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

for the ALICE collaboration

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki1Outline• Introduction• Jet shapes• Jet azimuthal anisotropy• Summary

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki2

Introduction

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

3Partons in heavy-ion collisions• hard partons are produced
early and traverse the hot
and 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 – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

4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 – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki5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• 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 …

6

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

Jet nuclear modification factor• strong suppression,
similar to hadron RAA→ parton energy notrecovered inside jetcone• 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

8Jet Shapes

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki8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 – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

10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
1.6

jet

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

2

1
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

−4

1 10
0.8

10−5

0.6

10−6

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

−7

0.4
0.2
0
0

10

10−8
10−9

0
10

ALI-SIMUL-101958

0.02 0.04 0.06 0.08
20

30

40

50

0.1
60

0.12 0.14 0.16
70

80

90

uncorrected g 11

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/N

jets

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/c

20

T

ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 11

15
10
5
0
0

ALI−PREL−101580

0.02

0.04

0.06

0.08

0.1

0.12

g
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki12• 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/c

4

T

3
2

0
0.3

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

6

T

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

0.12
0.1
0.08
0.06
0.02
0
0

5

10

p D
ALI−PREL−101584

T

ALI−PREL−101588

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

15

20

25

30

LeSub (GeV/c )13

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 – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

14

Jet azimuthal anisotropy

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

15

Reaction plane dependence• different medium thickness in- and out-of plane• sensitive to path length dependence of jet
quenching:pQCD radiative E-loss : ~L2collisional E-loss : ~Lstrong coupling (ADS/CFT) : ~L3

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

16

Analysis details• charged jets, R = 0.2• account for flow-modulation of background via

event-by-event fit and subtraction of local background density
200
Pb-Pb sNN = 2.76 TeV

ALICESingle event

150

ch

background fluctuations :separately for spectrain- and out-of-planeρ (ϕ) (GeV/c )• unfolding to account for100
50 0.15 < p

T, track0

track| < 0.9

ρ (ϕ )

ρ (1+2v cos(2[ϕ-Ψρρ (1+2v cos(3[ϕ-Ψch0< 5 GeV/ c , |η
0

2

0

0

1

3

2

3

Phys. Lett. B753 (2016) 511
Oliver Busch – XIIth Quark Confinement \and the Hadron Spectrum, Thessaloniki

]))

EP, 2

]))

EP, 3

4

5

6
ϕ (rad)17

Jet v2 : results• quantify azimuthal asymmetry via 2nd Fourier harmonic v2ch jet• central collisions: 1.5 - 2 sigma from v2ch jet = 0→ consistent with 0, but maybe hint for effect of initial density fluctuations ?

ch jet

v2

0.2

ALICE

0-5%, Stat unc.

Pb-Pb sNN = 2.76 TeV

Syst unc. (shape)

Syst (correlated)

Syst unc. (correlated)

R = 0.2 anti-k T, |η |<0.7
jet

0.1

ch jetv2

0.2

30-50%, Stat unc.

ALICE
Pb-Pb sNN = 2.76 TeV

Syst unc. (shape)

Syst (correlated)

R = 0.2 anti-k T, |η |<0.7
jet

Syst unc. (correlated)

0.1

0

0
(a)

20

jet
{EP, |∆η|>0.9 }
v ch
2

jet
{EP, |∆η|>0.9 }v ch
2• non-zero v2ch jet in semi-central collisions

30

p

40

50

T, track60> 0.15 GeV/c , p

70

T, lead(b)> 3 GeV/ c

80 90 100ch jetp(GeV/c )

20

p

30

40

50

T

T, track60> 0.15 GeV/c , p

70

T, lead> 3 GeV/ c

80 90 100ch jetp(GeV/c )T

Phys. Lett. B753 (2016) 511
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

18Comparison to JEWEL• good agreement with JEWEL in semi-central collisions

ch jet0-5%, JEWEL

ALICEch jet0-5%, Stat unc.

Pb-Pb sNN = 2.76 TeV

v2

0.2

v2

v 2ch jetv 2ch jet• clear indication of path-length dependence of energy loss

0.2

30-50%, JEWEL

ALICEch jet30-50%, Stat unc.

