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Away-side Modification and Near-side Ridge

Relative to Reaction Plane

at 200 GeV Au+Au Collisions

第十届全国粒子物理学术会议 （南京） Apr. 28th, 2008

Aoqi Feng, Fuqiang Wang, Yuanfang Wu

(for the STAR collaboration)

Institute of Particle Physics, Wuhan, China

Purdue University, USA

Lawrence Berkeley Lab, Berkeley, USA

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Outline

Short introduction 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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Short Introduction --- RHIC A phase transition between hadronic matter and exotic quark-

gluon plasma is predicted by QCD at energy density of ~ 1 .

Little bang at RHIC may produce QGP.

3GeV fm

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Short Introduction --- Jet signals

Jets are good probes of the dense nuclear matter.

In PP collisions, the hard scattering of quarks and gluons early in the collision leads to the production of jets.

In AA collisions, energetic partons, resulting from initial hard scattering are predicted to lose energy (jet quenching).

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Short Introduction --- Di-hadron Corr.

Why di-hadron azimuthal correlation? Standard: jet cone method

Heavy-ion collisions: di-hadron azimuthal correlation

very large amount of particles are produced. It is not possible to reconstruct jets event by event due to the large background. So in

heavy-ion collisions people reconstruct jet-like correlations through angular correlations in statistical basis.

What’s di-hadron correlation?Trigger particle: high pT(3<pT<4GeV/c); associate particles: lower pT.

1( ) ( , )

trigger

D d NN

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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

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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 Near-side(1)

A significant change in the near-side peak amplitude!

whereas naively little modification is expected due to the minimal amount of medium that the parton transverses.

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jet

ridge

Focus On Near-side (2)

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!

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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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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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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)