STAR Back-to-Back Di-Jet Triggered Multi-Hadron Correlations as Medium Probes in STAR Back-to-Back...

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


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


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


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 .


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.


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


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


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


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)


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


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


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


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%


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).


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


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


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


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


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


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


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.


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).



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

STAR Back-to-Back Di-Jet Triggered Multi-Hadron Correlations as Medium Probes in STAR Back-to-Back...

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