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Two-particle Correlations in pp and Pb-Pb Collisions with ALICE
Xiangrong Zhu, Ruina Dang(for the ALICE Collaboration)
Institute Of Particle Physics, Central China Normal University
The 9th Chinese Physical Society Conference on High Energy Physics18-23 April, 2014, Wuhan
Many thanks to Jan Fiete Grosse-Oetringhaus and Nicolas Arbor
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 1 / 33
Outline
Motivation
ALICE detectorTwo-hadron correlations
Modification of charged hadrons yield in Pb-Pb
Isolated photon-hadron correlations
Imbalance parameter xE extraction
Summary
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 2 / 33
Motivation
Hard scattered partons generated in the early stage lose their energy whenpropagating through the hot and dense QCD medium, Quark-Gluon-Plasma(QGP) (“jet quenching”).
Nuclear modification factorof hadron and jet yield:
Parton energy loss leads tohadron and jet yieldsuppression.Modification of partonfragmentation in comparisonto pp collisions?Provide crucial insight intothe nuclear medium effectsof energy loss in the QCDmatter.
Charged hadrons Full jets
Two-particle azimuthal correlations: sensitive to parton fragmentation, energyloss in the QGP ⇒ a powerful tool to investigate the properties of QGP.
Two-hadron correlationsphoton-charged hadron correlations: the “golden channel” to understand the QGP.
photons do not interact strongly with QGP.direct photon is dominated by quark-gluon Compton scatting and quark anti-quarkannihilation, which is balanced with the the recoil jet (parton).
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 3 / 33
ALICE detector
ITS (Inner Tracking System)
six cylindrical layers of silicondetectors, |η| < 0.9 and ∆φ = 2π
localize the primary vertexreconstruct the secondaryvertices of hyperons and D and Bmesonstrack and identify particles downto pT ∼ 100 MeV/c
TPC (Time Projection Chamber)
a cylindrical gas detector, |η| < 0.9and ∆φ = 2π
charged particle momentum(0.15 < pT < 100 GeV/c)particle identification (dE/dxresolution better than 10%)two-track separation (resolutionin relative momentum below 5MeV/c)
VZERO
Centrality determinationTrigger
EMCal (ElectroMagnetic Calorimeter)
a lead-scintillator sampling calorimeter,|η| < 0.7 and ∆φ = 5π
9
high energy jetshigh-pT neutral pions and photons
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 4 / 33
Two-hadron correlations
Datasets
Pb-Pb at√sNN = 2.76 TeV : ∼ 15 M events from 2010 data (Lint ≈ 1.8 µb−1)
pp reference at√s = 2.76 TeV : ∼ 55 M events from 2011 data (Lint ≈ 1.0 nb−1)
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 5 / 33
Correlation function
A particle at one pT region (“trigger particle”) correlated withparticles from another pT region (“associated particles”)where pT,assoc< pT,trig.
Per-trigger yield as a function of ∆φ and ∆η(∆φ = φtrig − φassoc, ∆η = ηtrig − ηassoc):
Y =1
Ntrig
d2Npaird∆φd∆η
(1)
Correlation function is obtained by event mixing correction fortwo-track acceptance in bins of centrality and vertex as
C(∆φ,∆η) = Nmixed(1
Ntrig
d2Nsamepair
d∆φd∆η)/Nsame(
1
Ntrig
d2Nmixedpair
d∆φd∆η) (2)
Per-trigger yields corrected fortracking efficiency andcontamination (no influenceon shapes)
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 6 / 33
Correlations at high pT (8.0 < pT,trig < 15.0 GeV/c)
Per-trigger yield as a function of ∆φ at highpT
Jet-like region (small collective effects):
8.0 < pT,trig < 15.0 GeV/c3.0 < pT,assoc < pT,trig
Different background subtractionmethod:
Zero Yield At Minimum (ZYAM) withconstant fitting inregion |∆φ− π/2| < 0.4elliptic flow from ALICE v2
η gap (1.0 < |∆η| < 2.0)
Per-trigger yield in two different region:
|∆φ| < 0.7 for Near-side|∆φ− π| < 0.7 for Away-side
0 2 4
0.4
0.6
0.8
1.0
c < 15 GeV/t,trig
p8 <
c < 6 GeV/t,assoc
p4 <
= 2.76 TeVNNs
a) not background subtracted
0 2 4
)1
(ra
dϕ
∆/d
assoc
N d
trig
N1
/
0.40
0.42
0.44
b) zoomed
(rad)ϕ∆0 2 4
0.0
0.2
0.4
0.6PbPb 05% centrality
PbPb 6090% centrality
pp
c) background subtracted
PRL 108 092301 (2012)
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 7 / 33
Modification of assoicated hadrons yield at high pT,assoc
Modification factor
Y =∫
1Ntrig
dNassocd∆φ
d∆φ ⇒ IAA = YPbPbYpp
and ICP =Y centralPbPb
YperipheralPbPb
Central events:Near-side enhancement (≈ 1.2):
Change of the fragmentationfunction?Change of the quark vs gluonjet ratio?Bias on the parton pt spectrum?
