jets in in pb-pb collisions at alice - tsukuba

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

Oliver Busch

University of Tsukuba Heidelberg University

for the ALICE collaboration

Jets in in Pb-Pb collisions at ALICE

1


• Introduction

• Jet shapes

• Jet azimuthal anisotropy

• Summary

Outline

2


Introduction

3


• hard partons are produced early and traverse the hot and dense QGP

• expect enhanced parton energy loss, (mostly) due to medium-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’

4

Partons in heavy-ion collisions


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 and are sensitive to its properties (energy density, , mean free path, coupling ... )

5

6

• 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 …

• 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

Jets at ALICE (LHC run 1)

central peripheral

• jet reconstruction in heavy-ion collisions : high underlying event background not related to hard scattering

• background is dominant at low jet and constituent pT

• background fluctuations are important

jet area ~ 0.5 (R = 0.4)

Underlying event in heavy-ion collisions

8

• strong suppression, similar to hadron RAA → parton energy not recovered 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 collisional energy loss

• different models reproduce observed jet suppression → study jet quenching through more differential measurements

Phys.Lett. B746 (2015) 1JEWEL: PLB 735 (2014)

YaJEM:PRC 88 (2013) 014905

Jet nuclear modification factor

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki 8

Jet Shapes


• radial moment ‘girth’ g, longitudinal dispersion pTD, difference leading - subleading pT LeSub

• shapes in Pb-Pb as probe of quenching of low-pT jets: characterise fragment distributions and are sensitive to medium induced changes of intra-jet momentum flow

• ‘event-by-event’ measure, sensitive to fluctuations

Jet shapes

10

0 10 20 30 40 50 60 70 80 900

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

guncorrected 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

g d

N/d

jet

1/N

9−10

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10

1

10

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

ALICE simulation

charged jetsT

R=0.2, Anti-k

c <60 GeV/jet,ch

T40 < p

ALI-SIMUL-101958

• 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)

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

11

Analysis details


Jet shapes in Pb-Pb

12

• fully corrected to charged particle level

• compare to PYTHIA reference, validated with results from pp collisions at 7 TeV

• g shifted to smaller values indicates more collimated jet core

g0 0.02 0.04 0.06 0.08 0.1 0.12

g d

N/d

jets

1/N

0

5

10

15

20

25

30

ALICE Data

Shape uncertainty

Correlated uncertainty

PYTHIA Perugia 11

= 2.76 TeVNNsPb-Pb

= 0.2R charged jets, TkAnti-

c < 60 GeV/jet,ch

Tp40 <

ALICE Preliminary

ALI−PREL−101580

→

Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki 13

• larger pTD in Pb-Pb compared to PYTHIA indicates fewer constituents in quenched jets • LeSub in Pb-Pb in good agreement with Pb-Pb: hardest splittings likely unaffected

DT

p0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

DT

p d

N/d

jets

1/N

0

1

2

3

4

5

6

ALICE Data

Shape uncertainty


PYTHIA Perugia 11

= 2.76 TeVNNsPb-Pb


ALICE Preliminary

c <60 GeV/jet,ch

Tp40 <

ALI−PREL−101584

)c (GeV/LeSub0 5 10 15 20 25 30

/GeV

)c

(LeSub

dN

/dje

ts1/N

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

ALICE Data

Shape uncertainty


PYTHIA Perugia 11

= 2.76 TeVNNsPb-Pb


ALICE Preliminary

c < 60 GeV/jet,ch

Tp40 <

ALI−PREL−101588

→

→


• 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

Jet shapes: model comparison

14


Jet azimuthal anisotropy

15


Reaction plane dependence• different medium thickness in- and out-of plane

• sensitive to path length dependence of jet quenching: pQCD radiative E-loss : ~L2

collisional E-loss : ~L

strong coupling (ADS/CFT) : ~L3

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

• unfolding to account for background fluctuations : separately for spectra in- and out-of-plane

Phys. Lett. B753 (2016) 511 (rad)ϕ0 1 2 3 4 5 6

)c

) (G

eV

/ϕ(

chρ

0

50

100

150

200 = 2.76 TeVNNsPb-Pb

Single event

| < 0.9 track

η, |c < 5 GeV/T, track

p0.15 <

]))EP, 2

Ψ-ϕcos(2[2

(1+2v0

ρ

]))EP, 3

Ψ-ϕcos(3[3

(1+2v0

ρ

)ϕ(ch

ρ

0ρ

ALICE

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 ?

• non-zero v2ch jet in semi-central collisions

Phys. Lett. B753 (2016) 511)c (GeV/ch jet

Tp

20 30 40 50 60 70 80 90 100

|>

0.9

}η

∆ {

EP

, |

ch je

t2

v

0

0.1

0.2

Syst (correlated)

ALICE

(a)

= 2.76 TeVNNsPb-Pb

|<0.7jet

η, |Tk = 0.2 anti-R

c > 3 GeV/T, lead

p, c > 0.15 GeV/T, track

p

0-5%, Stat unc.ch jet2v

Syst unc. (shape)

Syst unc. (correlated)

)c (GeV/ch jet

Tp

20 30 40 50 60 70 80 90 100

|>

0.9

}η

∆ {

EP

, |

ch je

t2

v

0

0.1

0.2Syst (correlated)

ALICE

(b)

= 2.76 TeVNNsPb-Pb

|<0.7jet


c > 3 GeV/T, lead


p


Syst unc. (shape)


