Jet Physics with ALICEat the LHC

Oliver Buschfor the ALICE collaboration

Oliver Busch – TGSW 2017/09/26

1

Outline• introduction• jet nuclear modification factor• jet shapes• subjet measurements at STAR, CMS and ALICE

Oliver Busch – TGSW 2017/09/26

2

Introduction

Oliver Busch – TGSW 2017/09/26

3

Jets: seeing quarks and gluons• jet: collimated bunch of hadrons• quasi-free parton scattering at high Q2:
the best available experimental equivalent to quarks and gluons
Oliver Busch – TGSW 2017/09/26

4

Jet fragmentation• initial hard scattering: high-pT partons• cascade of (anti-)quarks and gluons: parton shower• at soft scale (O(ΛQCD)): hadronization

Oliver Busch – TGSW 2017/09/26

5Jet reconstruction• Establish correspondence between
detector measurements /final state particles / partons• two types of jet finder:- iterative cone
- sequential recombination(e.g. anti-kT)• resolution parameter R

hep-ph/0802.1189

6

Partons in heavy-ion collisions• hard partons are produced early and traverse the hot and dense QGP• expect enhanced parton energy loss: ‘jet quenching’ (mostly) due tomedium-induced gluon radiation• ‘vacuum’ expectation calculable by pQCD : ‘calibrated probe of QGP’
• jets sensitive to properties ofthe medium (energydensity, , mean free path,coupling ... )• ... but also jet-medium

interaction not trivial(strong / weak coupling,
parton mass / type,fireball dynamics ...)

JET collaboration,
Phys. Rev. C 90 (2014) 014909

7

Jets at ALICE (LHC run 1)• charged particle tracking:- Inner Tracking System (ITS)- Time Projection Chamber- full azimuth, |η |< 0.9
pT > 150 MeV/c• EMCal :- neutral particles- Δφ = 107°, |η|<0.7
cluster ET > 300 MeV• jet trigger with EMCal and TRD• ‘charged’ (tracking) jets and ‘full’ jets
• full jets from charged particle tracking and EM energy:

conceptually different and complementary to traditional approach8

Jet nuclear modification factor•

Phys.Lett. B746 (2015) 1• strong suppression observed, similar to hadron RAAJEWEL: PLB 735 (2014)→ parton energy not recovered inside jet cone
• increase ofsuppressionwith centrality• JEWEL andYaJEM
jet quenchingmodels reproduce
suppressioncentral

YaJEM:PRC 88 (2013) 014905

semi-central

Jet RAA at 5 TeV• motivation: compare to previous results at 2.76 TeV to study√sNN dependence of RAA• with increasing √sNN
higher initial energy density• stronger jet quenching ?

• comparable RAA:

• with increasing √sNN

flatter parton spectra• effect of flattening
compensated bystronger suppression
ALI-PREL-114186

Oliver Busch – TGSW 2017/09/26

10

Jet Shapes

Oliver Busch – TGSW 2017/09/26

11

Jet shapes• radial moment ‘girth’ g, longitudinal dispersion pTD,

difference leading - subleading pT LeSub• shapes in Pb-Pb as probe ofquenching of low-pT jets:
characterise fragment distributions and are sensitive to medium inducedchanges of intra-jet momentum flow
• ‘event-by-event’ measure, sensitive to fluctuations

Oliver Busch – TGSW 2017/09/26

12

Jet shapes in Pb-Pb• fully corrected to charged particle level• compare to PYTHIA reference, validated with results from pp collisions
at 7 TeV

1/Njets

dN/dg• g shifted to smaller values → indicates more collimated jet core30

ALICE PreliminaryPb-Pb sNN = 2.76 TeV

25 Anti-k T charged jets, R = 0.2
40 < p jet,ch < 60 GeV/c20T

ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 111510500

ALI−PREL−101580

0.02

0.04

0.06

0.08

0.1

0.12

g
Oliver Busch – TGSW 2017/09/26

13• larger pTD in Pb-Pb compared to PYTHIA→ indicates fewer constituents in quenched jets
• LeSub in Pb-Pb in good agreement with pp:→ hardest splittings likely unaffectedPb-Pb sNN = 2.76 TeV

5 Anti-k T charged jets, R = 0.2

ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 1140 < p jet,ch <60 GeV/c4T3200.3