Away-side suppressed (≈ 0.6):Energy loss in medium
Peripheral events:Consistent with unity
Collective contribution small athigh pT
PRL 108 092301 (2012)
)c (GeV/t,assoc
p2 4 6 8 10
AA
I
0.0
0.5
1.0
1.5
2.0
Nearside
= 2.76 TeVNNs
a)
c < 15 GeV/t,trig
p < c8 GeV/
t,trigp <
t,assocp | < 1.0η|
)c (GeV/t,assoc
p2 4 6 8 10
0.0
0.5
1.0
1.5
2.0
Awayside ALICE
05% PbPb/pp 6090% PbPb/pp
Flat bkg Flat bkg bkg2v bkg2v
gapη gapη
)c (GeV/t,assoc
p2 4 6 8 10
(0
5%
/ 6
09
0%
)C
PI
0.0
0.5
1.0
1.5
2.0
Nearside
= 2.76 TeVNNs
b)
c < 15 GeV/t,trig
p < c8 GeV/
t,trigp <
t,assocp | < 1.0η|
)c (GeV/t,assoc
p2 4 6 8 10
0.0
0.5
1.0
1.5
2.0
Awayside ALICE
Flat bkg bkg2v
gapη
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 8 / 33
Modification of assoicated hadrons yield going to low pT,assoc
Alternative subtraction approachSignal yield (YS) from a cone with radiusR = 0.2Background yield (YB) estimated in twoR = 0.2 cones at large ∆ηGap > 1.1Subtract background YB from peak regionyield YS
Avoid flow modulation by using same ∆φregion
Calculate IAA as
IAA =(YS−0.5×YB)/Ntrig|Pb−Pb
(YS−0.5×YB)/Ntrig|pp
On Near-side, 20-50% enhancement in central Pb-Pb, compared to pp
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 9 / 33
Photon-hadron correlations
Data sets
pp at√s = 7 TeV : ∼ 10 M EMCal triggered events from 2011 data
(Lint ≈ 500 nb−1)
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 10 / 33
Measurement observables
EMCal trigger
Use EMCal trigger capabilities to enrich high-pT photons statistics
Trigger threshold ≈ 5.0 GeV/c
Imbalance parameter:
∆φ = φγtrig − φ
h±assoc approximate
p p
xE distribution describes fragmentation function for 0.2 < xE < 0.8
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 11 / 33
Isolation analysis
Isolated photon
Most of direct photons are isolated, while most of decay photons are not (jet)
Isolation parameters: a). cone radius R =√
(∆φ)2 + (∆η)2, b). pthresholdT
Isolation technique
No particles, including track andcluster, with pT > 0.5 GeV/c incone R = 0.4.
Isolation efficiency
(GeV/c)T
p8 10 12 14 16 18 20 22 24
tota
l/N
iso
= N
∈
0
0.2
0.4
0.6
0.8
1
1.2
=7 TeVsjet) γ Pythia (γ
=7 TeVs pp data 0π
<0.5 GeV/cthresh
TIsolation R=0.4, p
26/07/2012
ALI−PERF−31469
80% of direct photons are isolated.About 10% of π0 pass the isolation criteria(main background).
