Measurement of Inclusive Charged Jet
Production in pp and Pb-Pb collisionsat √sNN = 5.02 TeV with ALICE
Hiroki Yokoyama for the ALICE collaboration
LPSC, Université Grenoble-Alpes, CNRS/IN2P3
University of Tsukuba, Japan
Hard Probes 2016 24/09/2016
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Outline
Introduction
Jet quenching
ALICE experiment
Measurement of inclusive charged jets in √sNN = 5.02 TeV pp and Pb-Pb collisions
Jet cross section in pp
Underlying event density
Underlying event fluctuation
Jet cross section in Pb-Pb
Nuclear modification Factor
Comparison with previous results
Summary and Outlook

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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Jets in HI Collisions ( Hard Probes of the QGP )What’s a Jet ?

Jet Quenching

Collimated spray of hadrons producedby the hard scattering of partons at the initialstage of the collision

Attenuation or disappearance of observed Jets
in Pb-Pb
due to partons' energy loss in the QGP

high-Q2 process, pT ≳ 20 GeV

jet shape broadening

Why Jets ?
The QGP lifetime is so short ( ~10-23 s ) thatcharacterisation by external probes is ruled out

evaluation of the degree of theattenuation allows to assess QGPproperties

self-produced probes
Occur at early stage : τ ~ 1/Q
probe the entire medium evolution
Production rate calculable within pQCD
well calibrated probes
Large cross-section at the LHC
copious production
Reconstructed jet enables to access
4-momentum of original parton
jet structure (energy re-distribution)
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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )ALICE Jet Quenching Measurements in Pb-Pb
Nuclear modification factor : RAA
if RAA = 1, NO modification

High-pT Hadrons

Jetsstrong suppression : RAA~0.2

strong suppression : RAA~0.4

proxy for Jet ( parton ) : pT >10 GeV/c

Jet shape broadens?
where is the lost energy?
RAA

fragmentation of quenched partons
1.2

ALICE Pb-Pb

sNN = 2.76 TeV

Anti-kT R = 0.2| η | < 0.5
jetplead,ch> 5 GeV/ cT1
Data 0 - 10%

Correlated uncertainty

Data 10 - 30%

Shape uncertainty

0.8
0.6

Correlated uncertainty
Shape uncertainty

JEWEL

JEWEL

YaJEM

YaJEM

0.4
0.2
0050

ALI−PUB−92182

Centrality Dependence of Charged Particle Production at Large Transverse
Momentum in Pb-Pb Collisions at √sNN = 2.76 TeV , PLB 720 (2013) 52

100pT,jet (GeV/c)50100pT,jet (GeV/c)

Measurement of jet suppression in central Pb-Pb collisionsat √sNN = 2.76 TeV , PLB 746 (2015) 14Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )ALICE Jet Quenching Measurements in Pb-Pb
Nuclear modification factor : RAA
if RAA = 1, NO modification

High-pT Hadrons

Jetsstrong suppression : RAA~0.2

strong suppression : RAA~0.4

proxy for Jet ( parton ) : pT >10 GeV/c

Jet shape broadens?
We are interested in quantifying the jet suppression
energy loss)
where is the (partonlost energy?
fragmentation of quenched partonsRAA

as a function of Jet pT , collision energy and centrality
1.2
ALICE Pb-Pb

sNN = 2.76 TeV

Anti-kT R = 0.2| η | < 0.5

For higher √sNN , denser, hotter and1 longer-lived QGP is created.
Data 0 - 10%
Data 10 - 30%=> stronger jet 0.8suppression
JEWEL
JEWEL

plead,ch> 5 GeV/ cT

Correlated uncertainty

Correlated uncertainty

Shape uncertainty

Shape uncertainty

YaJEM

0.6jetYaJEM0.40.20050

ALI−PUB−92182

Centrality Dependence of Charged Particle Production at Large Transverse
Momentum in Pb-Pb Collisions at √sNN = 2.76 TeV , PLB 720 (2013) 52

100pT,jet (GeV/c)

50100pT,jet (GeV/c)

Measurement of jet suppression in central Pb-Pb collisionsat √sNN = 2.76 TeV , PLB 746 (2015) 14Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Jet Measurement in LHC-ALICE
ALICE detector : focus on Heavy-Ion Collisions
LHC Run2 period started from 2015√s = 13 TeV pp , √sNN = 5.02 TeV Pb-Pb, pp

Charged Particles : |η| < 0.9, 0<φ<2π
ITS : silicon tracking detector
TPC : Time Projection Chamber

Charged Jet

In this analysis,charged jets are measured using
ALICE central barrel detectors.

