Back-to-back Jets Tagged via 2+1 Correlation
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Transcript of Back-to-back Jets Tagged via 2+1 Correlation
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Back-to-back Jets Tagged via 2+1 Correlation
Hua Pei
Iowa State University
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Outline
• Motivation
• Method
• Current Analysis Result
• Summary
05/27/2008 RHIC-AGS Annual User Meeting 2008
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2-Particle Correlations Indicate Suppression
We have been studying jets using the 2-particle correlation.
Clear jet signal is observed on near-side even in the most central AA collisions.
Strong suppression is observed on the away-side.
More central arXiv:0801.4545v1
0-20% Au+Au 20-40% 60-92%
STARSTAR4<pT
trig<6 GeV/c, 2<pTassoc<pT trig
STAR PRL 91 (2003) 072304
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Theory Models Are There to Explain
• We all agree:
• Jet-quenching is due to partons lose energy in the hot and dense QCD medium
• We don’t agree on: 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)•….
• Important to study the back-to-back jets simultaneously: use di-jets events.
05/27/2008 RHIC-AGS Annual User Meeting 2008
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First Observations of Di-jets at RHIC
PRL 97 (2006) 162301
8<pT(trig)<15 GeV/cSTARSTAR
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Analysis Method
• Select only events with 2 back-to-back (within azimuthal space) high-pT hadrons.
• Then in the 2-D space, plot the azimuthal distribution of lower-pT particles around one of the two high-pT hadrons.
• The flow background and underlying events are taken into consideration.
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Advantage of Using Di-jets
Select events that have two high-pT hadrons back-to-back
Trigger hadron
The “2nd”
hadron at high-pt
MediumAssoc h
Trigger hadron
Assoc h
Shift distribution of hard scattering towards center of medium. Near-side parton travels through more medium
Removes some events where hard-scattering occurs near surface but not tangential (large difference between path lengths)
Path lengths comparable in dense medium.A.k.a., 2+1 correlations
vs.
05/27/2008 RHIC-AGS Annual User Meeting 2008
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First 2+1 / T1T2_A Measurement by STAR
Di-jets in d+Au 200 GeV Minimum bias data
Di-jet trigger
T2
A1
T1
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 jetsFrom QM08 talk of
Olga Barannikova
STARSTAR
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Di-jets Correlation in Most Central Au+Au
From QM08 talk ofOlga Barannikova
• One high-pT trigger only: Away-side modification.
• Di-jet trigger: jet peaks on both near and away side!
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
Di-jet trigger
T2
A1
T1
T2A1_T1
T2A1
-1-2 0 1 2 3 4 5
2
0 1
_d
N_
Ntr
ig d
)
-2
4
STAR Preliminary200 GeV ZDC central 12% Au+Au
From QM08 talk ofOlga Barannikova
STARSTAR
05/27/2008 RHIC-AGS Annual User Meeting 2008
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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 Correlation of Au+Au vs. d+Au
STARSTAR
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c• We see similar shapes between
d+Au and most central Au+Au
• The suppression is less comparing to “normal” 2-particle correlation.
• Was this due to tangential di-jets (surface-bias) or real punch-through jets, without much energy loss?
05/27/2008 RHIC-AGS Annual User Meeting 2008
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And Where does the Ridge Go?
STARSTAR
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
• Ridge is naturally considered as the energy lost (deposited) to the medium by jets.
• di-jets statistically remove those surface-biased jets, and keep either tangential di-jets or those punch-through from center of medium, in either case two partons travel comparable path-lengths.
• Lack of ridge, shall this remain us only tangential di-jets?
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
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Tangential Bias of Di-jets
T1: pT>5GeV/c
T2: pT>4GeV/c
• If the triggers are from tangential surface of medium,
we 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
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Model Calculation
Renk, 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
From QM08 talk ofOlga Barannikova
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Measurement: Di-jets Still Lose Energy
• PHENIX h-h correlation. This JAA is calculated on per-event basis instead of per-trigger.
• At right-end of top-left panel (most central) , the JAA (pTA pTB) approaches the inclusive RAA
2 (dash line), indicating an evident energy loss.
• We need to further quantize the di-jets.
arXiv:0801.4545v1
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Di-jets Correlation Shape Evolution with Centrality in Cu+Cu 200GeV
T2A1_T1
T2A1
-1
-2
0 1 2 3 4 5
2
0
1
_dN
_
N
trig
d
)
-2
4
STAR Preliminary
We show the near-side widths in di-jet correlation as function of centrality.
Inside each bin, it goes from most central Cu+Cu (most left) towards peripheral until p+p (most right)
Each bin represents a different pT region of “conditional” particle, or “T1”.
QM08 talk ofHua Pei
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Di-jets Correlation Yield Evolution with Centrality in Cu+Cu 200GeV
T2A1_T1
T2A1
-1
-2
0 1 2 3 4 5
2
0
1
_dN
_
N
trig
d
)
-2
4
STAR Preliminary
We show the near-side yields in di-jet correlation as function of centrality.
Inside each bin, it goes from most central Cu+Cu (most left) towards peripheral until p+p (most right)
Each bin represents a different pT region of “conditional” particle, or “T1”.
QM08 talk ofHua Pei
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Discussion on Di-jets Correlation Centrality Dependence
• Cu+Cu yields are higher than p+p at “non-required” conditional pT bin, is consistent to the Au+Au h-h correlation result (arXiv:0801.4545)
• Cu+Cu or p+p yield increase with conditional pT is possibly due to the Q2 shift. That is, if we fixed the trigger pT and increase the away-side conditional pT, we are increasing the Q2 of di-jets, thus the parton energy on near-side, and naturally increase the yields. A similar discussion was given in PHENIX paper Phys. Rev. D 74, 072002 (2006)
• On the other hand, the fact that Cu+Cu yields increase slower than p+p with conditional pT, indicates possible medium effect such as jet absorption. Existence of underlying events (random combination of T1 and T2) can also lower the yields especially in central AA.
• Far-side yields was not shown here since I am still in development of removing the bias introduced by this “conditional” particle.
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Outlook
• People at RHIC are analyzing their Run 2007 Au+Au data with much more statistics (e.g., PHENIX has more than 3-times of what it got in 2004 Au+Au, and better flow measurement with the new RXNP detector).
• Geometry dependence of di-jets (yields, ridge shapes, etc) can be extend to a broader range then.
• Methods of removing underlying events will be improved.
• Long-term scope: – (nearly) full di-jet reconstruction (STAR and PHENIX Upgrade)?– Direct jet correlation to ultimately remove the surface bias?
Can be the silver bullet.
05/27/2008 RHIC-AGS Annual User Meeting 2008
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Summary
• We already know from 2-particle correlation that jets are modified by medium, on both near and far side. Medium effect shows as jet suppression and varies on centrality and pT.
• New analysis of 2+1 correlations:a new method of controlling jet source via the surface-bias, especially on exploring the jet suppression.
• Preliminary results from both PHENIX and STAR show a di-jet structure, and statistically shift our observation to either tangential jets or punch-through (both sides) jets.
• New technique allows further exploration of the “surface bias”, and will probe the medium in a more controlled way.