Hard Probes of the QGP: What have welearned and what can we still learn?
Marco van Leeuwen, !
Nikhef and Utrecht University
Heavy Ion Cafe, Tokyo!
27 September 2014

Soft QCD matter and hard probes
Heavy-ion collisions produce
QCD matter
Dominated by soft partonsp ~ T ~ 100-300 MeV

Hard-scatterings produce ‘quasi-free’ partons⇒ Initial-state production known from pQCD
⇒ Probe medium through energy loss
‘Hard Probes’: sensitive to medium density, transport properties2

Hard Probes of the Quark Gluon Plasma
1. Understand* interactions between hard partons and the
Quark Gluon Plasma!
•

Is LPM interference important?!

•

Is interference between successive parton emissions important?!
Is there a dead cone effect for heavy quarks?!
Are the quark/gluon differences driven by CA/CF, as expected?!

••

2. Use this to learn about the properties of the Quark Gluon
Plasma!
••General properties: nature of the interactions: scattering centers or fields(strong coupling), effective degrees of freedom? etc!
Specific properties: e.g. density/temperature at RHIC vs LHC*As usual: there is some freedom in defining the question, e.g. we can decide that certain
aspects are not tractable and/or uninteresting3

Nuclear modification factor
RAA =dN / dpT Pb + Pb
N coll dN / dpT p + p

Suppression factor 2-6
Significant pT-dependence
Similar at RHIC and LHC?
So what does it mean?
4

From RHIC to LHC
RHIC: 200 GeV per nucleon pair
LHC: 2.76 TeV
Energy ~14 x higher

LHC: spectrum less steep,larger pT reach

RHIC: n ~ 8.2
LHC: n ~ 6.4

5

Back-of-the envelope considerations
Just to get a sense of the numbers:!
• Take the hadron spectra and apply ‘energy loss’!
!
!
!

•

Two scenarios:!
•

Constant relative energy loss ΔE/E!
• RAA constant for a power law spectrum!

•

Constant energy loss ΔE!
• RAA increases with pT for a power law spectrum

RAA depends on n, steeper spectra, smaller RAA
6

From RHIC to LHC
Back-of-the envelope considerations
RHIC

LHC

RHIC: n ~ 8.2

LHC: n ~ 6.4

6.2

4.4

(1 − 0.23) = 0.20

(1 − 0.23) = 0.32

Qualitative lessons:1) Similar RAA at LHC means larger energy loss
2) Increase of RAA with pT: relative energy loss not constantExpect: constant ΔE, or log increase + kinematic limits

7

Towards a more complete picture

Energy loss not single-valued, but a distribution!
• Geometry: density profile; path length distribution!
• Energy loss is partonic, not hadronic!
•

–
–

–

Full modeling: medium modified shower!
Simple ansatz for leading hadrons: energy loss followed byfragmentation!
Quark/gluon differences8

First generation models
Parton spectrum Energy loss distribution Fragmentation (function)dN=
dpT hadrdN
dE jets
known
pQCDxPDF⊗ P(ΔE ) ⊗ D( pT ,hadr / E jet )extract`known’ from e+e-This is where the information about the medium isP(ΔE) combines geometry
with the intrinsic process– Unavoidable for many observables
Notes:• This is the simplest ansatz – most calculation to date use it (except someMCs)
• Jet, γ-jet measurements ‘fix’ E, removing one of the convolutions
9

Finding qhat from data at RHIC and LHC
1

0.8

RHIC

q =0.8, 1.0, 1.2, 1.4, 1.7 GeV 2/fm
0

0.6

RAA

0.4

PHENIX 2008 (0-5%)
PHENIX 2012 (0-5%)

0.8

RHIC

0.6

0.4
0.2

0.2

0.8

0

CUJET2.0(αmax,fE,fM)

68

10 12 14pT (GeV/c)

0.6 18
16

0.4

0.2

LHC
20

8

10

12

14

16

p (GeV)

18

20

1

20

0.8

0

6

T

40
60
pT (GeV/c)

CMS 0-5%
ALICE 0-5%
(0.20,1,0)(0.21,1,0)(0.22,1,0)(0.23,1,0)(0.24,1,0)(0.25,1,0)(0.26,1,0)(0.27,1,0)(0.28,1,0)(0.29,1,0)(0.30,1,0)(0.31,1,0)(0.32,1,0)80

CMS (0-5%)
Alice (0-5%)
q =1.4, 1.8, 2.2, 2.6, 3.0 GeV 2/fm
0

0.6

RAA

0

RAA(pT)

RAA(pT)CUJET2.0(αmax,fE,fM)

0.4

LHC

0.2

100

Burke et al, JET Collaboration, arXiv:1312.5003

PHENIX 0-5% 2012
PHENIX 0-5% 2008
(0.20,1,0)(0.21,1,0)(0.22,1,0)(0.23,1,0)(0.24,1,0)(0.25,1,0)(0.26,1,0)(0.27,1,0)(0.28,1,0)(0.29,1,0)(0.30,1,0)(0.31,1,0)(0.32,1,0)

