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STARSTAR
Back-to-Back Di-Jet Back-to-Back Di-Jet Triggered Multi-Triggered Multi-
Hadron Correlations Hadron Correlations as Medium Probes in as Medium Probes in
STARSTAROlga BarannikovaUniversity of Illinois at Chicago
for the STAR Collaboration
Physics motivations Data analysis:
Results:
Summary
di-jet events background subtraction
correlations in d+Au and Au+Au interpretations
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 2 / 19
STARSTAR
di-hadron high pt suppression effect vanishes in peripheral/d+Au
collisions energy loss, jet quenching
4<pTtrig<6 GeV/c, 2<pT
assoc<pT trigSTAR PRL 91 (2003) 072304
Current state of experimentCurrent state of experiment
di-hadron low-pT enhancement Reappearance of away-side
correlation Double-hump structure hints at
additional physics phenomena
3<pTtrig<4 GeV/c, 1.3<pT
assoc<1.8M.v. Leeuwen, Hangzhou ‘06
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 3 / 19
STARSTAR
Partonic energy loss, jet-quenching in the hot and dense QCD medium
Energy loss mechanisms Mach cone (Stoecker, Casalderrey-Solana,
Shuryak, Teaney, Ruppert, Muller, Renk) Cherenkov gluon radiation (Dremin, Koch,
Majumder, Wang) Large angle gluon radiation (Vitev, Salgado) Flow-induced deflection (Armesto, Salgado,
Wiedemann) Parton multiple scattering (Chiu, Hwa) ….
Current understanding of theoryCurrent understanding of theory
Important to know the away-jet-axis: use di-jets.
Mach-Cone Double-hump structure
on away-side.
“near-side”
“away-side”
Jet Quenching Dissipation jet energy
propagating through the medium
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 4 / 19
STARSTAR
Knowing away-jet-axis should improve the sensitivity to this and other effects.
Use di-jets.
*by M. Andrews (UIC)
Unmodified Jet*
Simulation
Modified Jet*Simulation
Benefit of knowing away-jet-axis: Benefit of knowing away-jet-axis: Mach-cone simulationMach-cone simulation
unmodified jet
Mach-cone
away-side
direction
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 5 / 19
STARSTAR
PRL 97 (2006) 162301
8<pT(trig)<15 GeV/c
First di-jet observationFirst di-jet observation
Two high-pT triggers: di-jet observation
New analysis exploits this observation: back-to-back hadrons pin down away-jet-axis in and .
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 6 / 19
STARSTAR
8 < pT(trig) < 15 GeV/c
Moreover…Moreover…Away-side widths similar for d+Au and Au+Au.
lose some energy, and then fragment?
= 0.12 ± 0.08: cannot rule out ~1 GeV large angle (90o) single gluon or softer multi-gluon radiation.
1
2
2
finite prob. of not interacting: fragmentation as in vacuum?
1
Look for a third, softer particle around away-side high pT hadron.
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 7 / 19
STARSTAR
Aim of this analysis is to “pin” the jet axis.
Only select events with high-pT particles back-to-back in .
Correlate all particles with this away-axis.
associates
associates
primary
trigger (T1)
“jet-axis”
trigger (T2)
Signal sit atop of a largely
uncorrelated background
Data analysis: techniquesData analysis: techniques
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 8 / 19
STARSTAR
Raw, uncorrected signal
-1-2 0 1 2 3 4 5
a.u
.
0.6
0.4
0.2
T1: pT>5 GeV/c
T2: pT>4 GeV/c
T1T2 correlation Correlation between primary trigger
(T1) and “away-jet-axis trigger” (T2).
Require that the 2 highest pT particles are back-to-back in .
Assume this defines the jet-axis, look in 2D-space about the second trigger.