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 12 / 33
xE of isolated cluster and π0
Underlying events contribution is subtracted
xE from isolated cluster-h± correlation
Ex0 0.2 0.4 0.6 0.8 1
EdxdN
tr
igN
1
410
310
210
110
1
10
Isolated clusters
< 12 GeV/c trig
T 8 GeV/c < p
)1 < 16 GeV/c (x10trig
T12 GeV/c < p
)2 < 25 GeV/c (x10trig
T16 GeV/c < p
= 7 TeVspp,
25/07/2012
ALI−PERF−31645
Result is a mix of isolated photons andbackgroundNeed to know photon purity (see backup)
xE from isolated π0-h± correlation
Ex0 0.2 0.4 0.6 0.8 1
EdxdN
tr
igN
1
410
310
210
110
1
100
πIsolated
< 12 GeV/c trig
T 8 GeV/c < p
)1 < 16 GeV/c (x10trig
T12 GeV/c < p
)2 < 25 GeV/c (x10trig
T16 GeV/c < p
= 7 TeVspp,
ALI−PREL−34317
Background ∼ 95% π0 decay photonsUse π0 to evaluate background
8.0 < ptrigT < 12.0 GeV/c 12.0 < ptrigT < 16.0 GeV/c 16.0 < ptrigT < 25.0 GeV/c
Nucl. Phys. A 904 (2013) 697c QM 2012
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 13 / 33
xE of isolated photon
Sum up pT bins (∆pT = 1.0 GeV/c)
D(xγ isoE ) =
25 GeV/c∑i=8
(1
piDi(x
cluster isoE ) −
1 − pi
piDi(x
π0isoE )) −
25 GeV/c∑i=8
Di(xUEE ) (3)
Ex0 0.2 0.4 0.6 0.8 1
EdxdN
tr
igN
1
210
110
1
10 Isolated photons
< 25 GeV/ctrig
T8 GeV/c < p
= 7 TeVspp,
Fit (exponential)
Inverse slope : 7.8 +/ 0.9
ALI−PREL−34327
Nucl. Phys. A 904 (2013) 697c QM 2012
Exponential shape [0.2-0.8]
Baseline for the study of mediummodified parton fragmentation inPb-Pb collisions
Fit function: Ae−Bx
B = 7.8± 0.9
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 14 / 33
xE of isolated π0
Isolated π0: Eπ0 samples a large fraction of Eparton ⇔ < zπ0 >= 0.8 (Pythia +cuts)
Fit slope parameter of isolated π0
Ex0 0.2 0.4 0.6 0.8 1
EdxdN
tr
igN
1
410
310
210
110
1
100
πIsolated
< 12 GeV/c trig
T 8 GeV/c < p
)1 < 16 GeV/c (x10trig
T12 GeV/c < p
)2 < 25 GeV/c (x10trig
T16 GeV/c < p
= 7 TeVspp,
ALI−PREL−34322
Compare xE slopes with fragmentationfunction
(GeV/c)trig
Tp
8 10 12 14 16 18 20 22 24 26
slo
pe
EN
eg
ative
x
4
6
8
10
12
14 = 7TeVspp,
= 0.5 GeV/c)thres
T (R=0.4, p0πIsolated
DSS NL0 quark±π
DSS NL0 gluon±π
range [0.20.8]Ex
<z> = 1
<z> = 0.5
ALI−PREL−35420
8.0 < ptrigT < 12.0 GeV/c 12.0 < ptrigT < 16.0 GeV/c 16.0 < ptrigT < 25.0 GeV/c
Nucl. Phys. A 904 (2013) 697c QM 2012
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 15 / 33
Summary
Two-hadron correlations:Modification of charged hadrons yield in Pb-Pb at high pT :
Constraint for models: enhancement in near-side yield, but suppression at away-side.
Photon-hadron correlations:Establish global shape of fragmentation function through the measurement of isolatedphoton-hadron correlations with isolated photon trigger at 8.0 < pT < 25.0 GeV/c inpp collisions at
√s = 7 TeV .
Extract isolated π0 slope parameter to study fragmentation bias in using isolatedπ0-hadron correlations.Correlations measured in pp collisions will serve as a reference for future correlationmeasurements in Pb-Pb collisions. Work is ongoing.
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 16 / 33
Backup
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 17 / 33
Two-hadron correlations
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 18 / 33
Event and track selection
Data sets
PbPb at√sNN = 2.76 TeV : ∼ 15 M events from 2010 data taking period in 0-90%
centrality classpp reference: ∼ 55 M events from 2011 low energy run
Centrality selection: VZERO (2.8 < η < 5.1 and 3.7 < η < 1.7)
Tracking
TPC tracks constrained to the primary vertex
optimal azimuth (φ) acceptance = uniformity for angular correlationsMinimize twotrack cluster merging effects in the TPC
Two step correction procedure
2-track acceptance correction using mixed events ⇒ shapeSingle particle efficiency and contamination corrections ⇒ yield
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 19 / 33
Single particle corrections
Tracking efficiency Track contamination
Ncorrectedpair (∆η,∆φ, pT,trig, pT,assoc, C) = Nrawraw (∆η,∆φ, pT,trig, pT,assoc, C)
×Ctrkeff (pT,assoc, C)× Ctrkeff (pT,trig, C)× Ccont(pT,assoc)×Ccorrelatedcont(∆η,∆φ, pT,trig, pT,assoc)
Ncorrectedtrig (pT,trig, C) = Nrawtrig (pT,trig, C)× Ctrkeff (pT,trig)× Ccont(pT,trig)
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 20 / 33
Systematic uncertainties
Sourcesη range of flow subtractionTrack selectionVertex rangeInfluence of resonances and conversionsTwo-track effectWing (increase at large ∆η) correctionTwo different fit procedures
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 21 / 33
Photon-hadron correlations
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 22 / 33
Experimental aspects
Background from decay
Relative contributions of the quark-gluon Compton, q − q̄ annihilation andfragmentation subprocesses in NLO isolated photon production.