0

10

20

30

40

50

60

p (GeV)

70

80

90

100

T

Systematic comparison of energy loss models with data!
Medium modeled by Hydro (2+1D, 3+1D)

pT dependence matches reasonably well10

Fitting the jet quenching parameter
CUJET 2.0

HT-BW
PHENIX 08+12

4

CMS+ALICE

χ2/d.o.f2χ /d.o.f.(pT>8)

8
6
4

3
2
1

2

0
0

0.2

0.22 0.24 0.26 0.28
αmax

0.3

0.32

CUJET: 𝛼s is medium parameter!
Lower at LHC

1

1.5

2

2

2.5

3

3.5

Burke et al, JET Collaboration, arXiv:1312.5003

PHENIX 08+12
CMS+ALICE

10

q0 GeV /fm (LHC)

HT: transport coeff is parameter!
Higher at LHC
11

Summary of transport coefficient study
7

MARTINI
HT-BW
HT-M

6

(T=370 MeV)

McGill-AMY
GLV-CUJET

ˆ 3
q/T

5

LHC:

4
3

(T=470 MeV)

2

Expect factor 2.2 frommultiplicity + nuclear size

103qˆ N/Teff
(DIS)

0

0.1

Au+Au at RHIC
Pb+Pb at LHC
0.2

0.3

0.4

Burke et al, JET Collaboration, arXiv:1312.5003

RHIC:

0.5

T (GeV)values from different models consistent(NB: multiple-soft scattering omitted)

larger at RHIC than LHC: running of 𝛼s ?!
Or: limited validity of models?
12

Are we done with qhat?
Not at all!
There are significant conceptual problems with the baseline models
Main open questions for RAA-type observables: !
• Large angle radiation, kT ~ k"
•
•
•

•

Large x, ΔE ~ E"•••

Some large x results/estimates exist; still eikonal?!
Probably not important for medium-high pT!

Path averaging"•••Not treated in any of the ‘analytical’ calculations!
Important for phenomenology!
Path to solution: include NLO/recoil!

Not much work done; not simple due to interference!
Possible solution: brute force; integrate path integral over scattering centers (Zakharov)!

Multiple gluon emission"••

Most calculations use independent emission!
May suffice for leading hadrons, but jet observables need a more complete treatment

We can hope/argue that the impact of these on qualitative picture may be limited,!
but quantitative conclusions require a closer look13

Medium-induced radiation
Landau-Pomeranchuk-Migdal effectFormation time important
Zapp, QM09

radiated
gluon

propagating
parton2ωτf = 2kT

Lc = τf,maxRadiation sees
length ~τf at once

1
Energy loss depends on density: λ ∝ρand nature of scattering centers(scattering cross section)

Transport coefficient qˆ ≡If λ < τf, multiple scatterings
add coherently

2

ΔEmed ~ α S qˆL

q⊥2
λ
14

Large angle radiation
Emitted gluon distribution

TECHQM ‘brick report’, arXiv:1106.1106

Gluon perp momentum k⊥ (GeV)

Opacity expansion

kT < kGluon momentum k (GeV)

Calculated gluon spectrum extends to large k⊥ at small k
Outside kinematic limits
GLV, ASW, HT cut this off ‘by hand’
15Effect of large angle radiation!
!

Expand in powers of

Blue: kTmax = xE
Red: kTmax = 2x(1-x)EL
λ

Different definitions of x:
ASW:

ω
xE =
E

GLV:

x+ =

ω+

Horowitz and Cole, PRC81, 024909

Opacity expansion formalisms

Single-gluon spectrum

E+

Different large angle cut-offs:
kT < ω = xE E
kT < ω = 2 x+ E

Factor ~2 uncertainty
from large-angle cut-off

16

L-dependence; regions of validity?
Emission rate vs τ (=L)
Caron-Huot, Gale, arXiv:1006.2379

E = 16 GeV
k = 3 GeV
T = 200 MeV

GLV N=1
Too much radiation
at large L(no interference
between scatt centers)
Full =
numerical solution of
Zakharov path integral= ‘best we know’

AMY, small L,
no L2, boundary effect

H.O = ASW/BDMPS like (harmonic oscillator)
Too little radiation at small L(ignores ‘hard tail’ of scatt potential)

Agreement of medium density for !
AMY, GLV/CUJET fits is a coincidence!
Multiple soft tends to give smallest E-loss, !
but may be most accurate?
17

Energy loss formalisms•

Large differences between formalisms understood!
Large angle cut-off!
– Length dependence (interference effects)!
–•

Mostly (?) ‘technical’ issues; can be overcome!
Use path-integral formalism!
– Monte Carlo: exact E, p conservation!
–•

Full 2→3 NLO matrix elements!

•

Include interference!

