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Away-side Modification and Near-side Ridge

Relative to Reaction Plane

at 200 GeV Au+Au Collisions

QM2008, Jarpur, India Feb. 5th, 2008

Aoqi Feng for the STAR Collaboration

Purdue University

Institute of Particle Physics, Wuhan, China

Lawrence Berkeley Lab, Berkeley

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Outline

Motivation

Di-hadron correlation wrt reaction plane

Summary

Previous key measurements of di-hadron corr.

Path-length effect study via di-hadron corr.

Away-side discussion.

Near-side discussion.

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Motivation: the Away-side Modification

High pT di-hadron suppression

partonic energy loss.

Low pT di-hadron correlations strong jet-medium interaction

High pT di-hadron correlations (w.r.t RP) path-length dependent jet quenching.

PRL 90 (2003) 082302

PRL 95 (2005) 152301

PRL 93 (2004) 252301

Jet quenching: energy loss is path-length dependent.

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Motivation: the Near-side Ridge

In-plane

Out

-of-

plan

e

1

4

3

2

56

Non-central collision (20-60%):

overlap region like almond.

select trigger particle direction relative to reaction plane.

Ridge (long range correlation in )is observed on the near-side.

To gain more insights into the away-side modification and near-side ridge, we study RP dependence.

Au+Au 0-10%

STAR preliminary

The underlying physics is not understood yet!

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Ref: Phys. Rev C 69, 021901, 2004

Flow Background Subtraction

, ,2 2 4 41 2 cos(2 ) 2 cos(4 )

pairsasso trig R asso trig RdN

B v v v vd

sin( )cos( ) cos( )k s

kcT k k

kc

(1)

(2)

, | |2,4,6,...

2,4,6,...

1 2

trig trig trign n even n k n k n k

kRn trig

k kk

v T v v T

vv T

,

The contribution from v4 terms is about 10%, can not be neglected!

VnR is the trigger flow in the angular slice R.cos( )n

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Results: Correlations v.s. Reaction Plane

Away-side:

Evolves from single- to double-peak.

Near-side:

Amplitude drops.

3<pTtrig<4GeV/c, 20-60%

STAR Preliminary

in-plane S=0o out-of-plane S=90o

φS: the angle between trigger particle and reaction plane.

0.15

0.5

1.0

1.5

2.0

3.0

GeV

Histograms:

v2 uncertainty.

Red curves:

dAu data

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Mid-Central v.s. Central Collisions Comparison

top 5%

3<pTtrig<4GeV/c & 1.0<pT

asso<1.5GeV/c

20-60%

in-plane S=0 out-of-plane S=90o

• In 20-60%, away-side evolves from single-peak (φS =0) to double-peak (φS =90o).• In top 5%, double peak show up at a smaller φS.• At large φS, little difference between two centrality bins.

STAR Preliminary

STAR Preliminary

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3<pTtrig<4,1.0<pT

asso<1.5GeV/c

Focus On Away-side: Broadness

Slice 1: similar to dAu in 20-60%

broader than dAu in 5%.

Slice 6: no much difference in two

centrality bins.

Path-length effect

Slice 1: remains constant. not much broader than dAu.

Slice 6: higher than slice1.

increase with pTasso.

Double peak: strongest when more out-of-plane and associate particle is harder.

3<pTtrig<4GeV/c

RMS

STAR Preliminary

2( )i ii

ii

yRMS

y

v2{4}

v2{RP}

v2 sys. error

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Focus On Away-side: Amplitude

top 5%

πregion: drops with φs, similar between the two centrality bins.

double peak region: constant over φs.

top 5% > mid-central.

20-60%

3<pTtrig<4,1.0<pT

asso<1.5GeV/c

STAR Preliminary

πregiondouble peak

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jet

ridge

Focus On Near-side

Amplitude seems to change, whereas naively little modification is expected.

3<pTtrig<4, 1.5<pT

trig<2.0 GeV/c

Raw(| |<0.7) - C×Raw(| |>0.7)

Correlation in .

Ridge part: | |>0.7, flow background subtracted.

