Back-to-back Jets Tagged via 2+1 Correlation

05/27/2008 RHIC-AGS Annual User Meeting 2008

1

Back-to-back Jets Tagged via 2+1 Correlation

Hua Pei

Iowa State University


2

Outline

• Motivation

• Method

• Current Analysis Result

• Summary


3

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

http://arxiv.org/abs/0801.4555v1


4

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.


5

First Observations of Di-jets at RHIC

PRL 97 (2006) 162301

8<pT(trig)<15 GeV/cSTARSTAR


6

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.


7

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.


8

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


9

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!


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


STARSTAR


10

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?


11

And Where does the Ridge Go?

STARSTAR


• 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


12

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


13

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



14

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

http://arxiv.org/abs/0801.4555v1


15

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


16

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


17

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.


18

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.


19

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.

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Back-to-back Jets Tagged via 2+1 Correlation

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