Jet Physics
at ALICE

Oliver Busch
University of Tsukuba
Heidelberg University

Oliver Busch – Waldthausen, 26.08.20161

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Outline• Introduction• Results from pp collisions• Identified jet fragmentation in pp• Jets in heavy-ion collisions• Jet shapes in Pb-Pb collisions

Oliver Busch – Waldthausen, 26.08.20162Introduction

Oliver Busch – Waldthausen, 26.08.20163

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 – Waldthausen, 26.08.20164Jet fragmentation• initial hard scattering: high-pT partons• cascade of gluons: parton shower• at soft scale (o(ΛQCD)): hadronization

Oliver Busch – Waldthausen, 26.08.20165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
Oliver Busch – Waldthausen, 26.08.20166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 approach7

Results from pp collisions

Oliver Busch – Waldthausen, 26.08.2016

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Full jets in pp at= 2.76 TeV• good agreement with NLO calculations for R = 0.2 and R = 0.4• reference for Pb-Pb at same energy
R = 0.4

R = 0.2

Phys. Lett. B 722 (2013) 262
Oliver Busch – Waldthausen, 26.08.2016

9CMS jets at= 7 TeV• single inclusive jet cross sections compared to NLO:
agreement over 14 orders of magnitude• comparable theoretical and experimental uncertainties• complementary jet pT reach
PRD 87 (2012) 112002

Oliver Busch – Waldthausen, 26.08.201611pp jet fragmentation at•distributionsof charged particles incharged jets

• for z > 0.2 distributions consistent
for all jet pT: ‘scaling’• bulk production at low z:~ 5-10 charged particles per jet

PRD 91, 112012= 7 TeVPID in jets : `TPC coherent fit’
• particle identification via specific ionisation in TPC ('dE/dx'):• TPC coherent fit:use energy loss model parameterisation as input,
adjust model
parameters and particle
fractions `on the fly’during fit• regularisation requiringcontinuity ofparticle fractions• complementary andconsistent:
multi-template fit
Oliver Busch – Waldthausen, 26.08.201613

Particle identified fragmentation• identified charged hadrons in charged jets at= 7 TeV• π, K, p, 5 < pTch jet < 20 GeV/c• scaling for zch > 0.2 for higher jet pT bins

Oliver Busch – Waldthausen, 26.08.201614

Particle ratios in jets
• leading baryons suppressed• strangeness content strongly enhanced for zch → 1

Oliver Busch – Waldthausen, 26.08.201615

Event generator comparison• comparison to PYTHIA(pT ordered parton shower,Lund string fragmentation)• data reasonably welldescribed• best reproducedby Perugia tune without
color reconnections

Jets and Quark-Gluon Plasma

Oliver Busch – Waldthausen, 26.08.201617

Hadrons in heavy-ion collisions• hard partons are produced early and traverse the hot and dense QGP• expect enhanced parton energy loss, (mostly) due tomedium-induced gluon radiation: ‘jet quenching’• high-pT hadrons ‘proxy’ for jet

PLB 720 (2013) 250• jet quenching for charged hadrons,

ALICE Pb-Pb sNN = 2.76 TeV
charged particles, |η|<0.8

Pb-Pb collisions at √sNN = 2.76 TeV• measure RAA for fully reconstructedjets to mitigate fragmentation bias
and assess parton kinematicsR AA

10-5%
70-80%
10−10ALI−DER−105710

Oliver Busch – Waldthausen, 26.08.2016

1020304050

p (GeV/c )T18

Underlying event in heavy-ion collisions• jet reconstruction in heavy-ion collisions :high underlying event backgroundnot related to hard scattering• background is dominant at low jet and constituent pT• background fluctuations are importantcentraljet area ~ 0.5 (R = 0.4)
peripheral

19Jet nuclear modification factor• strong suppression,

similar to hadron RAA→ parton energy notrecovered inside jet cone• increase of suppression
with centrality,
weak pT dependence• JEWEL:- microscopic pQCD
parton shower + gluon
induced emissions• YaJEM:- detailed fireball model- parameterisation of radiative and collisionalenergy loss

Phys.Lett. B746 (2015) 1
JEWEL: PLB 735 (2014)
YaJEM:PRC 88 (2013) 014905• different models reproduce observed jet suppression→ study jet quenching through more differential measurements

