-Soft
 Bkg1  Bkg1
Soft-Soft
 Bkg1  Bkg2
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3-particle correlation (||<0.7)
dAu
dAu
STAR Preliminary
dAu
dAu
3<pTTrig<10 GeV/c
1<pTAsso<3 GeV/c
STAR Preliminary
STAR Preliminary
2-particle Correlation
STAR Preliminary
AuAu
40-80%
AuAu
40-80%
AuAu
40-80%
AuAu
40-80%
0.7<R<1.4
STAR
Preliminary
AuAu
0-12%
AuAu
0-12%
Shaded :
sys. error.
Line :
v2 uncer.
AuAu
AuAu
0-12%
0-12%
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Comparison (Projections)
3<pTTrig<10 GeV/c
1<pTAsso<3 GeV/c
|| <0.7
STAR Preliminary
STAR Preliminary
0.7<R<1.4
AuAu 0-12% is higher than dAu and AuAu 40-80%
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Summarizing … 3-particle correlation in -
3<pTTrig<10 GeV/c,
1<pTAsso<3 GeV/c, ||<0.7
dAu
STAR Preliminary
AuAu 40-80%
AuAu 0-12%
STAR Preliminary
STAR Preliminary
The ridge is approximately uniform or broadly falling with .
No significant structures along diagonals or axes.
Ridge
+
=
Ridge is uniform event by event.
Jet
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Potential for away-side analysis
STAR Preliminary
3<pTTrig<10 GeV/c
1<pTAsso<3 GeV/c
|-| <0.7
Another tool to study Ridge
3<pTtrig<4GeV/c
1.0<pTasso<1.5GeV/c
STAR Preliminary
Study the ridge with the help of
Di-hardon correlation w.r.t. reaction
plane.
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Summary : Medium Response
Strong jet-medium interaction observed.
 Signals of conical emission observed in central Au+Au
collisions at 200 GeV in 2-component approach
 Medium responds through ridge formation.
 New observations should provide significant constrains
on the mechanism of ridge formation
 Particle ratios in ridge similar to inclusive measurements
 Di-hadron correlations with respect to reaction plane
indicates - ridge is dominated in-plane, consistent with
medium density effect
STAR Preliminary
Ridge vs. Bulk
STAR Preliminary
Jet Cone vs. Bulk
STAR Preliminary
STAR Preliminary
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Summary: Meduim Response
 Three-particle correlation in - can potentially identify the underlying
physics of the ridge.
 Correlation peak at =~0, characteristic of jet fragmentation, is
observed in d+Au, Au+Au 40-80% and central Au+Au 0-12%.
 The peak sits atop of pedestal in central Au+Au. This pedestal, composed
of particle pairs in the ridge, is approximately uniform or broadly falling with
 in the measured acceptance. No significant structures along diagonals or
axes.
Significant step forward in experimental study. Quantitative theoretical
calculations are needed for further understanding.
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Thanks
Thanks to STAR Collaboration
Argonne National Laboratory
Institute of High Energy Physics - Beijing
University of Birmingham
Brookhaven National Laboratory
University of California, Berkeley
University of California - Davis
University of California - Los Angeles
Universidade Estadual de Campinas
Carnegie Mellon University
University of Illinois at Chicago
Creighton University
Nuclear Physics Inst., Academy of Sciences
Laboratory of High Energy Physics - Dubna
Particle Physics Laboratory - Dubna
Institute of Physics. Bhubaneswar
Indian Institute of Technology. Mumbai
Indiana University Cyclotron Facility
Institut Pluridisciplinaire Hubert Curien
University of Jammu
Kent State University
University of Kentucky
Institute of Modern Physics, Lanzhou
Lawrence Berkeley National Laboratory
Massachusetts Institute of Technology
Max-Planck-Institut fuer Physics
Michigan State University
Moscow Engineering Physics Institute
City College of New York
NIKHEF and Utrecht University
Ohio State University
Panjab University
Pennsylvania State University
Institute of High Energy Physics - Protvino
Purdue University
Pusan National University
University of Rajasthan
Rice University
Instituto de Fisica da Universidade de Sao Paulo
University of Science and Technology of China
Shanghai Institue of Applied Physics
SUBATECH
Texas A&M University
University of Texas - Austin
Tsinghua University
Valparaiso University
Variable Energy Cyclotron Centre. Kolkata
Wayne State University
Warsaw University of Technology
University of Washington
Institute of Particle Physics
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Yale University
University of Zagreb
Back up
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2-particle correlation
AuAu ZDC central (0-12%) triggered data,
3<pTTrig<10 GeV/c, 1<pTAsso<3 GeV/c
Black : Raw signal
Pink : Mixed-event background
Blue : Scaled bkgd by ZYA1
Red : Raw signal – bkgd
STAR Preliminary
||<0.7
||<0.7
 acceptance corrected
Ridge
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