Transcript Folie 1

Azimuthal Correlations in Pb+Pb Collisions
at sNN = 2.76 TeV measured with ALICE
First 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
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 < pT,trig < 6
2 < pT,assoc < pT,Trig
STAR, PRL 91 (2003) 072304
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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Dihadron Correlations
• Study two-particle correlations
with per-trigger yields
1 dNassoc
N trig d
and
1 d 2 N assoc
Ntrig dd
• 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
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
Pb+Pb 2.76 TeV
trigger particle
associated particle
Pb+Pb 2.76 TeV
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A Large Ion Collider Experiment
EMCAL
γ, π0, jets
T0/V0
Trigger
L3 Magnet
ACORDE
Cosmic trigger
HMPID
PID (RICH) @ high pT
TRD
Electron ID (TR)
TOF
PID
PMD
γ multiplicity
Dipole
TPC
Tracking,
PID (dE/dx)
PHOS
γ, π0, jets
FMD
Charged
multiplicity
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
ITS
Low pT tracking
PID + Vertexing
MUON
μ-pairs
Not shown: ZDC (at ±114m)
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• 12M Pb+Pb MB collisions used
• Tracking with Time Projection
Chamber and Inner Tracking
System in || < 0.8
Number of tracks
Analysis
ALICE performance, 14.03.11
Pb+Pb 2.76 TeV, TPC tracks
– Flat f acceptance  No mixed
events needed for acceptance
correction (in f)
– Weakly centrality dependent
– Two-track effects small but
considered
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
Tracking efficiency
• Centrality determination with
V0 (forward scintillators) and
hits in pixel detector
• Corrections applied for
efficiency and contamination
f (rad.)
ALICE performance
14.03.11
MC Pb+Pb 2.76 TeV
TPC tracks
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 < pT,trig < 15 GeV/c)
 Indicate difference in
measurement if v2 was subtracted
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
different
pedestals
v2 contribution
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Yield Extraction
• After pedestal (and optionally v2)
subtraction), integrate to obtain
yield Y
0-5%
60-90%
– Near side -0.7 < f < 0.7
– Away side -0.7 < f – p < 0.7
• In bins of associated pT: pT,assoc
• Divide yields to obtain ICP and IAA
I CP ( pT ,trig ; pT ,assoc ) 
I AA ( pT ,trig ; pT ,assoc ) 
AA
Ycentral
( pT ,trig ; pT ,assoc )
AA
Yperipheral
( pT ,trig ; pT ,assoc )
Y AA ( pT ,trig ; pT ,assoc )
Y pp ( pT ,trig ; pT ,assoc )
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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
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
• Different pedestal determination schemes
• Integration window (between ±0.5 rad. and ±0.9 rad.)
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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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
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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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
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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IAA Reference
• Interesting to study yield with
respect to unquenched (pp) case
0.9 TeV
– 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)
7 TeV
Data
Pythia
 Use scaled Pythia reference to
calculate IAA,Pythia
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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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
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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Near-Side Enhancement
• 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!
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
Toy power law example
A/(pT – pT)n
pT (GeV/c)
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IAA,Pythia: ALICE vs. RHIC
8 < pT,trig < 15
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 d+Au reference
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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Summary
• Azimuthal dihadron
correlations extracted from
Pb+Pb 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 
 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!
Measurement of ICP and IAA - Jan Fiete Grosse-Oetringhaus
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