Jet Properties
in Pb-Pb collisions at ALICE
Oliver Busch
University of Tsukuba
Heidelberg University

for the ALICE collaboration

Oliver Busch – LHC Seminar 05/2016

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Outline• Introduction• Jets in heavy-ion collisions• Strangeness production in jets
• Jet shapes• Summary

Oliver Busch – LHC Seminar 05/2016

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Introduction

Oliver Busch – LHC Seminar 05/2016

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QCD phase transition• in heavy-ion collisions at ultra-relativistic energies,

a quasi macroscopic fireball of hot, strongly interacting matterin local thermal equilibrium is created• lattice QCD predicts phase transition to deconfined,chirally symmetric matter• energy densityfrom the lattice:
rapid increase around TC,indicating increase ofdegrees of freedom(pion gas →quarks and gluons)• TC = 154 ± 9 MeV

EC = 340 ± 45 MeV/fm3

HotQCD, PRD 90, 094503

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

Phys. Lett. B 754 (2016) 235
Oliver Busch – LHC Seminar 05/2016

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Partons in heavy-ion collisions• hard partons are produced
early and traverse the hotand dense QGP• expect enhanced partonenergy loss, (mostly) due tomedium-induced
gluon radiation: ‘jet quenching’• jet: ‘collimated bunch of hadrons’• the best available experimental equivalent to quarks and gluons• ‘vacuum’ expectation calculable by pQCD: ‘calibrated probe of QGP’

Oliver Busch – LHC Seminar 05/2016

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Parton fragmentation• initial hard scattering: high-pT partons with high virtuality• virtuality evolution through parton shower• hadronisation at a scale (O(ΛQCD))• jets probe the medium at a variety of scales andare sensitive to its properties(energy density, ,mean free path, coupling ... )Oliver Busch – LHC Seminar 05/2016

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Hadrons in heavy-ion collisions• high- pT hadrons ‘proxy’ for jet• jet quenching for charged hadrons,

Pb-Pb collisions at √sNN = 2.76 TeV

PLB 720 (2013) 250
ALICE Pb-Pb sNN = 2.76 TeV
charged particles, |η|<0.8R AA

10-5%
70-80%
10−10ALI−DER−105710

Oliver Busch – LHC Seminar 05/2016

1020304050p (GeV/c )T8• baryons / meson RAA a probe ofgluon / quark energy loss?

• would expect stronger radiative

R AA

Identified hadrons in heavy-ion collisions
arXiv: 1506.07287
1
ALICE 0-5% Pb-Pb sNN=2.76 TeV
π++πK++K
p+p
Charged0.8

energy loss for gluons than for quarks
0.6- subtle cancellations?
- hadron observable biased
0.4
towards hard fragmentation?
0.2• study jets to improve our02468

101214

ALI−DER−86149

161820p (GeV/ c )Tunderstanding of parton energy loss:- PID in reconstructed jets mitigates fragmentation biases- enhanced sensitivity to medium effects measuring soft particles in jets• 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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Jets at ALICE (LHC run 1)• charged particle tracking:- Inner Tracking System (ITS)- Time Projection Chamber- full azimuth, |η |< 0.9pT > 150 MeV/c• EMCal :- neutral particles- Δφ = 107°, |η|<0.7cluster ET > 300 MeV• full jets: assess full parton energy• charged (tracking) jets:- full azimuthal coverage- measures parton energy deposited into charged fragments- good definition of jet axis: well suited for fragmentation, jet structure, PID …

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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

Strangeness Productionin Jets

Oliver Busch – LHC Seminar 05/2016

3Strange hadron reconstruction• neutral strange particles reconstructed via
decay topology (‘V0’):• signal extraction frominvariant mass distributions

Oliver Busch – LHC Seminar 05/201613Strangeness production in nuclear collisions• Baryon / Meson ratio enhanced in Pb-Pband p-Pb collisions- collective effects ?
- parton recombination ?
- jet fragmentation ?