Plenty of room for interesting and relevant theory work!
Current progress on:• Interference in multiple gluon emission: ‘antenna radiation’• Some work on non-eikonal propagation• Monte-Carlo approaches for E, p conservation(JEWEL, q-PYTHIA, YaJEM, MARTINI)

18

Path length dependence: ‘surface bias’
Near side trigger,
biases to small E-loss

Away-side large L

Away-side (recoil) suppression IAA samples
longer path-lengths than RAA
NB: other effects play a role: quark/gluon composition, spectral shape (less steep for recoil)19

Di-hadron modelling
Model ‘calibrated’ on single hadron RAA

T. Renk, PRC, arXiv:1106.1740

L2 (ASW) fits data
L3 (AdS) slightly below

L (YaJEM): too little suppresion
L2 (YaJEM-D) slightly above

Modified shower
generates increase at low zTDi-hadron suppression is (probably?) a more robust probe of path length
dependence than the more obvious observables: v2, centrality dependence20Di-hadrons and single hadrons at LHC
Need simultaneous comparison toseveral measurements
to constrain geometry and E-loss
Here: RAA and IAA

Three models:
ASW: radiative energy loss
YaJEM: medium-induced virtuality
YaJEM-D: YaJEM with L-dependent
virtuality cut-off (induces L2)
NB: would like to see a more precise
evaluation a la RHIC results
21

Heavy flavour energy loss
M. Djordjevic2, PRL 112, 042302

Latest calculations of radiative+collisional E-loss forheavy and light quarks agree with data
RAA similar for light hadrons and D mesons due to interplaysof spectra shape, fragmentation with dead cone effect22

Jets and parton energy loss
Motivation: understand parton energy loss by tracking the lost energy

Qualitatively two scenarios:
1) In-cone radiation: RAA = 1, change of fragmentation
2) Out-of-cone radiation: RAA < 1

As usual: reality is somewhere in-between
23

Jets at LHC
ALICETransverse energy map of 1 eventϕη

Clear peaks: jets of fragmentsfrom high-energy quarks and gluons
And a lot of uncorrelated ‘soft’ background24

Charged and full jets
Reconstructed energyCounts / 2 GeV

1200

100 GeV jets (particle level)

1000

•

Full jets: charged + neutral
particles (except neutrinos)!
•••Hadronic + Electromagnetic Calorimetry(ATLAS)!
• + tracking (particle flow; CMS)!
Tracking + EMCal (ALICE)!

Charged jets: only charged
particles!
•

Charged + a
Charged800

Leading charged particle

600

400

200

0
0

10

20

30

40

50

60

70

80

90
100
Econe
[GeV]
T

Charge to neutral fluctuations!

Used by ALICE because of limited acceptance of EMCal

Full jets preferred for original goal: recover jet energy!
In practice, differences are small, however….
25

Detector corrections

Standard practice:!
•

Charged jets are corrected to charged jets at
the particle level!
••main effect: tracking efficiency!

Full jets are corrected to full jets at the particle
level!
••

Probability/Bin(0.04)

Definitions:!
Particle level: as generated by event generator, e.g. Pythia!
Detector level: as reconstructed (Pythia+detector simulation)!
(Parton level: parton energy; ill-defined?)
0.4
0.35

ALICE simulation
partp(GeV/c )T,ch jet

0.3
0.25
0.2
0.15

30 - 40
50 - 60
70 - 80
Anti-k T R = 0.3
p track > 0.15 GeV/ c
T

0.1
0.05
0
-1
ALI−PUB−64226

-0.8

-0.6

-0.4

-0.2

0

0.2

(p det - p part )/p partT,ch jetT,ch jetT,ch jetCalorimetric jets: HCal response!
Tracking+EMCal: Unmeasured hadrons (neutrons, K0L, tracking efficiency)

26

PbPb jet background
Background density vs multiplicity

Jet finding illustration

Cacciari et al

η-ϕ space filled with jets
Many ‘background jets’

Background contributes up to ~180 GeV per unit area
Subtract background:subT , jetpraw
T , jet=p−ρ A

Statistical fluctuations remain after subtraction

27

Pb+Pb jet RAA
Jet RAA measured by
ATLAS, ALICE, CMS

Good agreement
between experiments
Despite different methods:
ATLAS+CMS: hadron+EM jets
ALICE: charged track jets

RAA < 1: not all produced jets are seen;
out-of-cone radiation and/or ‘absorption’
For jet energies up to ~250 GeV; energy loss is a very large effect28

Comparing hadrons and jets
R CP

2 Ch. particles
Jets
1.8
ALICE
ALICE Ch. Jets R =0.3
1.6
(0-10%)/(50-80%)(0-10%)/(50-80%)
CMS
ATLAS Calo Jets R =0.3
1.4
(0-5%)/(50-90%)(0-10%)/(60-80%)
1.2
1Pb-Pb s NN=2.76 TeV
0.8
0.6
0.4
0.2
0
2
10
10
jet
track
p
, p (GeV/c )T
TSuppression of hadron (leading fragment) and jet yield similar!
Is this ‘natural’? No (visible) effect of in-cone radiation?
29

