Dependence of Ridge Formation on Trigger Azimuth

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Dependence of Ridge Formation on Trigger Azimuth

• Rudolph C. Hwa
• University of Oregon

Ridge Workshop Brookhaven National Lab September
22, 2008
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The study of high pT physics in heavy-ion
collisions --- the effect of medium on the
properties of jets.
Here, the reverse --- the effect of semihard jets
on the medium.
Intermediate pT physics 1.5 lt pT lt 6 GeV/c
The focus of this talk is on the ridge structure
in azimuthal angle at mid-rapidity in
particular, on the dependence on trigger
direction.
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Soft ridge in auto-correlation without triggers
integrates over ?1?2 , so it is insensitive to
the jet direction.
We do not consider the problem of long-range
rapidity correlation found by PHOBOS.
It is a different problem.
In pp collisions the correlation between
transverse-momenta (pT1-pT2) is due to a
mechanism different from that between
longitudinal-momenta (y1-y2), so they are
essentially unrelated.
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Correlation on the near side
Putschke, Quark Matter 2006
STAR
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STAR data on Correlations vs. Reaction Plane
Feng QM08
0.15
3ltpTtriglt4GeV/c, 20-60
0.5
1.0
Away-side Evolves from single- to
double-peak. Near-side Amplitude drops.
1.5
STAR Preliminary
2.0
3.0
GeV
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Four features about Ridges
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P. Fachini, arXiv0808.3110
B/M1.7 up to pT11 GeV/c!
How is it to be explained by fragmentation?
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Hadronization by recombination
pT distributions of ? and p
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4. Baryon/Meson ratio in the Ridge is higher
than in the Inclusive distribution
in recombination/coalescence model (Reco)
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What partons?
Thermal partons correlated to jets
Inverse slope T (for R) gt T (for inc.)
?T40-50 MeV/c
quark exp(-qT/T) hadron exp(-pT/T) RF
?(pT-?i qiT) ? T same for quarks and hadrons
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For pT,trig as low as 3 GeV/c, the semihard
parton is created not far from the surface
because of medium opacity.
1. Centrality dependence
TT
Enhanced thermal partons are strongly dependent
on medium
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Partonic basis for ridge formation
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Trigger 3 lt pT lt 4 GeV/c Associated 1.5 lt
pT lt 2 GeV/c
Not hard enough for pQCD to be reliable, too
hard for hydrodynamics.

• Physical processes involve
• semihard parton propagating through dense
  medium
• energy loss due to soft emission induced by
  medium
• enhancement of thermal partons
• hydro flow and hadronization
• ridge formation above background

We have no reliable theoretical framework in
which to calculate all those subprocesses.
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Work done in collaboration with Charles Chiu
(University of Texas, Austin)
arXiv 0809.3018
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If not, then the effect of soft emission is
spread out over a range of surface area, thus the
ridge formation is weakened.
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Geometry
Ellipse
y
h
x
w
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Ridge particle distribution
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Quark Matter 2008 -- A. Feng (STAR)
Dependence on trigger azimuthal angle
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Ridge and Jet components are separated.

Ridge shapes in ?? are similar.
20-60
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Jet and Ridge Yield
STAR Preliminary
3ltpTtriglt4, 1.5ltpTtriglt2.0 GeV/c
jet part, near-side ridge part, near-side
jet part, near-side ridge part, near-side
20-60
top 5
Ridge seem to decrease with fs . More
significant in 20-60 than top 5. Jet seem to
slightly increase with fs .
Strong near-side jet-medium interaction in
reaction plane, generating sizable ridge? Minimal
near-side jet-medium interaction perpendicular to
reaction plane?
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Yield per trigger
N encapsules all uncalculable effects of the soft
processes involved in the ridge formation, and is
not essential to the study of the ?s dependence.
Adjust N to fit overall normalization for top
5 relative normalization for 20-60 not
adjustable.
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3ltpTtriglt4, 1.5ltpTtriglt2.0 GeV/c
Data Feng QM08
Vary t0 and ?
Chiu-Hwa (0809.3018)
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?? distribution
?? ?-?s
Yield involves integration over ?
? --- degree of fluctuation of ? around ?
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Mid-Central v.s. Central Collisions Comparison
3ltpTtriglt4GeV/c 1.0ltpTassolt1.5GeV/c
STAR Preliminary
20-60
STAR Preliminary
top 5

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

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?1
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Dependence on ?
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- ?/2 lt ?s lt 0 right shift
0 lt ?s lt ?/2 left shift
? gt ?s
? lt ?s
?? ? ?-?s
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0 lt ?s lt ?/2
20-60
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Ridge yield integrates over ??, so it is not
sensitive to the shifts in ?? for specific ?s.
The shifts are properties of the ?s-?
correlation, which can be tested directly by
experiment by use of a new measure.
We propose the inside-outside asymmetry function
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Asymmetry function
outside
inside
RP
inside
outside
A(?s)0 along x y axes
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Inside-outside Asymmetry
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STAR data on ?s dependence
3ltpTtriglt4, 1.5ltpTtriglt2.0 GeV/c
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Why does the yield decrease as b decreases?
Fix ?S5 deg, study yield/trigger vs impact
parameter b.
b0
So yield/trigger should decrease with decreasing
b, when ?S5 deg.
But not for all ?S
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Yield averaged over all ?S has a dip at high Npart
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PHENIX data before Ridge is separated
Has always been a puzzle until now.
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Need further study
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Tangential jets should not lead to ridge formation
for any centrality
Should look for lack of ridge structure in events
triggered by dijets.
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STAR - Study of dijet events
associates
primary trigger (T1)
jet-axis trigger (T2)
associates
Courtesy of R .Hollis
Signal sit atop of a largely uncorrelated
background

• Barannikova ISMD 07 QM 08

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T1 pTgt5GeV/c T2 pTgt4GeV/c A pTgt1.5GeV/c
Shape Modifications
Barannikova ISMD-2007

• Centrality dependence
• Df projection no significant shape/yield
  modifications
• Dh projection no apparent ridge

ZYAM normalization
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T1 pTgt5GeV/c T2 pTgt4GeV/c A pTgt1.5GeV/c
Surface Effects
Barannikova ISMD-2007

• Number of triggers per event
• (per number of binary collisions)
• Single triggers and (all qualified) pairs behave
  similar to inclusives

• If the triggers have tangential bias expect a
  term related to the surface
• Surface R2 Npart2/3

Surface R2 Npart2/3
From the Npart2/3 dependence, it can be inferred
that these dijets are mainly tangential jets.
STAR Preliminary
Shown are statistical errors only
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We predict that as pT1-pT2 increases, a ridge
will begin to develop.
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We have not used recombination explicitly, but
hadronization of thermal partons is based on it.
TS recombination is also suppressed if their
directions are not parallel --- for a different
reason Reco Function
SS recombination (i.e., fragmentation) would be
independent of ?, so there would be no ?s
dependence.
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Summary
The Correlated Emission Model (CEM) reproduces
the data on the ?s dependence of the ridge yield.
The main input is the correlation between the
directions of semihard parton and soft-parton
flow.
Predictions inside-outside asymmetry a dip in
centrality dependence at large Npart
development of ridge structure in dijet events