Pb-Pb sNN = 2.76 TeV

v2

R = 0.2 anti-k T, |η |<0.7

Syst unc. (shape)

R = 0.2 anti-k T, |η |<0.7

Syst unc. (shape)

ch jet

v2

jet

jet

Syst unc. (correlated)

Syst unc. (correlated)

0.1

0.1

0

0

(a)

20

p

30

40

50

T, track60> 0.15 GeV/c , p lead > 3 GeV/ c
T70

80 90 100ch jetp(GeV/c )

(b)

20

p

30

40

50

T

T, track60> 0.15 GeV/c , p lead > 3 GeV/ c
T70

80 90 100ch jetp(GeV/c )T

Phys. Lett. B753 (2016) 511
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

19Summary• 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 described by quenching models like JEWEL• jet azimuthal anisotropy- significant v2ch jet in semi-central collisions consistent with path-length
dependence of energy loss
- good agreement with JEWEL (semi-central)

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

20

- Backup -Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki21

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 0

0.08
0.06

2

10

T

0.1

jets

3

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

0.14

1/N

1/N

15

ALICE Data
Shape uncertainty
Correlated uncertainty
PYTHIA Perugia 11
PYTHIA Perugia 0

1/N

T

20

5

T

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

25

jets

dN/dp DdN/dg

• fair agreement with PYTHIA simulations:validates PYTHIA as reference for Pb-Pb

0.04

1

5

0

ALI−PREL−101515

0.02

0.04

0.06

0.08

0.1

0.02

0.04

0.06

0.08

0.1

0.12

0.12

g

0

0
0.3
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0

0.3

ALI−PREL−101510

0.4

0.5

0.6

0.7

0.8

0.9

0.4

0.5

0.6

0.7

0.8

0.9

1

1

p D
T

Data/MC

0

20
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0

Data/MC

Data/MC

0.02

20
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0

0

ALI−PREL−101520

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

5

10

15

5

10

15

20

25

30

20

25

30

LeSub (GeV/c )22

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

25
20
15

7
ALICE Simulation

6

Anti-k T charged jets, R =0.2
jet, chPYTHIA Perugia 0
40 < p< 60 GeV/ c
T

5

ALI−SIMUL−101543

0.02

0.04

0.06

0.08

0.1

0.12

jets

1

40 < p< 60 GeV/ c

PYTHIA Perugia 0
pp s = 2.76 TeV
Gluon jets
Quark jets0.02

0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

ALI−SIMUL−101547

T

0.04

0

0
p D
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessalonikigjet, ch1/N

5Anti-k T charged jets, R =0.2

0.06

3
2

ALICE Simulation

0.1

0.08

4

10

0
0

pp 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/ c

T

dN/dp D

35

1/N

1/N

jets

dN/dg

• LeSub characterises hardest splitting, insensitive against backgroundT

ALI−SIMUL−101551

5

10

15

20

25

30

LeSub (GeV/c )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

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

24

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

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
ALI-DER-102103

ALI-DER-102107

25

Comparison to previous results• ALICE + CMS single particles, ATLAS full jets :different energy scales !

• non-zero v2 up to high pT

CMS, PRL 109 (2012) 022
ATLAS, PRL 111 (2013) 152

0.3

ALICE

ch jetPb-Pb sNN = 2.76 TeV
R = 0.2 anti-k T, |η |<0.7jetv2

0-5%, Stat unc.

Syst unc. (shape)
Syst unc. (correlated)calo jet
ATLAS v 2
5-10%
part

0.2

CMS v2 {|∆η|>3} 0-10%v 2part, v 2jetv 2part, v 2jet

ALICE, Phys. Lett. B753 (2016) 511
ALICE, Phys. Lett. B719 (2013) 18
0.3

ALICE

ch jetPb-Pb sNN = 2.76 TeV
R = 0.2 anti-k T, |η |<0.7
jetcalo jet

0.1

0.1

0

0

0

50

T, track> 0.15 GeV/c , p

T, lead> 3 GeV/ c

100
150
part
jet
p , p (GeV/c )T

Syst unc. (correlated)

p

(b)

0

30-50%part
CMS v 2 {|∆η|>3} 30-50%
part
ALICE v 2 {|∆η|>2} 30-50%

part

p

30-50%, Stat unc.

Syst unc. (shape)
ATLAS v 2

0.2

ALICE v 2 {|∆η|>2} 0-5%

(a)

v2

50

T

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki

T, track> 0.15 GeV/c , p

T, lead> 3 GeV/ c

100
150
part
jet
p , p (GeV/c )T

T26