Inclusive charged jet spectra in pp and Pb-Pbcollisions at √SNN = 5.02 TeV with ALICE

Ritsuya Hosokawa
University of Tsukuba, JapanLPSC CNRS/IN2P3, Université Grenoble Alpes,FranceOutline• Introduction- Jet- Physics motivation• ALICE experiment• Results: Inclusive charged jet spectra measurements• Summary and outlook

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Jet
Di-jet candidate event√S NN = 5.02 TeV Pb-Pb collision at ALICE(Ch track pT > 3 GeV/c)• Collimated spray of hadrons originated from hard scattered partons atthe initial stage of collision
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Physics motivations• pp collisions- Good test of pQCD calculations and MC generators for high energy physics- Reference for heavy ion collisions• Pb-Pb collisions- Jets are well established probe for Quark-Gluon-Plasma (QGP) properties– QGP lifetime in heavy ion collisions is very short (~10-23)→ Self produced probes, like jets, allows to access QGP properties– Jets are produced at an very early stage of collision→ entire QGP evolution can be proved- Jets are modified while traversing the QGP→ Jet quenching effect– QGP properties can be probed by evaluating
the effect (Nuclear modification factor(RAA), Jet shape…)

https://www.star.bnl.gov/

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ALICE experiment• Specialized for measurements of heavy ion collisions• Explores QGP properties

ITS

EMCAL
V0A

• Charged particle tracking- Time Projection Chamber (TPC)- Inner Tracking System (ITS)

PHOS

DCAL
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• Minimum bias event triggeringand centrality determination- V0A,CTPCV0C• Neutral components measurement- Electro Magnetic Calorimeters– EMCAL, DCAL, (PHOS)

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ALICE experiment• Specialized for measurements of heavy ion collisions• Explores QGP properties

ITS

EMCAL
V0A

• Charged particle tracking- Time Projection Chamber (TPC)- Inner Tracking System (ITS)

PHOS

DCAL
24 January 2017

• Minimum bias event triggeringand centrality determination- V0A,C

TPC

V0C

Charged jets were measured inthis analysis with central barrel
charged tracking detectors
𝐴𝑐𝑐𝑒𝑝𝑡𝑎𝑛𝑐𝑒: 0 < 𝜑 < 2π , |0.9| < η
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6Jet reconstruction
correspondence
Jet• Establish
reconstruction
between detector measurements /final state particles / partons• two types of jet finder:- iterative cone
- sequential recombination
http://cms.web.cern.ch(e.g. anti-kT)
ITS,TPC• Signal jets were reconstructed byanti-kt algorithm• Background was estimated with jets
Anti-kt reconstructedalgorithm by kt algorithm(p.12 in this slide)
EMCAL,DCAL,PHOS• resolution parameter R

1) Include all particles in the cluster list.

hep-ph/0802.1189

2) Calculate
Where, p=-1,*,
and are respectively thetransverse momentum, rapidity and azimuth of particle i. R isradius parameter.
3) Set minimum value ofandas. If=,calculate the sum of four-momentum of cluster i and j which isweighted by energy, then set the cluster i and j as one cluster.
If=, consider
a Jet and then remove
fromcluster list.
hep-ph/0802.1189

Oliver Busch– HI café, Tokyo 2017/01
15/07/17

Journal Club22*(p=-1: anti-kt algorithm, p=1: kt algorithm)

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Results: pp 5.02 TeV

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8Inclusive charged jet cross section• Differential charged jet cross section- Detector effects in real data were corrected by the SVD unfolding method withdetector response extracted from MC simulation- Well described by POWHEG NLO calculation within systematic uncertainties• Jet cross section ratio of R = 0.2 and R = 0.4 jets
- Sensitive to the jet structure- Indicates stronger jet collimation at higher jet pT- Well described by POWHEG and PYTHIA
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9Comparison with pp and p-Pb results• No significant dependency on collision energy and collision system- Supports preceding study (Phys. Lett. B749 (2015) 68-81)

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10Results: Pb-Pb 5.02 TeV

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11Underlying event in Pb-Pb collisions• Difficulty on heavy ion collisions- Large background (Underlying event, UE)
to be subtracted
H.Yokoyama• Background density
𝜌 = 𝑚𝑒𝑑𝑖𝑎𝑛78𝑝2,456

𝐴9,45678- here, 𝑝2,456 are jets reconstructed bykt algorithm- 𝐴456: jet area
- excluding the highest pT tow jets

Central collision

Peripheral collision• Background subtraction:;<<<=>
𝑝2,456= 𝑝2,456− 𝜌 @ 𝐴456
H.Yokoyama

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12Underlying event in Pb-Pb collisions
H.Yokoyama• Underlying event fluctuation: 𝛿𝑝2 DE
𝛿𝑝2 = B 𝑝2C<=:7 − 𝐴 @ 𝜌

RC: random cone9

• 3 methods were tested
1) Simply apply random cone without any limitations
2) RC apart from leading jet (Δr > 1.0)
- can be reduced contributions from signal jets component
Δ𝑟 =H.Yokoyama(𝜂DE − 𝜂JKC ) M +(𝜙 DE − 𝜙 JKC ) M

3) randomized track(η,φ)- to exclude flow effect

In this analysis, 2) was selected as UE fluctuation
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pp data / POWHEG simulationconsistent within uncertainties

Charged jet nuclear modification factor: RAA

ross section

y bins (0-10, 10-30, 40-50, 50-90%)
uncertainty

ed among pT bins

ncorrelated ) uncertainty

to the shape of spectrum

12Hiroki Yokoyama

H.Yokoyama• Spectra in Pb-Pb collisions are corrected by SVD unfolding withdetector(from MC) and background response(from data)• Strong suppression at central collisions- Centrality dependence of the suppression