PHOS
5Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Analysis Flow ( pp collisions )Dataset√s = 5.02 TeV, pp collisions

Event Selection

MB triggered events ( 25.5M events )Charged track selection

Track Selection| η | < 0.9, pTtrack > 0.15 GeV/c
Jet reconstruction
Jet Reconstruction

anti-kt jet reconstruction algorithm
R = 0.2, 0.4| η | < 0.7, ( pTlead > 5 GeV/c for RAA ref. )Unfoldingto correct for detector effects
Inclusive jet spectrum

UnfoldingPYTHIA detector RM

fully corrected to charged particle level
Inclusive Jet spectrum

6Jet finding eﬃciency
Jet matching eﬃciency

Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Analysis Flow ( Pb-Pb collisions )Dataset√sNN = 5.02 TeV, Pb-Pb collisions

Event Selection

MB triggered events( 3.36M events, ~3% of full statistics)
Track SelectionCharged track selection| η | < 0.9, pTtrack > 0.15 GeV/c
Jet reconstruction

Jet Reconstruction

UE density : ρanti-kt jet reconstruction algorithm
R = 0.2

Underlying Eventsubtraction| η | < 0.7, pTlead > 5 GeV/c

UE fluctuation : δpT

Underlying Event subtraction
background is subtracted from reconstructed
jetUnfolding
PYTHIA detector RM

Unfoldingto correct for detector effects andbackground fluctuations
Inclusive jet spectrum, RAA
fully corrected to charged particle level,
assess nuclear modification
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Inclusive Jet spectrum

Jet finding eﬃciency

RAA

pp inclusive Jet spectrum

Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )pp Inclusive Jet Cross Section
Jet cross section

Ratio of cross sections

well described by POWHEG NLO
calculations within systematic uncertainties

σ(R=0.2) / σ(R=0.4)
sensitive to the jet structurestronger collimation at high pT

poster : Measurement of inclusive charged jet cross sectionin pp collisions at √s = 5.02 TeV with ALICE at the LHC

well described by PYTHIA6, PYTHIA8 and POWHEG

8Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Underlying Event Density
Challenge in Heavy-Ion Collisions
large background contribution to jet energy
dNch/dη ~ 1300 ( 0-10% centrality )Jet Background Subtraction
background density : ρ
median kT excluding the highest two clusters

ρ ~ 145 GeV/c for 0-10% ( ~18 GeV/c for R=0.2 jets )background subtraction
background is estimated event-by-event and subtracted
from each jet
minimum leading constituent pT > 5 GeV/c requirement
suppresses combinatorial jets in low momentum

9Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Underlying Event Fluctuation
UE fluctuation : δpT
δpT is used as a measure for background fluctuations

Random Cone Method1) random selection
2) RC apart from leading jet ( Δr > 1.0 )to reduce jet component.
Δr = √(ηRC - ηjet)2 + (φRC - φjet)23) use ηφ randomised tracks
to exclude flow effectδpT width (magnitude of UE fluctuation)
fluctuations larger in central than in peripheral collisions~5 GeV/c for R=0.2, 0-10% centrality
2) RC with Δr > 1.0 is selected as UE fluctuation
10Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Inclusive Jet Cross Section
pp Jet cross section ( reference for RAA )Pb-Pb Jet cross section

pp reference run ( √sNN = 5.02 TeV )4 centrality bins (0-10, 10-30, 40-50, 50-90%)

POWHEG simulation

correlated uncertainty

to compensate for the limited statistics ofdata ( extend kinematic reach )correlated among pT bins
shape ( uncorrelated ) uncertainty
related to the shape of spectrum

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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Nuclear Modification Factor : RAA
RAA in each centrality bin
Increased suppression
from peripheral ~0.8 to central ~0.4
diﬀerence of pp reference
pp data / POWHEG simulationconsistent within uncertainties

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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Comparison of RAA with √sNN = 2.76 TeVCharged Jet RAA in √sNN = 5.02 TeV ( R=0.2 ) iscompared withRAA (5.02TeV) is comparable to RAA (2.76TeV)

1. Full Jet RAA in √sNN = 2.76 TeV ( R=0.2 , ALICE )2. Jet RAA in √sNN = 2.76 TeV ( R=0.4 , ATLAS )Jet energy scale is diﬀerent,( resolution parameter R is diﬀerent in ATLAS )13denser medium stronger jet suppression
smaller RAA
harder collision flatter jet spectrum
larger RAA
eﬀect of flattening of the spectrum
compensated by stronger jet suppression

Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )SummaryFirst measurement of jet RAA at √sNN = 5.02 TeVCharged jet, R=0.2, pTlead > 5 GeV/c
pp cross section , σ(R=0.2) / σ(R=0.4)
well described by POWHEG NLO simulation
Evaluation of Underlying Event density / fluctuationlarge fluctuating underlying event in most central collisions
Nuclear Modification Factor : RAA
strong suppression in most central collisions
RAA value is comparable to √sNN = 2.76 TeV
effect of flattening of the spectrum compensated by stronger jet suppression
EMCal

OutlookMore statistics to extend kinematic reach( Few percent of full statistics is used in this analysis. )DCalFull-jet measurement
enable direct comparison with Run1 results
three electromagnetic calorimeters in ALICE
PHOS
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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )BACKUPQuark-Gluon Plasma
What is QGP ?
Quark-Gluon Plasma (QGP)
Hot & dense color thermalized QCD matter prevailing at the early Universe ~1μs after
big bang
Deconfined state of quarks and gluons
Theoretically inferred through lattice gauge simulations of QCD