Comparison to JEWEL energy loss MC
R AA

R=0.2
0.6
JEWEL+PYTHIAALICE data

0.5
0.4
0.3

2
0.2

CMS data, 0-5% centrality
ALICE data, 0-5% centrality

1.8
1.6

0.1

JEWEL+PYTHIA

0
1.4

1.4

1.2

charged hadrons

Ratio

1
0.8

1

0.6

0.8

30

40

50

60

70

0.6

80

90

100
110
p⊥ [ GeV ]

R AA

R=0.3

0.4

0.6

0.2

0.5

0

0.4

10

100
pt [GeV]

ALICE data
JEWEL+PYTHIA

1000
0.3
0.2
0.1

JEWEL shows the same feature:
jet RAA ~ hadron RAA

0
1.4
1.2
Ratio

RAA

1.2

1
0.8
0.6
40

50

60

70

80

90

100
110
p⊥ [ GeV ]30

Generic expectations from energy loss
Ejet

kT~µ
λ
fragmentation
after energy loss?

• Longitudinal modification:– out-of-cone ⇒ energy lost, suppression of yield, di-jet energy
imbalance– in-cone ⇒ softening of fragmentation• Transverse modification– out-of-cone ⇒ increase acoplanarity kT– in-cone ⇒ broadening of jet-profile

Out-of-cone effects are large, so expect combination of all of the above31

Changes in fragmentation
Longitudinal

Transverse
RD(z)

1.5

ATLAS
Pb+Pb

1.6

sNN=2.76 TeV

1.5

-1

0.14 nb

anti-kT R=0.2

1.4

p >85 GeV
0-10%/60-80%

1

0.9

0.9

10-1

T

0-10%/60-80%

10-1

1
z

T

1.5

ATLAS
Pb+Pb

1.6

sNN=2.76 TeV

1.5

-1

0.14 nb

anti-kT R=0.2

1.4

jet

p >85 GeV

1.3

Relation rjetp >92 GeV

0.8

RD(p )1zT

RD(p )

1.6

Enhancement at large r, low pT"
ratio ~1 at small r, large pT

anti-kT R=0.3

1.1

1

0.8

0.14 nb

1.2

Data
Systematic
Uncertainty

1.1

sNN=2.76 TeV
-1

1.3

T

1.2

Pb+Pb

1.4

jet

1.3

ATLAS

ATLAS, arXiv:1406.2979

CMS, arXiv:1310.0878

1.6

RD(z)

fragment distributions

fragment distributions

T

0-10%/60-80%

1.1

1.1

1

1

0.9

0.9

102
p [GeV]T

sNN=2.76 TeV-10.14 nb

anti-kT R=0.3
jet

p >92 GeV

1.3
1.2

10

Pb+Pb

1.4

1.2

0.8

ATLAS

0.8

T

0-10%/60-80%

10102p [GeV]T

pT: large pT is at small r

No modification at small R, large pT: physics or auto-correlation?
32

ATLAS

Pb+Pb s NN = 2.76 TeV

1.5 0-10 % Centrality∫

L dt = 7 µb -1

1
158 < p < 182 GeV
T

0.5
0
1
89 < p < 103 GeV
T

0.5
0
1

0.2
R
RCP
/RCP

2

Ratio of spectra with different R
ATLAS, arXiv:1208.1867

R CP

Jet broadening: R dependence of RAA2

0 - 10 %Pb+Pb s NN = 2.76 TeV
1.8∫

L dt = 7 µb-1

40

50

ATLAS

R = 0.3
R = 0.4
R = 0.5

1.6

1.4

1.2

50 < p < 58 GeV
T

0.5

1
0
1
38 < p < 44 GeV

0.8

T

0.5

60

70

100

0
R = 0.2

R = 0.3

R = 0.4

T

R = 0.5

Larger jet cone: ‘catch’ more radiation àp [GeV]200

Jet broadening

However, R = 0.5 still has RAA < 1– Hard to see/measure the radiated energy330-10% central 2.76 ATeV PbPb

T. Renk, arXiv:1212.0646

anti-kT R=0.3, Eobs = 100-110 GeV

2full result
no shower bias

near

1.5

IAA

Ratio of fragment distributions

Comparison to models: YaJEM

Selection on reconstructed
jet energy

1

0.5

0
0

20

40
60
PT [GeV]80

100

Naive expectation: no
selection on reconstructed
jet energy

Thorsten Renk: the increase towards R = 1 at z = 1 isnatural consequence of the jet energy selection (bias effect)34

q-PYTHIA results
See also: L. Apolinario, Lisbon jet workshop

2.4

PbPb

/ρ

pp

3

Transverse distribution

ρ

Ratio PbPb/pp
dN/dz

Longitudinal fragment distribution
2

10
2.5

100 < pT,jet

< 120 GeV/c

2.2
2

102

1.8
1.6

1.5
1

10

1.4
1.2

-11

q-Pythia Pb+Pb
q-Pythia pp

1

0.5
-2
10

0.8
0.6

0
00

0.1
0.1

0.2
0.2

0.3
0.3

0.4
0.4

0.5
0.5

0.6
0.6

0.7
0.7

0.8
0.8 0.9
0.9 11z=p/pz=pT,part /pT,jet
T,part

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35
r

T,jet

q-Pythia: enhancement at small z,!
~1 at intermediate z

q-Pythia: enhancement at large r,!
suppression at small r

https://indico.cern.ch/event/304078/session/3/contribution/19/material/slides/0.pdf