• measurement of identified particles

in jets helps to constrain hadronisationand energy loss scenarios

Phys. Lett. B 728 (2014) 25

Phys. Rev. Lett. 111 (2013) 223001

Strangeness in jets
• candidate - jet matching (V0 in jet cone)• jet R = 0.2, acceptance |ηV0 | < 0.7• candidate - bulk matching: underlying event V0• signal extraction from invariant mass distributions• correct for efficiency and feed-down• subtract underlying event from spectra

Oliver Busch – LHC Seminar 05/201615

Underlying event subtraction• subtract underlying event contribution to K0S, Λ spectra in jets

• various methods with different sensitivity to acceptance, event plane
correlations, presence of additional jets, …

• differences used to estimate systematic uncertainty

Oliver Busch – LHC Seminar 05/201616

(Λ+Λ)/2K0s ratio in jets
• Pb-Pb collisions at= 2.76 TeV•(Λ + Λ)/2K0S• jet R = 0.2
pTjet > 10 GeV/c (20 GeV/c)• leading constituent bias
pTleading > 5 GeV/c
to reject ‘fake’ jets

2ALICEPreliminary

1.8

Pb−Pb, s NN = 2.76 TeV, 0−10 %in jets, p

1.6

in jets, pT> 10 GeV/ c> 20 GeV/ c0inclusive Λ/K S, ALICE,(0−5 %, |y 0| < 0.5)

1.2

V1|η 0| < 0.7V

anti-k t, R = 0.2|η| < 0.50.8

than for inclusive caseTjet,chfeed-down uncertainty

1.4• no significant jet pTjet dependence• ratio in jets significantly lower

jet,chp

0.6

jet,chleading track> 5 GeV/ cTp track > 150 MeV/c
T

0.4
0.2
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ALI−PREL−93799

Oliver Busch – LHC Seminar 05/2016

24681012p (GeV/ c )T17Comparison to p-Pb• compare Pb-Pb results to reference from p-Pb collisions at 5.02 TeV:
agreement within uncertainties

p-PbS

systematics for lowest
K0S, Λ pT(Λ + Λ)/2K0• ongoing efforts to improve

Pb-Pb
2ALICEPreliminary

1.8

Pb−Pb, s NN = 2.76 TeV, 0−10 %in jets, p

1.6

in jets, pjet,chT
jet,chT> 10 GeV/ c> 20 GeV/ cfeed-down uncertainty

1.40inclusive Λ/K S, ALICE,(0−5 %, |y 0| < 0.5)

1.2

V1|η 0| < 0.7V

anti-k t, R = 0.2|η| < 0.50.8p

0.6jet,chleading track> 5 GeV/ cTp track > 150 MeV/c
T0.4
0.2
0ALI-PREL-80712

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ALI−PREL−93799

Oliver Busch – LHC Seminar 05/2016

4681012p (GeV/ c )T18

Jet Shapes

Oliver Busch – LHC Seminar 05/2016

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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 pp collisions at 7 TeV:
- constrain QCD calculations of small-R jets(‘microjets’: M. Dasgupta, F. Dreyer, G. Salam, G. Soyez)- validate MC simulations• shapes in Pb-Pb as IRC safe 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 – LHC Seminar 05/201621

Jet shapes as quenching signatures• compare quarks and gluons: gluon jets broader and softer• g is pT weighted width of the jet:- broadening (collimation) → enhanced (reduced) g• pTD measures pT dispersion:- less constituents / ‘more democratic’ splitting → reduced pTD

T

Quark jets

jets

2520157
ALICE Simulation

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Anti-k T charged jets, R =0.2jet, ch
PYTHIA Perugia 040 < p< 60 GeV/ c
T5

ALI−SIMUL−101543

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gjets

1jet, chT

< 60 GeV/ c

PYTHIA Perugia 0pp s = 2.76 TeVGluon jets
Quark jets

0.04
0.02

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

ALI−SIMUL−101547 Oliver Busch – LHC Seminar 05/2016

40 < p1/N

5Anti-k T charged jets, R =0.20.06

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ALICE Simulation

0.10.08

41000pp s = 2.76 TeV
Gluon jets
Quark jets

dN/dLeSub (c /GeV)PYTHIA Perugia 0
ALICE Simulation
30 Anti-k T charged jets, R =0.2 pp s = 2.76 TeV
Gluon jets
40 < p jet, ch < 60 GeV/ cT

dN/dp D35

1/N

1/Njets

dN/dg• LeSub characterises hardest splitting, insensitive against background

p D
T

00
ALI−SIMUL−101551

51015202530

LeSub (GeV/c )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

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

2110−1

1.4

10−2

1.2

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

10−5

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10−6

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

−70.4
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00

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ALI-SIMUL-101958

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4050

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600.12 0.14 0.16708090uncorrected g 23