18


Comparison to JEWEL

• good agreement with JEWEL in semi-central collisions

• clear indication of path-length dependence of energy loss

Phys. Lett. B753 (2016) 511

)c (GeV/ch jet

Tp

20 30 40 50 60 70 80 90 100

ch je

t2

v

0

0.1

0.2 0-5%, JEWEL

ch jet2v

0-5%, Stat unc. ch jet2v

Syst unc. (shape)


ALICE

= 2.76 TeVNNsPb-Pb

|<0.7jet


c > 3 GeV/lead

Tp, c > 0.15 GeV/

T, trackp(a)

)c (GeV/ch jet

Tp

20 30 40 50 60 70 80 90 100

c

h je

t2

v0

0.1

0.2 30-50%, JEWEL

ch jet2v


Syst unc. (shape)


ALICE

= 2.76 TeVNNsPb-Pb

|<0.7jet


c > 3 GeV/lead

Tp, c > 0.15 GeV/

T, trackp(b)

19


• 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 and have 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)

Summary

20


- Backup -

21


Jet shapes in pp

22

0 0.02 0.04 0.06 0.08 0.1 0.12

g d

N/d

jets

1/N

0

5

10

15

20

25 ALICE Preliminary = 7 TeVspp


c < 60 GeV/jet,ch

Tp40 <

ALICE Data

Shape uncertainty


PYTHIA Perugia 11

PYTHIA Perugia 0

g0 0.02 0.04 0.06 0.08 0.1 0.12

Da

ta/M

C

00.20.40.60.8

11.21.41.61.8

2

ALI−PREL−101515

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

DT

p d

N/d

jets

1/N

0

1

2

3

4

5ALICE Preliminary

= 7 TeVspp


c < 60 GeV/jet,ch

Tp40 <

ALICE Data

Shape uncertainty


PYTHIA Perugia 11

PYTHIA Perugia 0

DT

p0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Da

ta/M

C

00.20.40.60.8

11.21.41.61.8

2

ALI−PREL−101510

• fully corrected to charged particle level • fair agreement with PYTHIA simulations: validates PYTHIA as reference for Pb-Pb

0 5 10 15 20 25 30

/Ge

V)

c d

N/d

Le

Su

b (

jets

1/N

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14 ALICE Preliminary

= 7 TeVspp


c < 60 GeV/jet,ch

Tp40 <

ALICE Data

Shape uncertainty


PYTHIA Perugia 11

PYTHIA Perugia 0

)cLeSub (GeV/0 5 10 15 20 25 30

Da

ta/M

C

00.20.40.60.8

11.21.41.61.8

2

ALI−PREL−101520


• 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 • LeSub characterises hardest splitting, insensitive against background

Jet shapes as quenching signatures

g0 0.02 0.04 0.06 0.08 0.1 0.12

g d

N/d

jets

1/N

0

5

10

15

20

25

30

35PYTHIA Perugia 0

= 2.76 TeVspp Gluon jetsQuark jets

c < 60 GeV/jet, ch

Tp40 <

=0.2R charged jets, TkAnti-

ALICE Simulation

ALI−SIMUL−101543

DT

p0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

DT

p d

N/d

jets

1/N

0

1

2

3

4

5

6

7

PYTHIA Perugia 0 = 2.76 TeVspp

Gluon jetsQuark jets

c < 60 GeV/jet, ch

Tp40 <

=0.2R charged jets, TkAnti-ALICE Simulation

ALI−SIMUL−101547)c (GeV/LeSub

0 5 10 15 20 25 30

/GeV

)c

(LeSub

dN

/dje

ts1/N

0

0.02

0.04

0.06

0.08

0.1

PYTHIA Perugia 0 = 2.76 TeVspp

Gluon jetsQuark jets

c < 60 GeV/jet, ch

Tp40 <

=0.2R charged jets, TkAnti-

ALICE Simulation

ALI−SIMUL−101551

→

→


• 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

24

ALI-PREL-101608ALI-PREL-101616

Qualitiative discussion


• jet quenching = jet pT shift + vacuum fragmentation?

• if yes, would expect shapes to agree with vacuum shapes from higher pT jets

• g agrees qualitatively with this picture, however pTD does not

25ALI-DER-102107ALI-DER-102103

Qualitiative discussion II


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

• non-zero v2 up to high pT ALICE, Phys. Lett. B753 (2016) 511 ALICE, Phys. Lett. B719 (2013) 18

)c (GeV/ jet

Tp, part

Tp

0 50 100 150

jet

2v

, p

art

2v

0

0.1

0.2

0.3ALICE

= 2.76 TeVNNsPb-Pb

|<0.7jet


c > 3 GeV/T, lead


p


Syst unc. (shape)


5-10% calo jet2vATLAS

0-10%|>3}η∆{|part

2CMS v

0-5%|>2}η∆{| part2vALICE

(a)

)c (GeV/ jet

Tp, part

Tp

0 50 100 150

jet

2v

, p

art

2v

0

0.1

0.2

0.3ALICE

= 2.76 TeVNNsPb-Pb

|<0.7jet


c > 3 GeV/T, lead


p


Syst unc. (shape)


30-50% calo jet2vATLAS

30-50%|>3}η∆{| part2vCMS

30-50%|>2}η∆{| part2vALICE

(b)

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

26

jets in in pb-pb collisions at alice - tsukuba

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