Data analysis: di-jet selectionData analysis: di-jet selection
± 0.2
Di-jet trigger
T2T1
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 9 / 19
STARSTAR Flow backgroundFlow backgroundT1: pT>5GeV/c
T2: pT>4GeV/c
A: pT>1.5GeV/c
Raw correlation
STAR Preliminary
Mixed-event background
2 1 1 12 2 2 2
1 21 2
2 2
sin 22 2
21 cos(2 )sin 2
1 22
T A T A
A TT T
av v v v
aa
v va
Flow background for T2A1_T1
0 - 10%
10-20%
20-30%
30-40%
Phys. Rev. C72 (2005) 014904
T2A1_T1:
where a=0.2
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 10 / 19
STARSTAR Background NormalizationBackground NormalizationT1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
-1-2 0 1 2 3 4 5
a.u
. 85
80
75
-1-2 0 1 2 3 4 5
1.0
0.5
0.5
1.0
0
STAR PreliminarySTAR Preliminary
T2A1:Mixed-event bkgdZDC central Au+Au
Raw T2A1_T1: – Flow backgroundT2A1:
T2 direction(away-side)
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 11 / 19
STARSTAR Correlated BackgroundCorrelated Background
Signal + Background
Background
Signal
a.u
. T1T2 Accounting for correlated
background on T1 and T2 side Correlated background give by
di-hadron correlations
T1: pT>5GeV/c, T2: pT>4GeV/c, A: pT>1.5GeV/c
-1-2 0 1 2 3 4 5
2
1
0 1
_d
N_
N
trig
d
)
STAR Preliminary
T2A1_T1
flow subt.T2A1 T1A1
0 2 4
-1-2 0 1 2 3 4 5
2
0
1
_d
N_
N
trig
d
)
-2
4
STAR Preliminary
Final T2A1_T1 correlation
ZDC central Au+Au 12%
Nd
ijet
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 12 / 19
STARSTAR
Di-jets in d+Au 200 GeV Minimum bias data
Preliminary result: min-bias d+AuPreliminary result: min-bias d+Au
Di-jet trigger
T2
A1
T1
Single trigger
A1
T2
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
T2A1_T1
T2A1
-1-2 0 1 2 3 4 5
1
0
1
_d
N_
Ntr
ig d
) 2
STAR Preliminary
Difference in associated yield Di-triggers sample higher energy jets
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 13 / 19
STARSTAR
One high-pT trigger only: Away-side modification.
Di-jet trigger: jet peaks on both near and away side!
Preliminary result: central Au+AuPreliminary result: central Au+Au
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
Di-jet trigger
T2
A1
T1
Single trigger
A1
T2
T2A1_T1
T2A1
-1-2 0 1 2 3 4 5
2
0 1
_d
N_
Ntr
ig d
)
-2
4
STAR Preliminary
200 GeV ZDC central 12%
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 14 / 19
STARSTAR Au+Au vs d+Au comparisonAu+Au vs d+Au comparison
T1A1_T2
T2A1_T1
-1-2 0 1 2 3 4 5
2
0 1
_d
N_
Ntr
ig d
)
STAR Preliminary
200 GeV Au+Au, 12% central
Di-jets are suppressed. Once select di-jets, away-side associated particles NOT suppressed. Shapes of near- and away-sides similar. Central Au+Au ~ d+Au.
No energy loss for triggered di-jets!Tangential di-jets (or punch-through without interactions).