ALICE Preliminary
0.18 Pb-Pb sNN = 2.76 TeV

0.04

1/N

1
0.4

0.5

0.6

0.7

0.8

0.9

1

0.2

dN/dLeSub (c /GeV)

jets

1/N

ALICE Preliminary

jets

6T

dN/dp D• collimation through emission of soft particles at large angles

0.16 Anti-k T charged jets, R = 0.2jet,ch0.14 40 < p TT

ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 11

0.12
0.1
0.08
0.06
0.02
00p D
ALI−PREL−101584< 60 GeV/c

ALI−PREL−101588

Oliver Busch – TGSW 2017/09/26

51015

202530

LeSub (GeV/c )14

Subjets

Oliver Busch – TGSW 2017/09/26

15

Subjets at LHC• declustering and soft drop grooming to identify hard jet substructure• subjet momentum balance> 0.1• in vacuum, dσ/dzg ~ splitting function• CMS: strongest
suppression forlower pTjet at high zg

CMS PAS HIN 16-006
16

Subjets in STAR
• STAR: RHIC accelerator (USA), √sNN = 200 GeV• select dijet pairs matching to ‘hard core’ jets reconstructed with highconstituent cut pTconst > 2 GeV/c• no suppression observed• role of different kinematics, STAR selection bias, subjet ΔR cut ?

STAR, HP 2016

17

ALICE subjets• charged jets, kt declustering• subjettiness•2/

N: how consistent is a jet with having N subjets

1 : no significant modification

18

ALICE subjet ΔR• subjet distance ΔR - observable potentially sensitive to medium response(J. G. Milhano, U. A. Wiedemann, K. C. Zapp, arXiv: 1707.04142)• data uncorrected for detector effects and background fluctuations
compared to PYTHIA embedded reference• no significant modification observed relative to reference,full correction to particle level in progress

19

Summary• hard probes allow to probe properties of the QGP• first insights on dynamics parton of energy loss from jet

nuclear modification factor and jet shape measurements• collimation through emission of soft particles at large angles• exploring potential of subjet observables• intriguing jet pT dependence• dN/dzg from ALICE under preparation

Oliver Busch – TGSW 2017/09/26

20

- Backup -Oliver Busch – TGSW 2017/09/26

21

Jet reconstruction• Establish correspondence between
detector measurements /final state particles / partons• two types of jet finder:- iterative cone
- sequential recombination(e.g. anti-kT)• resolution parameter R

hep-ph/0802.1189

22

QCD matter at LHC• direct photons:

prompt photons from hard scattering+ thermal radiation from QCD matter• low-pT inverse slope parameter:

Teff = 297 +/- 12stat. +/- 42syst. MeV/c• indicates initial temperature way
above TC

arXiv 1509.07324 [nucl-ex]
Oliver Busch – TGSW 2017/09/26

23

Hadrons in heavy-ion collisions
PLB 720 (2013) 250• high- pT hadrons `proxy’ for jet• jet quenching for charged hadrons,

Pb-Pb collisions at √sNN = 2.76 TeV• hadron observables biased towards
leading fragment→ study the effect for fully reconstructed jets

Oliver Busch – TGSW 2017/09/26

24

Analysis details• charged jets from charged particle tracks, pTconst > 150 MeV/c
in pp MinB at 7 TeV and Pb-Pb 10% central at 2.76 TeV• R=0.2, 40 < pTjet < 60 GeV/c, no leading constituent cut• novel background subtraction methods (Pb-Pb)- area subtraction (G. Soyez et al, Phys. Rev. Lett 110 (2013) 16)- constituent subtraction (P. Berta et al, JHEP 1406 (2014) 092)
1/N dN/dg

10

1.8
1.6jet• 2D unfolding to correct
for background fluctuations
and detector effects

21
10−1

1.4

10−2

1.2

10−3

ALICE simulation
R=0.2, Anti-kT charged jets
jet,ch40 < pT <60 GeV/c−41 10
0.8

10−5

0.6

10−6

0.4
0.2
00

Pythia Det. Level
Pythia Embedded Area. Sub (2nd order)
Pythia Embedded Const. Sub
Pythia Embedded Unsubtracted

10−7
10−8
10−9010

ALI-SIMUL-101958

0.02 0.04 0.06 0.08
2030

40

50

0.1
60

0.12 0.14 0.16
70

8090

uncorrected g25

Underlying event in heavy-ion collisions• jet reconstruction in heavy-ion collisions :difficult due to the high underlying event backgroundnot related to hard scattering• correct spectra for background fluctuations and detector effects
via unfolding• not possible down to lowest jet pT

centraljet area ~ 0.5 (R = 0.4)
peripheral

Jet shapes: model comparison• trends reproduced by JEWEL jet quenching model:

collimation through emission of soft particles at large angles

JEWEL: K.C. Zapp, F. Kraus, U.A. Wiedemann, JHEP 1303 (2013) 080
Oliver Busch – TGSW 2017/09/26

27