35

JEWEL results for jet structureLongitudinal distribution

JEWEL: more fragments at high z
for medium-modified shower

Radial distribution

JEWEL: more fragment pT at!
small r for medium-modified shower

Trends opposite to q-PYTHIA!
retain relation small r large z
36Models for jet quenching•

q-PYTHIA: medium-induced branching!
••

JEWEL: medium-induced branching with MC formation time!
•••

Includes momentum exchange with medium!
Complication: recoil; momentum leaks out of the jet (need to defineboundary between medium and jet for calculational reasons)!

YaJEM: medium-induced virtuality!
••

Energy-momentum conserved in shower!

Model somewhat ad-hoc, but describes much of the observations!

PYQUENCH: medium-induced branching; ad-hoc
implementation

Virtuality evolution may be a key concept?

37Virtuality evolution and jet mass
Idea: jet mass is determined by virtuality of showering parton
Majumder, Putschke, Verweij

JEWEL

q-PYTHIA

jet mass spectrum R=0.4

jet mass spectrum R=0.4

-3

× 10

0.004

p 10 - 15 GeV/ c
T

0.0035

M. Verweij, private communication

pp JEWEL
PbPb JEWEL (no re

0.003

0.2

p 20 - 25 GeV/ c
T

0.18

pp JEWEL
PbPb JEWEL (no re

0.16
0.14

0.0025

0.12
0.002

0.1
0.08

0.0015

0.06

0.001

0.04
0.0005

0.02

0
0

5

10

15

20
25
30
jet mass (GeV/c 2 )00

jet mass spectrum R=0.4

-6

5

15

20
25
30
jet mass (GeV/c 2 )jet mass spectrum R=0.4

-9

× 10

10

× 10
40

2

p 50 - 60 GeV/ c

1.8

T

pp JEWEL
PbPb JEWEL (no re

1.6

p 100 - 120 GeV/c
T

35

pp JEWEL
PbPb JEWEL (no re

30

1.4
25

1.2
1

20

0.8

15

0.6
10
0.4
5

0.2

q-PYTHIA: medium effects
increase jet mass

0
0

5

10

15

20
25
30
jet mass (GeV/c 2 )

0
0

5

10

15

20
25
30
jet mass (GeV/c 2 )JEWEL: medium effects
decrease jet mass
38

Other ideas•

Iancu, Blaizot et al:!
•
•
•

•

Rajagopal, Casalderrey-Solana, Milhano et al:"
•
•
•

•

At some soft scale om ω = αs ωc, branching probability
becomes ~1!
Medium-induced gluon splittings favor x~0.5 (quasi-democratic)!
Both effects would strongly modify in-medium showers; are they
compatible with phenomenology? What are the limits of applicability?!
What about strong coupling?!
Use AdS/CFT estimates and implement in PYTHIA for
phenomenology!
Main observational impact from !

Tywoniuk, Mehtar-Tani, Salgado et al!
•••

Improve treatment of multiple gluon emissions in medium!
No angular ordering for medium-induced radiation!
Important interplay of scales: opening angle of jet vs Qs etc

39Conclusion•

Inclusive particle RAA follows expectations!
•••larger at RHIC than LHC!
Modelling/calculations still need to be refined!

Jets, main observations:!
••

RAA < 1, significant out-of-cone radiation!
Longitudinal, transverse fragment distributions modified:!
Enhancement at small z, large r!
• Suppression at intermediate z, r!
• Very little change at small r, large z!
••

First comparisons to MC models:!
JEWEL shows too much suppression at large r, small z!
• q-PYTHIA: too much suppression at small r, large z!
••

Connection to virtuality, jet mass to be explored
Clearly, there is physics in the jet fragmentation"
No clear picture yet, but we have a handle!
40

Extra slides

Hadron trigger vs jet trigger
Are jets an unnecessary complication?
If hadron and jet RAA are similar, why not use hadron observables?
Hadron trigger: strong “surface bias”
maximizes recoil path length

Hadron trigger

partially cancelled by bkg fluctuationsJet trigger

T.Renk, PRC85 064908

Full jet trigger: no geom. bias

Biases are different! Can be exploited to constrain models
42Summary• Jets: a ‘new’ tool for parton energy loss measurements– Large out-of-cone radiation (R = 0.2-0.4)• Energy asymmetry• RAA < 1, similar to hadrons• IAA < 1
• Radial shapes– Remaining jet has small modifications:• Longitudinal and transverse structure similar at small r, large z• Deviations at large r, low z– Most of the radiation is at low pT• Scale set by medium temperature?
• Democratic branchings?