R. Ichou et al. arXiv:1005.4529[hep-ph]
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 23 / 33
Cluster in EMCal
ElectroMagnetic Shower Shower shape: long axis λ20, short axis λ2
1
λ20(1) = 0.5(dxx + dzz)±
√0.25(dxx − dzz)2 + d2xz
(dxx: cluster position in x direction weighted by the cell energy)
Photon identification with shower shape
photon:0.1 < λ2
0 < 0.27
π0: λ20 > 0.5
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 24 / 33
Shower shape parameters
η̄ =∑i cell
wi cell ηi cellwtotal
(4)
φ̄ =∑i cell
wi cell φi cellwtotal
(5)
wi cell = max(0, 4.5 + lnEi cellEclester
), wtotal =∑i cell
wi cell (6)
d2ηφ =
∑i cell
wi cell ηi cell φi cellwtotal
− η̄φ̄ (7)
λ20 = 0.5(dφφ + dηη) +
√0.25(dφφ − dηη)2 + d2
φη (8)
λ21 = 0.5(dφφ + dηη)−
√0.25(dφφ − dηη)2 + d2
φη (9)
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 25 / 33
π0 identification
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 26 / 33
Imbalance parameter extraction strategy
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 27 / 33
Isolation photon purity estimation
Shower shape method
Isolated clusters sample = isolated photons + background.
Binned likelihood fit of the shower shape distribution:⇒ combined signal (MC) and background (data) shower shape to fit data.
20λ
0.1 0.15 0.2 0.25 0.3 0.35 0.4
En
trie
s
0
200
400
600
800
1000
1200
1400
25/07/2012
< 0.5 GeV/cthres
TIsolation : R = 0.4, p
< 25 GeV/cT
16 < E
= 7 TeVspp
Combined fit
Background
ALI−PERF−31629
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 28 / 33
Isolated π0 fraction
Isolated π0 : Eπ0 samples a large fraction of Eparton ⇔ < zπ0 >= 0.8 (Pythia +cuts)
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 29 / 33
Systematic uncertainties
Main systematic uncertainties are :Shower shape MC / DataLikelihood fit parameters (binning, range)Background template composition (signal contamination, shower shape)Underlying event subtractionDetectors effects correction
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 30 / 33
Isolated π0 slopes: KKP
(GeV/c)trig
Tp
8 10 12 14 16 18 20 22 24 26
slo
pe
EN
egative x
4
6
8
10
12
14 = 7TeVspp,
= 0.5 GeV/c)thres
T (R=0.4, p0πIsolated
KKP NL0 quark±π
KKP NL0 gluon±π
range [0.20.8]Ex
<z> = 1
<z> = 0.5
ALI−PREL−35424
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 31 / 33
Isolation photon analysis in Pb-Pb
Photon identification in PbPb Collision:Started to separate isolated photons from background, estimate isolation
Photon purityNeed to understand the more complex background (flow)
20λ
0.1 0.15 0.2 0.25 0.3 0.35
En
trie
s
100
200
300
400
500 < 3 GeV/cthres
TIsolation : R = 0.3, p
< 25 GeV/cT
10 < E
= 2.76 TeVNN
s020% PbPb
Combined fit
Background
25/07/2012
ALI−PERF−31633
0 0.2 0.4 0.6
0
1
< 7 GeV/cγ
T5 < p
Centrality 0-20%
/5 rad)π| < π - φ ∆Head Region (|
Tz0 0.2 0.4 0.6
0
1
< 12 GeV/cγ
T9 < p
0 0.2 0.4 0.6
< 9 GeV/cγ
T7 < p
T = 0.5 pµZOWW, NLO,
= 1.48, 1.68, 1.88 GeV/fm0
∈
Tz0 0.2 0.4 0.6
< 15 GeV/cγ
T12 < p
AA
I
PHINEX Phys. Rev. C 80, 024908 (2009)
Xiangrong Zhu, Ruina Dang (CCNU) Two-Particle Correlations in pp and Pb-Pb with ALICE April 21, 2014 32 / 33