away-side modification and near-side ridge relative to reaction plane
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Away-side Modification and Near-side Ridge Relative to Reaction Plane at 200 GeV Au+Au Collisions. Aoqi Feng for the STAR Collaboration. Purdue University Institute of Particle Physics, Wuhan, China - PowerPoint PPT PresentationTRANSCRIPT
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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!
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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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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)