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
Au+Au
d+Au
-1-2 0 1 2 3 4 5
1
0
1
_d
N_
Ntr
ig d
)
STAR Preliminary
2
3
200 GeV Au+Au & d+Au
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 15 / 19
STARSTAR Au+Au centrality comparisonAu+Au centrality comparison
projection: no ridge
J. Putschke, QM2006
12% Central40-60% MB60-80% MB
-1-2 0 1 2 3 4 5
1
_d
N_
Ntr
ig d
)
2
STAR Preliminary
0
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
projection: no significant centrality dependence
T2A1_T1
Au+Au 12% central
||<0.7
T2A1_T1T1A1_T2
-1-1.5 0 1-0.5 0.5 1.5
1
_d
N_
Ntr
ig d
)
STAR Preliminary
0
ZYAM normalization in
0.5
1.0
T: 3-4 GeV/c
A: 1.5-3 GeV/c
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 16 / 19
STARSTAR
Surface effectSurface effect
T1: pT>5GeV/c
T2: pT>4GeV/c
If the triggers have tangential bias:
expect a term related to the surface: ~ R2 ~ Npart2/3
STAR Preliminary
T1= 5 GeV/c
1000 200 300 Npart
0
N
trig__
Nev
t Np
art
2/3
0.4
d+Au
x10 -3
STAR Preliminary
#T1T2 pairs / #Single triggers
#Di-Jets / #Single triggers
1000 200 300
0.015
0.05
0.01
Npart
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 17 / 19
STARSTAR Model calculationModel calculationRenk, Phys. Rev. C 75, 054910 (2007)
<E> deposition for back-to-back jets
Thorsten Renk, private comm.
2 density models
T1 & T2 energy deposition by T2
T1
T2
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 18 / 19
STARSTAR
Increase T1-T2 asymmetryIncrease T1-T2 asymmetryEMC-triggered data access to higher pT triggers
T1: pT > 7 GeV/c T2: pT > 4 GeV/c A1: pT > 1.5 GeV/c
~10% Centrality
-1-2 0 1 2 3 4 5
a. u
.
STAR Preliminary T2A1_T1:
T1T2
High-pT triggers ~ no combinatoric background
Di-jet triggered correlations: near-side ~ away-side no significant modifications? no associated particle suppression?
~10% Centrality
-1-2 0 1 2 3 4 5
1
_d
N_
Ntr
ig d
)
2
STAR Preliminary
0
4
T2A1
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 19 / 19
STARSTAR
Summary:Summary:
New analysis of 2+1 correlations:Events triggered with 2 high-pT particles (T1: pT>5 GeV/c, T2: pT>4 GeV/c) and look at a third, associated particle (A1: pT>1,5 GeV/c).
Preliminary results show a di-jet structure, with no modification of associated particles tangential jets, or punch-through without interactions.
New technique allows further exploration of the “surface bias”, and will probe the medium in a more controlled way.
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 20 / 19
STARSTAR Au+Au vs d+Au comparisonAu+Au vs d+Au comparison
T1A1_T2
T2A1_T1
-1-2 0 1 2 3 4 5
2
0 1
_d
N_
Ntr
ig d
)
STAR Preliminary
200 GeV Au+Au, 12% central
sameaway
sameaway
Au+Au
d+Au
-1-2 0 1 2 3 4 5
1
0
1
_d
N_
Ntr
ig d
)
STAR Preliminary
2
3
200 GeV Au+Au & d+Au
Di-jets are suppressed. Once select di-jets, away-side associated
particles NOT suppressed. Shapes of near- and away-sides similar. Central Au+Au ~ d+Au.
No energy loss for triggered di-jets!
Tangential di-jets (or punch-through without interactions).
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
Quark Matter 2008, Jaipur Olga Barannikova, University of Illinois at Chicago 21 / 19
STARSTAR
STAR Preliminary **
Surface effectSurface effect
T1: pT>5GeV/c
T2: pT>4GeV/c
A: pT>1.5GeV/c
If the triggers have tangential bias: expect a term related to the surface
Surface ~ R2 ~ Npart2/3
** Shown are statistical errors only
Number of triggers per event (per number of binary collisions)
Single triggers and (all qualified) pairs behave similar to inclusives
STAR Preliminary **
T1= 5 GeV/c
1000 200 300 Npart
0
N
trig__
Nev
t Np
art
2/3
0.4
d+Au
x10 -3
STAR Preliminary **
#T1T2 pairs / #Single triggers
#Di-Jets / #Single triggers
1000 200 300
0.015
0.05
0.01
Npart