Interplays of many effects: impossible to read simple conclusions off theplots"
Need (detailed) calculations to draw conclusions
e.g. JEWEL and YaJEM energy loss MCs agreewith many of the observed effects"
Does this constrain the energy loss mechanism(s)?"Ongoing work…

43

PbPb jet backgroundToy Model

Main challenge: large fluctuations of uncorrelated background energy
Size of fluctuations depends on pT cut, cone radius
44

Background jets
Raw jet spectrum
Event-by-event background subtracted

Low pT: ‘combinatorial jets’- Can be suppressed by requiring
leading track- However: no strict distinction
at low pT possible

Next step: Correct for backgroundfluctuations and detector effects
by unfolding/deconvolution

45

Removing the combinatorial jets
Raw jet spectrum

Fully corrected jet spectrum
ALICE arXiv:1311.0633

Correct spectrum and remove combinatorial jets by unfolding

Results agree with biased jets: reliably recovers all jets and removed bkg
46

PbPb jet spectra
Charged jets, R=0.3
ALICE arXiv:1311.0633

RCP, charged jets, R=0.3

Jet reconstruction does not‘recover’ much of the radiated energy
Jet spectrum in Pb+Pb: charged particle jets
Two cone radii, 4 centralities

47

Comparing to energy loss models
Jet observables: need explicit modelling of multi-particle final states

ATLAS data
JEWEL+PYTHIA

1

R=0.2

10−1

R=0.3 (×10−1)

10−2

R=0.4 (×10−2)

10−3

R=0.5 (×10−3)

40

60

80

100

120

140

160

180

Mehtar-Tani, Tywoniuk, arXiv:1401.8293
K. Zapp et al, arXiv:1212.1599

RCP

JEWEL: RCP vs R

200
p⊥ [GeV]

JEWEL gets the right suppression for R=0.2,! Fragment distributions sensitive
but not the increase with R!
to coherence effects(NB: no geometry model yet)(Treatment of recoil partons?)48

Again: background fluctuations
Toy model spectrum

Fragment distributions (simulation)
3

10
1/Njet dNh/dj

PythiaPythia+Hydjet
z-subtracted

102
1/Njet dNh/dz

Cacciari et al, arXiv:1209.6086

10

10
1

10
1

10

10-1

2

PbPb 3s=2.76 TeV-1

0-10% (Hydjet)
anti-kt, R=0.4, |y|<3

10-2

0

0.2

0.4

0.6

0.8

1

10-2

0

1

2

z

Background fluctuations
migrate yield to higher pT"
1/Njet dNh/dz

10

2

10
1

4
j

5

6

7

8

At fixed p10T2: pick up
above-average background contributions"ξ≳4

1/Njet dNh/dj

103

3

10

pT ≲ 2 GeV
1Current measurements mostly pT > 2 GeV
10

-1

10

-1

10

-2

10

-2

49

Situation at RHIC, ca 2008

Large density:
AMY: T ~ 400 MeV
Transverse kick: qL ~ 10-20 GeV

PHENIX, arXiv:1208.2254

2
q
=
10
−
20
GeV
/fm
ASW: ˆ
HT:qˆ = 2.3 − 4.5 GeV 2 /fm
AMY: qˆ ≈ 4 GeV 2 /fm

Bass et al, PRC79, 024901

3 main calculations; comparison
with same medium density profile

Large uncertainty inabsolute medium density
One aspect: scattering potential/momentum transfer;see recent work by Majumder, Laine, Rothkopf on lattice50

Modelling azimuthal dependence
A. Majumder, PRC75, 021901

RAA

RAA

pT (GeV)pT (GeV)RAA vs reaction plane sensitive to geometry model51

RAA vs ϕ and elastic eloss

In Plane

Elastic E-loss gives
small v2
Data require L2 orstronger path length
dependence

T. Renk, PRC76, 064905, J. Auvinen et al, PRC82, 051901

Out of Plane

However, also quite sensitive to medium density evolution52

Path length dependence: RAA vs ϕ
PHENIX, arXiv:1208.2254

Out of PlaneIn Plane

Suppression depends on angle, path length
Not so easy to model: calculations give different results
53