Jet shapes in pp• fully corrected to charged particle level

dN/dLeSub (c /GeV)

ALICE Preliminary
pp s = 7 TeV
Anti-k T charged jets, R = 0.2

440 < pjets

jet,chT

0.12< 60 GeV/ c
ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 11
PYTHIA Perugia 0

ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 11
PYTHIA Perugia 00.08
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210T0.1

jets

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ALICE Preliminary
pp s = 7 TeV
Anti-k T charged jets, R = 0.2jet,ch40 < p< 60 GeV/ c

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1/N

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ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 11
PYTHIA Perugia 01/N

T205T

ALICE Preliminary
pp s = 7 TeV
Anti-k T charged jets, R = 0.2jet,ch40 < p< 60 GeV/ c25jets

dN/dp DdN/dg• fair agreement with PYTHIA simulations:validates PYTHIA as reference for Pb-Pb

0.04

150

ALI−PREL−101515

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g

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ALI−PREL−101510

Oliver Busch – LHC Seminar 05/2016

Data/MC

020
1.8
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Data/MC

Data/MC

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ALI−PREL−101520

5101551015202530

202530

LeSub (GeV/c )24

Jet shapes in Pb-Pb• fully corrected to charged particle level

1/Njets

dN/dg• g shifted to smaller values compared to PYTHIA reference→ 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

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0.12g
Oliver Busch – LHC Seminar 05/201625

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

Pb-Pb sNN = 2.76 TeV

5 Anti-k T charged jets, R = 0.2

ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 11

40 < p jet,ch <60 GeV/c4T3
200.3

ALICE Preliminary
0.18 Pb-Pb sNN = 2.76 TeV

0.04

1/N

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dN/dLeSub (c /GeV)

jets

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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
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00p D
ALI−PREL−101584< 60 GeV/cALI−PREL−101588

Oliver Busch – LHC Seminar 05/2016

51015202530

LeSub (GeV/c )26

Jet structure: 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 – LHC Seminar 05/2016

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Qualitiative discussion• characterise degree of dispersion and broadening

in terms of ‘quark-like’ and ‘gluon-like’• observed effects favour ‘quark-like’ scenario• quenching mechanism or change of quark/gluon composition
caveat: jet pT not equal parton pT

ALI-PREL-101608

ALI-PREL-101616

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Qualitiative discussion II• jet quenching = jet pT shift + vacuum fragmentation?

• if yes, would expect shapes to agree with vacuum shapes fromhigher pT jets
• g agrees qualitatively with this picture, however pTD does not

ALI-DER-102103

ALI-DER-102107

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Summary• strangeness production in jets in Pb-Pb collisions- significant difference between the Λ/K0S ratio of inclusive particles
and the ratio in charged jets• measurement of jet shapes- characterise modifications of intra-jet momentum flow by QGP- results indicate that jet cores in Pb-Pb are narrower and harder andhave fewer constituents than PYTHIA pp reference- results in qualitative agreement with quark-like fragmentation anddescribed by quenching models like JEWELOliver Busch – LHC Seminar 05/2016

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- Backup -Oliver
OliverBusch
Busch––LHC
TsukubaSeminar201405/2016/03/1328

LHC run 2• LHC run 2: 2015 - 2017, heavy-ion run November 2015

• increased CMS energy for Pb-Pb collisions from 2.76 → 5.1 TeV• quenching strength ~ s ~ ε3/4
• expect (modest) increase in ε, T

→ measure energy density
dependence of jet quenching• note: also a dependence onparton ‘input spectrum’(increased RAA ??? )Oliver Busch – TGSW 2015 /09/30

ALICE, PRL 105, 252301

ALICE in run 2: DCal

• run 2: DCal upgrade- significantly extended jet acceptance- back-to-back in azimuth (di-jet topology)
OliverOliverBusch
Busch– –LHCTGSW
Seminar2015 05/2016/09/30

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Jet structure• ‘jet structure ratio’ R=0.2 / R=0.3 for charged jets

• sensitive to potential broadening of jet shape• consistent with PYTHIA pp:
no modification observed
within small radii(jet core)

JHEP 03 (2014) 013

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

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Oliver Busch – LHC Seminar 05/2016