Jet part:

acceptance

factor

STAR Preliminary

in-plane S=0 out-of-plane S=90o

Rid

ge

Jet

3<pTtrig<4, 1.5<pT

trig<2.0 GeV/c

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Jet and Ridge Yield

20-60% top 5%jet part, near-side

ridge part, near-side

jet part, near-side

ridge part, near-side

Ridge: seem to decrease with φs . More significant in 20-60% than top 5%.

Jet: seem to slightly increase with φs .

Strong near-side jet-medium interaction in reaction plane, generating sizable ridge?

Minimal near-side jet-medium interaction perpendicular to reaction plane?

STAR Preliminary

3<pTtrig<4, 1.5<pT

trig<2.0 GeV/c

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Ridge In Two Centralities

STAR Preliminary

3<pTtrig<4GeV/c 4<pT

trig<6GeV/c

Collision geometry? Gluon density?

At φS=0o: Ridge yields are similar in two centralities.

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Summary

Both near- and away-side are modified. The modification depends on the trigger particle direction relative to RP.

Away-side:

==> path-length dependence of jet quenching.

Near-side:

==> near-side strong jet-medium interaction in-plane. collision geometry? Gluon density effect?

In 20-60%, it evolves from single peak (φs =0o) to double peak(φs =90o).

In top 5%, double peak shows up at a small φs .

At large φs , little difference between the two centralities.

Ridge drops with φs , Jet slight increase.

At φs =90o, there appears small or no ridge in 20-60%.

At φs =0o, strong ridge generation.

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Thank you!

15

backup

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Flow background is suggested to be: (Phys. Rev C 69,

021901, 2004)

Flow Background Estimation

1

1 2 cos( )pairs

Rn n

n

dNB v v n

d

, | |2,4,6,...

2,4,6,...

, | |2,4,6,...

sin( ) sin( )cos( ) cos( ) cos( ) cos( )

sin( )1 2 cos( ) cos( )

1 2

trig trig trign n even s k n k n s

kRn

trigk s

k

trig trig trign n even n k n k n k

k

nc kcv n n v v k k

nc kcv

kcv k k

kc

v T v v T

2,4,6,...

trigk k

k

v T

sin( )cos( ) cos( )k s

kcT k k

kc

(1)

(2)

(3)

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Something Relative to the Analysis

Determination of Event Plane:

modified reaction plane reduce non-flow effect;

associate pT range excluded avoid auto-correlations.

Corrections to raw correlation function:

tracking efficiency is corrected for the associated particles;

2-particle acceptance is corrected for by the event-mixing technique.

Systematic errors:

v2: average v2 as default results, v2_{4} and v2_{RP} as sys. estimation.

resolutions: random sub-event and charge sign sub-event.

B: from 3 different fitting methods.

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Systematics Errors

From v2

use v2_{EP}, average v2 and v2_{4} to estimate.

From event plane resolution

it’s smaller than that from v2.

From B

2, 4 and 6 lowest data points are used to get 3 B values.

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Fitting Method

J: jet signal

F: [1+2v2trig,Rv2

assocos(2Δφ)]

Real Flow: B*F = B* [1+2v2trig,Rv2

assocos(2Δφ)]

Raw: raw signal = J+B*F

Define: Y= Raw/F = (J+B*F)/F = B+ J/F

Find 2(4/6) continuous lowest points as the fitting range.

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2 points

6 points4 points

Raw signal/(1+2*v2*v2*cos(2*dphi))

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4<pTtrig<6 GeV/c, 20-60%

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3<pTtrig<4GeV/c, top 5%

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4<pTtrig<6GeV/c, top 5%

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Two Methods: Consistent

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Ridge Comparison

4<pTtrig<6, 1.5<pTasso<2.0GeV/c3<pTtrig<4, 1.5<pTasso<2.0GeV/c

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dPhi x dEta and Projection

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Jet width

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Details

Near-side amplitude:

|Δφ|<0.52 (-30o,30o)

πregion:

2.75<Δφ<3.53 (180o-22.5o,180o+22.5o)

Double-peak region:

1.44<Δφ<2.49 and 3.80< Δφ<4.84

(82.5o,112.5o) and (217.5o,277.5o)