Post on 16-Oct-2021
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
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
• Introduction
• Jet shapes
• Jet azimuthal anisotropy
• Summary
Outline
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
Introduction
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
• 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’
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Partons in heavy-ion collisions
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
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 ... )
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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
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• 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
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
• 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
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Analysis details
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
Jet shapes in Pb-Pb
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• 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
Correlated uncertainty
PYTHIA Perugia 11
= 2.76 TeVNNsPb-Pb
= 0.2R charged jets, TkAnti-
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
Correlated uncertainty
PYTHIA Perugia 11
= 2.76 TeVNNsPb-Pb
= 0.2R charged jets, TkAnti-
ALICE Preliminary
c < 60 GeV/jet,ch
Tp40 <
ALI−PREL−101588
→
→
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
• 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
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
Jet azimuthal anisotropy
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
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
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
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
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
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
η, |Tk = 0.2 anti-R
c > 3 GeV/T, lead
p, c > 0.15 GeV/T, track
p
30-50%, Stat unc.ch jet2v
Syst unc. (shape)
Syst unc. (correlated)
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
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)
Syst unc. (correlated)
ALICE
= 2.76 TeVNNsPb-Pb
|<0.7jet
η, |Tk = 0.2 anti-R
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
30-50%, Stat unc.ch jet2v
Syst unc. (shape)
Syst unc. (correlated)
ALICE
= 2.76 TeVNNsPb-Pb
|<0.7jet
η, |Tk = 0.2 anti-R
c > 3 GeV/lead
Tp, c > 0.15 GeV/
T, trackp(b)
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
• 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
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
- Backup -
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Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
Jet shapes in pp
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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
= 0.2R charged jets, TkAnti-
c < 60 GeV/jet,ch
Tp40 <
ALICE Data
Shape uncertainty
Correlated 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
= 0.2R charged jets, TkAnti-
c < 60 GeV/jet,ch
Tp40 <
ALICE Data
Shape uncertainty
Correlated 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
= 0.2R charged jets, TkAnti-
c < 60 GeV/jet,ch
Tp40 <
ALICE Data
Shape uncertainty
Correlated 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
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
• 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
→
→
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
• 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
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ALI-PREL-101608ALI-PREL-101616
Qualitiative discussion
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
• 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
Oliver Busch – XIIth Quark Confinement and the Hadron Spectrum, Thessaloniki
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
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)
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
η, |Tk = 0.2 anti-R
c > 3 GeV/T, lead
p, c > 0.15 GeV/T, track
p
30-50%, Stat unc. ch jet2v
Syst unc. (shape)
Syst unc. (correlated)
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
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