20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,sensitive to medium inducedchanges of intra-jet momentum flow
• ‘event-by-event’ measure, sensitive
to fluctuations

Oliver Busch – Waldthausen, 26.08.201621

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)
21/N dN/dg

10

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

1−1

10

1.4

10−2

1.2

10−3

ALICE simulation
R=0.2, Anti-kT charged jets
jet,ch40 < pT <60 GeV/c−4

1 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

10−710−810−9010ALI-SIMUL-101958

0.02 0.04 0.06 0.08
20304050

0.1
600.12 0.14 0.16708090uncorrected g22

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

1/Njets

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

ALICE Preliminary
Pb-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

0.12gOliver Busch – Waldthausen, 26.08.201623

• larger pTD in Pb-Pb compared to PYTHIA→ indicates fewer constituents in quenched jets

Pb-Pb sNN = 2.76 TeV
Anti-k T charged jets, R = 0.2
40 < p4jet,chT

ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 11<60 GeV/c3200.3

ALICE Preliminary
0.18 Pb-Pb sNN = 2.76 TeV

0.04

1/N1
0.4

0.5

0.60.70.80.9

10.2

dN/dLeSub (c /GeV)

jets

1/N5

ALICE Preliminaryjets

6TdN/dp D• LeSub in Pb-Pb in good agreement with Pb-Pb:→ hardest splittings likely unaffected

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

T

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 – Waldthausen, 26.08.2016

51015202530

LeSub (GeV/c )24

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 – Waldthausen, 26.08.201625

Summary• Jets at ALICE: a rich and interesting Physics programme !
• jet cross sections and properties studied with ALICE in pp collisions• identified jet fragmentation distributions in pp collisions• jet nuclear modification factor• jet shapes in Pb-Pb… and much more that wasn’t shown today !

Oliver Busch – Waldthausen, 26.08.201626

Happy Birthday !

Oliver Busch – Waldthausen, 26.08.201627

- Backup -Oliver Busch – Waldthausen, 26.08.2016

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ALICE jet response (pp)• full jets: JE correction ~ 20%, ‘resolution’ ~18%
• JES uncertainty ~3.6%

Oliver Busch – Waldthausen, 26.08.2016

pp charged jet cross-sections• measured in minimum bias collisions at= 7 TeV• good agreement with ATLAS charged jet measurements(despite slightly different acceptance and track pT range)
R = 0.4

R = 0.6

PRD 91 (2015)
112012

Oliver Busch – Waldthausen, 26.08.201610

Multi Template Fit• TPC multi-template fit- best possible description of dE/dx from external reference- parametrize dependences on η, TPC nClusters- templates in transverse momentum (z, xi) slices• dE/dx in one z slice (0.6 < z < 0.65), 10-15 GeV/c fitted with
4 templates

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Oliver Busch – Waldthausen,
26.08.2016

Method comparison• uncorrected hadron fractions from Multi-Template Fit andTPC Coherent Fit• 2 complementary methods
obtain consistent results

Oliver Busch – Waldthausen, 26.08.2016

Hard probes and medium properties• jet quenching sensitive to properties ofthe medium (energy
density, , 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, 014909 (2014)
Oliver Busch – Waldthausen, 26.08.201633

• baryons / meson RAA a probe ofgluon / quark energy loss?

arXiv: 1506.07287
1• would expect stronger radiative

energy loss for gluons than for quarks- subtle cancellations?
- hadron observable biased
towards hard fragmentation?

• study jets to improve ourR AA

Identified hadrons in heavy-ion collisions
ALICE 0-5% Pb-Pb sNN=2.76 TeV
π++πK++K
p+pCharged

0.8
0.6
0.4
0.2

02

4

68

10

1214

ALI−DER−86149

1618p (GeV/ c )understanding of parton energy loss:- PID in reconstructed jets mitigates fragmentation biases- enhanced sensitivity to medium effects measuring soft particles in jetsT• note: medium effects likely strongest at scales of ~ medium Temperature(J.G. Milhano, K. C. Zapp, hep-ph/1512.0819, T. Renk, Phys. Rev. C 81, 014906,
B. Mueller, hep-ph/1010.4258)
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