Reaction plane dependence at LHC: High-pT v2
CMS, arXiv:1204.1850
0-10%

v2

0.2

PbPb sNN = 2.76 TeV|η|<1

CMS 2011

10-20%

Model: B. Betz, M. Gyulassy, arXiv:1201.0281	

0.2

v2

CMS Lint = 150 µ b-1

CMS 2010, |η|<0.8
0.1

0.1

ATLAS

0.0

0.0

20

40

p (GeV/c) 20-30%
T40

p (GeV/c) 30-40%
T

v2

0.2

v2

0.2

20

0.1

0.1

0.0

0.0

20

40

p (GeV/c) 40-50%
T

0.2

20

40

p (GeV/c) 50-60%
T

Reasonable agreement between calculationand data for pT > 10 GeV!
(NB: simplified geometry, E-loss;
paper claims scale-dependence of 𝛼s main effect)

v2

v2

0.2

0.1

0.1

0.0

0.0

20

40

p (GeV/c)
T

20

40

p (GeV/c)T

54

Di-hadrons at high-pT: recoil suppression
d+Au

Au+Au 20-40%

Au+Au 0-5%
pTassoc > 3 GeV
pTassoc > 6 GeV

High-pT hadron production in Au+Au dominated by (di-)jet fragmentation

Suppression of away-side yield in Au+Au collisions: energy loss
55

Dihadron yield suppression
8 < pT,trig < 15 GeV

Near side

Yield in balancing
jet, after energy loss

Yield of additional
particles in the jet

trigger

Away side

trigger
Near side
associatedSTAR PRL 95, 152301

Away side associatedNear side: No modification⇒ Fragmentation outside medium?

Away-side: Suppressed by factor 4-5⇒ large energy loss

56

Four formalisms
Multiple gluon emission•

Hard Thermal Loops (AMY)
Dynamical (HTL) medium!
– Single gluon spectrum: BDMPS-Z like path integral!
– No vacuum radiation!
–•

Fokker-Planck
rate equations

Multiple soft scattering (BDMPS-Z, ASW-MS)
Static scattering centers!
– Gaussian approximation for momentum kicks!
– Full LPM interference and vacuum radiation!
–•

Opacity expansion ((D)GLV, ASW-SH)
Static scattering centers, Yukawa potential !
– Expansion in opacity L/λ(N=1, interference between two centers default)!
– Interference with vacuum radiation!
–•

Poisson ansatz(independent emission)

Higher Twist (Guo, Wang, Majumder)Medium characterised by higher twist matrix elements!
– Radiation kernel similar to GLV!
– Vacuum radiation in DGLAP evolution–

DGLAP
evolution

All formalisms can be related to the same BDMPS-Z path
integral formalism; different approximations used

See also: arXiv:1106.1106

57

The Brick Problem
TECHQM: Theory-Experiment Collaboration on Hot Quark Matter
arXiv:1106.1106

Gluon(s)Compare energy-loss in a well-defined model system:
Fixed length L = 2, 5 fm
Density T, q
Quark, E = 10, 20 GeV

Compare outgoing gluon, quark distributions- Same density
Two types of comparison:- Same suppression

and interpret/understand the differences
58

Multiple soft scattering: BDMPS, AMY
L=5 fm Single gluon spectra10

L = 2 fm
T = 300 MeV
q = 1.34 GeV2/fm
E = 20 GeV

1

dN/dk (GeV-1)

dN/dk (GeV-1)

L=2 fm Single gluon spectra
10

1

10-1

10-1

10-2

10-2

10-3

10-4
0

10-3

AMY
ASW-MSBDMPS-Z
BDMPS-Z (low-x)
5

10

15

20
k (GeV)L = 5 fm
T = 300 MeV
q = 1.34 GeV2/fm
E = 20 GeV

10-4
0

AMY
ASW-MSBDMPS-Z
BDMPS-Z (low-x)
5

10

15

20
k (GeV)AMY: no large angle cut-off+ sizeable difference at intermediate ω at L=2 fm
Large x treatment in AMY more accurate
Using qˆ (T ) based on AMY-HTL scattering potential

59Single gluon spectra
Same temperature
L = 2 fm

L = 5 fm@Same temperature: AMY > OE > ASW-MS
Size of difference depends on L, but hierarchy stays
60

Multiple gluon emission — Poisson ansatz
Average number of gluons:

Poisson convolution exampledI
N gluon = ∫dωdωPoisson fluctuations:n − N gluon
1
P ( n) =N gluon en!

(assumed)

Total probability:

Main other approach: build into DGLAP (used for HT)
61

Outgoing quark spectra
Same temperature: T = 300 MeV@Same T: suppression AMY > OE > ASW-MS
Note importance of P062

1/Nbin d2N/d2pT

Nuclear modification factor RAA‘Energy loss’‘Absorption’p+p

Downward shift

Shift spectrum to left

A+A

pT

Measured RAA is a ratio of yields at a given pT
The physical mechanism is energy loss; shift of yield to lower pT
The full range of physical pictures can be
captured with an energy loss distribution P(ΔE)

63

Geometry
Density profile

Profile at τ ~ τform known

Density along parton path

Longitudinal expansion
dilutes medium⇒ Important effect

Space-time evolution is taken into account in modeling64Getting a sense for the numbers – RHIC
π0 spectra

Nuclear modification factor
PHENIX, PRD 76, 051106, arXiv:0801.4020

Oversimplified calculation:-Fit pp with power law-Apply energy shift or relative E loss
Not even a model !