H.Yokoyama• Difference of pp reference (POWHEG or Real data)- Consistent within uncertainties
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RAA comparison with √SNN = 2.76 TeV

H.Yokoyama

H.Yokoyama•

Results at √S NN = 5.02 TeV are compared with…
- Full jet R AA in √S NN = 2.76 TeV collisions at ALICE (R=0.2)- Full jet R AA in √S NN = 2.76 TeV collisions at ATLAS (R=0.4)•Results at 5.02 TeV is comparable to the results at 2.76 TeV- Generally, more denser, hotter and ling time QGP formation is expected at higher √S NN→ Stronger suppression → decrease the R AA
- More harder (high pT) jet generation is expected at higher √S NN→ flatter jet spectrum → increase the R AA

→ effect of spectrum flattening is compensated by the stronger jet suppression
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15Summary• First measurements of charged jet spectra and RAA have been performedfor LHC Run2 data at ALICE.
• pp collisions• Inclusive charged jet differential cross sections are well described by NLO
calculation (POWHEG)• Jet cross section ratio is well described by POWHEG and PYTHIA• Reference for Pb-Pb collisions is established (~60 GeV/c)• Pb-Pb collisions• Larger Underlying Event fluctuation is observed at most central collisions incomparison with peripheral collisions• Nuclear modification factor- Strong suppression is observed in central collisions- Comparable with the results in √SNN = 2.76 TeV collisions- effect of spectrum flattening is compensated by the stronger jet suppression

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16Jet Measurement in LHC-ALICEOutlook

ALICE detector : focus on Heavy-Ion

LHC Run2 period started from 2015√s = 13 TeV pp• Extend jet pT reach with more statistics √s = 5.02 TeV Pb-Pb, pp: |η| < 0.9, 0<(pp: ~25 % of full statistics, Pb-Pb: ~ fewCharged% of Particles
full statistics)NN

ITS : silicon tracking detector
TPC : Time Projection Chamber• Full jet measurement with calorimetersCharged constituents- Allows direct comparison with Run1(2.76 TeV) results Full JetPHOS
EMCalNeutral particles : |η| < 0.7Charg

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 Probe

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Back up

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Data set and event selections: pp• Data Set• Data: LHC15n pass2 lowIR(~25M events)
MC: LHC15l1b2 (PYTHIA6, pp 5 TeV, Perugia-2011)
MC: LHC15l1a2 (PYTHIA8, pp 5 TeV, Monash2013)
MC: LHC16e1 (PYTHIA8, pp 5 TeV, Monash-2013, PtHard production)• Event selection
•••
•
•
•

MB event selection (kINT7, V0A and V0C trigger)|VtxZ| < 10 (cm)Number of tracklets contributing to the primary vertex is at least 2
Pileup event cut|VtxZtrack - VtxZSPD| < 0.5 (cm)VtxZSPD reconstruction resolution is better than 0.25 (cm) and the dispersion is less than 0.04 *• Jets• Charged tracks• Hybrid track (2011 version)• Utilized FastJet package
FastJet v3.1.3• Anti-Kt algorithm• Cone radii R=0.2,0.4• Fiducial cut24 January 2017

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19Data selection
Dataset and event selections: Pb-Pb
Data sets

Event Selection

Pb-Pb data

kINT7

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

LHC16e1(pthard-binned, jet production PYTHIA8),√s = 5.02 TeV pp
LHC15l1b2(MB, PYTHIA6 Perugia-2011),√s = 5.02 TeV pp

to compensate for inefficiency in SPD| η | < 0.9, pT > 0.15 GeV/cJet Reconstruction
R=0.2 , anti-kt algorithm, pt-scheme

HIJING ( tracking eff. )| η | < 0.7

LHC15k1a1, LHC15k1a2 , √sNN = 5.02 TeVPb-Pb

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Jet Area > 0.6 π R2
to reduce fake jet contamination at lowpT, jet20

kt clusters

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Charged jet cross section (R = 0.4)

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The technique for estimate a t
Unfoldingobserved distribution

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PYTHIA pthard-binned data

uced by tracking efficiency ratio to HĲING.

Response matricesof centrality bin and track pTV/c on detector level.

M

Inputs forunfolding
δpT distribution( w/olead. jet ) by RooUnfold software package(arXiv:1105.1160)

ciencyjet finding eﬀ. for 0-10% centrality
BKG RM for LHC15o data (0-10%)g

PYTHIA pthard-binned data

combined RM for 0-10% centrality
detector RM for 0-10% centrality

reduction and pTlead selection with detector

with PYTHIA MB.

HIA pthard-binned data=✖

: efficiency ratio to HĲING.
dUnfoldingby tracking

ng, k=5 bin and track pT
centralityrange: 30-120GeV/cdetectorlevel.
T on

range : 0-200 GeV/c

distribution ( w/o lead. jet )ncy

jet finding eﬀ. for 0-10% centrality

combined RM for 0-10% centrality

HIA pthard-binned datauction and pTlead selection with detector• Input for pp spectrum correction

PYTHIA MB.
24 January 201731• Input for Pb-Pb spectrum correction
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24Hiroki
Yokoyama : Physics Forum - Hard Probes Previews ( 16/09/2016)

Systematic uncertainties: pp
R=0.2
5-6 GeV
Efficiency
Resolution
SecondaryUE subtraction(Minusonly)
Unfolding
Normalization
Total (Minus, quadr. sum)
Total (Plus, quadr. sum)
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Systematic uncertainties: Pb-Pb

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