Azimuthal Correlations in Pb Pb Collisions at

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Azimuthal Correlations in PbPb Collisions at
ÖsNN 2.76 TeV measured with ALICEFirst
Measurement of ICP and IAA at LHC

• Jan Fiete Grosse-Oetringhaus, CERN
• for the ALICE collaboration
• Rencontres de Moriond QCD and High Energy
  Interactions
• La Thuile, 2011

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Motivation

• Ultrarelativistic heavy-ion collisions probe QCD
  matter at unprecedented energy densities
• Characterize the hot and dense medium
  (quark-gluon plasma) by comparing quenched (with
  plasma) and unquenched collisions
• Use high pT partons as probes that "feel" the
  medium ? jet tomography
• Assess modifications of dijet structure by
  dihadron two-particle azimuthal correlations
• STAR (RHIC) found disappearance of away-side peak

4 lt pT,trig lt 6 2 lt pT,assoc lt pT,Trig
STAR, PRL 91 (2003) 072304
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Dihadron Correlations

• Study two-particle correlations with per-trigger
  yields
• Lower pT
• Ridge
• Hydrodynamics, flow
• High pT
• Quenching/suppression, broadening
• Calculate near side (around f 0) and away side
  (f p) yields
• Compare central and peripheral ? ICP
• Compare AA and pp ? IAA

PbPb 2.76 TeV
and
trigger particle associated particle
PbPb 2.76 TeV
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A Large Ion Collider Experiment
Dipole
Not shown ZDC (at 114m)
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Analysis

• 12M PbPb MB collisions used
• Tracking with Time Projection Chamber and Inner
  Tracking System in h lt 0.8
• Flat f acceptance ? No mixed events needed for
  acceptance correction (in f)
• Centrality determination with V0 (forward
  scintillators) and hits in pixel detector
• Corrections applied for efficiency and
  contamination
• Weakly centrality dependent
• Two-track effects small but considered

Number of tracks
ALICE performance, 14.03.11 PbPb 2.76 TeV, TPC
tracks
f (rad.)
ALICE performance14.03.11 MC PbPb 2.76 TeV TPC
tracks
Tracking efficiency
0-20 20-40 40-90
pT (GeV/c)
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Pedestal and Flow

• To calculate yields, pedestal needs to be
  determined
• Fit in region around p/2 (ZYAM)
• Different ways to estimate uncertainty
• Estimate radial flow (v2) contribution using
  ALICE flow measurement
• Flow subtraction quite controversial
• Measure in a region where the signal dominates
  over pedestal and v2 modulation(8 GeV/c lt
  pT,trig lt 15 GeV/c)
• Indicate difference in measurement if v2 was
  subtracted

different pedestals
v2 contribution
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Yield Extraction

• After pedestal (and optionally v2) subtraction),
  integrate to obtain yield Y
• Near side -0.7 lt f lt 0.7
• Away side -0.7 lt f p lt 0.7
• In bins of associated pT pT,assoc
• Divide yields to obtain ICP and IAA

0-5 60-90
integration windows
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Systematic Uncertainties

• Detector efficiency and two-track effects
• Different detectors for centrality determination
• pT resolution
• Fold associated pT distribution with momentum
  resolution
• Different pedestal determination schemes
• Integration window (between 0.5 rad. and 0.9
  rad.)

Detector efficiency 5-8
Centrality selection 2-8
pT resolution 3
Pedestal calculation 7-20
Integration window 0-3
Ranges indicate different values for
ICP/IAA,Pythia and near/away side
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ICP

• Flat pedestal subtraction ? data points
• v2 subtracted ? line
• Difference only at low pT
• Statistical and systematic uncertainties (shaded
  area) shown

flat pedestal
v2 subtracted
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ICP (2)

• Slightly enhanced near-side ICP 1.2
  unexpected and interesting
• Away side suppressed ICP 0.6 expected from
  in-medium energy loss
• v2 contribution small except in lowest bin, there
  v3 subtraction may be significant

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IAA Reference
0.9 TeV

• Interesting to study yield with respect to
  unquenched (pp) case
• No pp data taken at 2.76 TeV, yet
• Use a MC
• Pythia6 tune Perugia-0 has been found to describe
  dihadron correlations at 0.9 and 7 TeV well
• Using a scaling factor between 0.8 and 1
• Interpolate to 2.76 TeV
• Factor 0.93 13 (stat/syst)
• ? Use scaled Pythia reference to calculate
  IAA,Pythia

7 TeV Data Pythia
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IAA,Pythia

• Central events
• Near side enhanced IAA,Pythia 1.5
• Away side suppressed IAA,Pythia 0.5 0.7

• Peripheral events
• Near side enhanced IAA,Pythia 1.2
• Away side IAA,Pythia consistent with 1

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Near-Side Enhancement
Toy power law example A/(pT DpT)n

• Near side enhancement in ICP and IAA,Pythia in
  central events
• Near side is modified ? trigger particle sees
  the medium
• Possible explanation
• In the presence of quenching same trigger pT
  might probe higher parton pT
• Change of normalization of power law (relative
  energy loss) leads to constant ratio
• Different result for different parton spectra ?
  sensitivity to initial parton spectrum
• Increased parton pT ? increased yield ? increased
  IAA,Pythia/ICP
• IAA,Pythia/ICP on away side would be even lower
  without this effect!

pT (GeV/c)
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IAA,Pythia ALICE vs. RHIC
PHENIX, PRL 104, 252301 (2010)
STAR, PRL97,162301 (2006)

• PHENIX subtracts v2 ? compare ALICE line with
  PHENIX
• STAR measurement in slightly different variable
  (zT) and dAu reference

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Summary

• Azimuthal dihadron correlations extracted from
  PbPb collisions
• pT-region studied where the background (pedestal
  v2) contribution is small
• ICP and IAA,Pythia measured
• Near side enhanced (slightly in ICP, factor 1.5
  in IAA,Pythia)
• Might be something interesting
• Factor 1.5 might be due to the PYTHIA reference ?
  LHC pp run at 2.76 TeV will tell
• Away side suppressed (ICP and IAA,Pythia) by
  about a factor 2

• Dihadron correlations will be further studied
• In more pT regions
• Characterization of near and away side peak
  shapes
• Differentially in Dh ? Ridge physics
• These measurements show the presence of a hot and
  dense medium at LHC and allow to constrain
  energy-loss models (together with RAA and other
  measurements)

Thank you for your attention!