How to create ?
‘Little Bang’high-energy head-on nucleus-nucleus collisions
at particle accelerators
Recreate QGP droplets for a brief period of timeto quantitatively map out the QCD phase diagram

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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Jet Measurement in LHC-ALICE
ALICE detector : focus on Heavy-Ion Collisions
LHC Run2 period started from 2015√s = 13 TeV pp√sNN = 5.02 TeV Pb-Pb, pp

Charged Particles : |η| < 0.9, 0<φ<2π
ITS : silicon tracking detector
TPC : Time Projection Chamber
Charged constituents

Full Jet
PHOS
EMCal

Neutral particles : |η| < 0.7

Charged Jet

EMCal, (DCal : Run2 from 2015-)
DCal

Pb-Scintillator sampling calorimeter
PHOS
lead-tungsten crystal (PWO) basedcalorimeter
Neutral constituents

PHOS
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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )pp Inclusive Jet Cross Section

R=0.2

R=0.418

Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Data selection
Data sets

Event Selection

Pb-Pb data

kINT7
| vzSPD - vzPRI | < 0.1 cm( to avoid UE density mis-estimation )LHC15o, √sNN = 5.02 TeV Pb-Pb
pass2 low-IR, AOD (3.36M events)| vz | < 10 cm

MC simulation data

Charged track selection

PYTHIA( tracking eff. Jet finding eff. detector RM )Hybrid track selection in which same
parameters used with LHC11h.

LHC16e1(pthard-binned, jet production PYTHIA8),√s = 5.02 TeV pp

to compensate for inefficiency in SPD| η | < 0.9, pT > 0.15 GeV/c

LHC15l1b2(MB, PYTHIA6 Perugia-2011),√s = 5.02 TeV pp

Jet Reconstruction
R=0.2 , anti-kt algorithm, pt-scheme

HIJING ( tracking eff. )| η | < 0.7

LHC15k1a1, LHC15k1a2 , √sNN = 5.02 TeV
Pb-Pb

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Jet Area > 0.6 π R2
to reduce fake jet contamination at low
pT, jetHiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Inclusive Jet Spectrum, RAA

Inclusive Jet Spectrum

RAA in each centrality bin

scaled by nuclear overlap function : TAA

strong suppression in 0-10% than 10-30%
centrality

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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Underlying Event Density
Charged Track Multiplicity
large background superimposed to the Jets
dNch/dη ~ 1300 ( 0-10% centrality )Underlying Event Density
background density : ρ
median of kT clusters except largest two

ρ ~ 145 GeV/c for 0-10% ( ~18 GeV/c for R=0.2 jets )The average background energy density ρ scales
linearly with track multiplicity.

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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )Systematic Uncertainty
source of systematic uncertainties
RC selection (exclude RC w/ dr < 1.0)

Regularisation parameter ( k = 5 )exclude RC w/ dr < 0.5, 1.5

k = 4, 6

w/o 1st, 2nd lead. jets

Flow Bias

Unfolding method (SVD)± 4 GeV/c for background density

Bayesian unfolding

Generator selection (Pythia8)

measured pT range ( 30 - 120 GeV/c for 0-10%
centrality )PYTHIA6: Perugia0, HERWIG
Tracking Efficiency± 5 GeV/c for lower limit± 4 % from nominal value± 10 GeV/c for upper limit

evaluated by fast simulation

unfolded pT range ( 0 - 200 GeV/c for 0-10% centrality )+10 GeV/c for lower limit± 25 GeV/c for upper limit
0-10 % centrality
Shape Uncertainties
Unfolding Method
Regularisation Parameter
Measured pT Range
Unfolded pT Range
Generator
Shape Uncertainties : Total

30-40 [GeV/c]

50-60 [GeV/c]

4.2
0.4+0.1
-3.2
+0.1
-0.7
4.2+6.0-6.8

4.2
3.3+2.1
-1.2
+0.5
-0.1
5.2
+7.8
-7.6+5.1-1.9
6.4
1.5+8.3
-6.8

+3.8
-0.9
4.6
5.9+8.4-7.5

10-30 % centrality
Shape Uncertainties
Unfolding Method
Regularisation Parameter
Measured pT Range
Unfolded pT Range
Generator
Shape Uncertainties : Total

Correlated Uncertainties
δpT selection
Flow Bias
TrackingEfficiency
Correlated Uncertainties : Total

30-40 [GeV/c]

50-60 [GeV/c]

2.2
1.3
+1.4
-0.1
+0.0
-1.2
4.2
+5.1
-5.1

2.2
3.9
+1.1
-2.4
+0.6
-0.1
5.2
+7.0
-7.3

+5.4
-1.9
6.0
1.5+8.2
-6.5

+4.0
-1.5
4.7
5.9
+8.5
-7.7

Correlated UncertaintiesδpT selection
Flow Bias
TrackingEfficiency
Correlated Uncertainties : Total

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Hiroki Yokoyama : Hard Probes 2016 ( 24/09/2016 )