Ball-park numbers: ΔE/E ≈ 0.2, or ΔE ≈ 3 GeV
for central collisions at RHIC
65

Looking outside the jet cone
In Cone R<0.8//

p T ,miss = ∑ pT cos(ϕ − ϕ jet )tracksPYTHIA+HYDJETCMS measured

CMS, arXiv:1102.1957

Momentum imbalance
restored by hadrons atlarge angle R>0.8 andsmall pT < 2 GeV/c

Out of Cone R>0.8

66

0.14 nb-1

3

10

1.6

ATLAS

1.5

0.14 nb-1

Pb+Pb

sNN=2.76 TeV

p >85 GeV

10

0-10%/60-80%

1

10-1

10-2

1

1

0.9

0.9

0.8

10-1

6

0-10% × 2
10-20% × 25
20-30% × 24
30-40% × 23
40-50% × 22
50-60% × 21
60-80%
Systematic
Uncertainty

ATLAS

1.5

0.14 nb-1

Pb+Pb

1
z

sNN=2.76 TeV
anti-kT R=0.2
jet

p >85 GeV

1.3

-1

10

T

0-10%/60-80%

0.8

10-1

T

0-10%/60-80%

1.6

ATLAS

1.5

0.14 nb-1

1
z

1.1

1.1

1

1

0.9

0.9

102
p [GeV]T

sNN=2.76 TeV
anti-kT R=0.3
jet

p >92 GeV

1.3
1.2

10

Pb+Pb

1.4

1.2

0.8

1
z

jet

p >92 GeV

T

1.6

1.4

anti-kT R=0.4
pjet> 100 GeVT

anti-kT R=0.3

1.1

T

10

sNN=2.76 TeV

1.2

Data
Systematic
Uncertainty

1.1

0.14 nb-1

Pb+Pb

1.3

T

1.2

2

1.5
1.4

jet

1.3

ATLAS

0.8

ATLAS, arXiv:1406.2979

anti-kT R=0.2

1.4

1.6

RD(p )

sNN=2.76 TeV

RD(z)

Pb+Pb

RD(z)

ATLAS

104

RD(p )

D(z)Longitudinal fragment distributions

T

0-10%/60-80%

10102p [GeV]T

Characteristic pattern observed: !
• enhancement at low pT < 3 GeV!
• suppression at intermediate pT: 5-15 GeV!
• enhancement (or close to 1) at high pT > 20 GeV
67

Hadron-recoil jet measurements

68

Hadron-triggered recoil jet distributions
G. de Barros et al., arXiv:1208.1518

pT,jet< 20 GeV/c:
No change with trigger pT
Combinatorial backgroundpT,jet> 20 GeV/c:
Evolves with trigger pT
Recoil jet spectrum

69

Background subtraction: Δrecoil
Remove background by
subtracting spectrum withlower pTtrig:
Δrecoil =[(20-50)-(15-20)]
Reference spectrum (15-20)
scaled by ~0.96 to accountfor conservation of jet
density

Δrecoil measures the change of the recoil spectrum with pTtrig

Unfolding correction for background fluctuations and detector response70Ratio of Recoil Jet Yield ΔIAAPYTHIA
pp reference: PYTHIA(Perugia 2010)
R=0.4!
Constituents:
pTconst > 0.15 GeV/c!
no additional cuts
(fragmentation bias) on
recoil jets

Recoil jet yield ΔIAAPYTHIA ≈0.75, approx. constant with jet pT
71

PYTHIA
Recoil Jet ΔIAA
: R dependence
R=0.4

R=0.2

Similar ΔIAAPYTHIA for R=0.2 and R=0.4
No visible broadening within R=0.4(within exp uncertainties)

72

Hadrons vs jets II: recoil
Hadrons

Jets
PRL108 092301

Hadron IAA = 0.5-0.6

Jet IAA = 0.7-0.8

In approx. agreement with models;elastic E-loss would give larger IAA

Jet IAA > hadron IAA
Not unreasonable

NB/caveat: very different momentum scales !
73

Model comparison IAA
1.6= ∆Pb-Pb
recoil / ∆ recoil

PythiaJEWEL: Zapp et al., EPJ C69, 617
constanti-kT , R=0.4, pT > 0.15 GeV/c
Pb-Pb 0-20% sNN = 2.76 TeV
ALICE ∆ IPYTHIA(20-50)-(15-20)
AA
YAJEM IPYTHIA(20-50) 0-10%
AA
PYTHIAJEWEL ∆ IAA(20-50)-(15-20)

1.2

∆IAA

Pythia

1.4

1

0.8
0.6
0.4
0.2
0
0
ALI−DER−44225

JEWEL correctly describes
inclusive jet RAA

10

20

30

40

50

60

70
pch
(GeV/c)T,jet

JEWEL ΔIAA~0.4, below measured
YAJEM agrees with measurement
Difference in energy loss or geometry?
74