two-particle correlations in pp and pb-pb collisions with...

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


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).


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


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)


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)


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)


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


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


Photon-hadron correlations

Data sets

pp at√s = 7 TeV : ∼ 10 M EMCal triggered events from 2011 data

(Lint ≈ 500 nb−1)


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


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).


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


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


T12 GeV/c < p


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


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


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


T12 GeV/c < p


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


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.


Backup


Two-hadron correlations


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


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)


Systematic uncertainties

Sourcesη range of flow subtractionTrack selectionVertex rangeInfluence of resonances and conversionsTwo-track effectWing (increase at large ∆η) correctionTwo different fit procedures


Photon-hadron correlations


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]


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


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)


π0 identification


Imbalance parameter extraction strategy


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


Isolated π0 fraction

Isolated π0 : Eπ0 samples a large fraction of Eparton ⇔ < zπ0 >= 0.8 (Pythia +cuts)


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


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


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)


Medium mofified fragmentation function


two-particle correlations in pp and pb-pb collisions with...

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