Hard probes of the Quark Gluon Plasma
Marco van Leeuwen,
Nikhef and Utrecht University

Tsukuba Global Science Week!
28-30 September 2014

A nucleus-nucleus collision

Colored spheres: quarks!
White spheres: hadrons, i.e. bound quarks

In a nuclear collision, a Quark-Gluon Plasma (liquid) is formed!
Study this new state of matter2

Probing the Quark-Gluon Plasma

photons, or particles

Not feasible:!
Short life time!
Small size (~10 fm)

Detector

Probe beam

Use self-generated probe:!
quarks, gluons from hard scatteringlarge transverse momentum

3

RHIC and LHC
RHIC, Brookhaven
Au+Au √sNN= 200 GeV

LHC, Geneva

Pb+Pb √sNN= 2760 GeVSTARFirst run: 2000

First run: 2009/2010

STAR, PHENIX,!
PHOBOS, BRAHMS

Currently under maintenance!
Restart 2015 with higher energy:
pp √s = 13 TeV, PbPb √sNN = 5.12 TeV

ALICE, ATLAS,CMS, (LHCb)

4

Collision centrality
Nuclei are large compared to the range of strong force

Peripheral collision
top/sideview:

Central collisionbb finite

b~0 fmfront view:

This talk: concentrate on central collisions
5

Centrality continued
peripheral

centralMultiplicity distribution

Experimental measure of centrality: multiplicity
Need to take into account volume of collision zone for production rates

6

Testing volume (Ncoll) scaling in Au+Au
Direct γ spectra

PHENIX, PRL 94, 232301
PHENIX

Centrality

Scaled by Ncoll

Direct γ in A+A scales with Ncoll

A+A initial production is incoherent superposition of p+p for hard probes
7

0π R

AA – high-pT suppression

PHENIX@RHIC!
√sNN = 200 GeV

γ: RAA = 1

π0: RAA ≈ 0.2

γ: no interactions
RAA = 1

RAA < 1

Hard partons lose energy in the hot matter

Hadrons: energy loss

8

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
9

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 ~ 8.2LHC: n ~ 6.4
Fractional energy loss:

RAA depends on n, steeper spectra, smaller RAA
10

From RHIC to LHC
RHIC

LHC

RHIC: n ~ 8.2

LHC: n ~ 6.4

6.2

4.4

(1 − 0.23) = 0.20

(1 − 0.23) = 0.32

Energy loss at LHC is larger than at RHIC
(RAA is similar due to flatter spectra)
11

Towards a more complete picture• Geometry: couple energy loss model to model ofevolution of the density (hydrodynamics)• Energy loss not single-valued, but a distribution• Energy loss is partonic, not hadronic– Full modeling: medium modified shower– Simple ansatz for leading hadrons: energy loss followed byfragmentation– Quark/gluon differences

12Medium-induced radiation
radiatedgluon

propagatingparton

Key parameter:
Transport coefficientqˆ ≡q2⊥

λ2

ΔEmed ~ α S qˆLMean transverse kick per unit path length1
Depends on density ρ through mean free path λ λ ∝ρ
13Fitting the model to the data
1PHENIX 2008 (0-5%)
PHENIX 2012 (0-5%)

RHIC

0RAA

0.6
0.40.206810

121416p (GeV)

1820

T

10.8

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

0.60.4

LHC0.20102030405060p (GeV)
T708090100

Burke et al, JET Collaboration, arXiv:1312.5003

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

PHENIX 08+124

CMS+ALICE

χ2/d.o.f

0.8

𝜒2 of data wrt model

32
10

1

1.5

2

2.5

3q0 GeV2/fm (LHC)

3.5

Clear minimum: found best value
for transport coefficient
Factor ~2 larger at LHC than RHIC
14Comparing several models
7MARTINIHT-BW
HT-M

6(T=370 MeV)

McGill-AMY
GLV-CUJET

5ˆ 3q/T

LHC:

432(T=470 MeV)
Expect factor 2.2 frommultiplicity + nuclear size

3qˆ N/Teff(DIS)

1000.1

Au+Au at RHIC
Pb+Pb at LHC
0.2

0.3

0.4

Burke et al, JET Collaboration, arXiv:1312.5003

RHIC:

0.5T (GeV)

values from different models agree
larger at RHIC than LHC
15

Transport coefficient and viscosity
Transport coefficient:!
momentum transfer per unit path length

basically measures the density
General relation:

Viscosity:

Expectfor a QCD medium

Majumder, Muller and Wang, PRL99, 192301
16

Relation transport coefficient and viscosity
Transport coefficient from RAA

0.02
00MC-KLNReaction Plane
2040centrality

RHIC: η/s=0.16
LHC: η/s=0.16
LHC: η/s=0.20
LHC: η/s=0.24
60

80(Scaled) viscosity slightly larger at LHC??
??

432NLO SYM

0.045??

v20.06

6Au+Au at 0.2 TeV
103
ˆqN/Teff(DIS)

0≈STARv {4}ALICE 27ˆ 3
q/T0.08

H. Song et al, PRC83, 054912

0.1≈Elliptic flow

0.20.25Pb+Pb at 2.76 TeV
0.30.35

0.4

0.45

0.5

T (GeV)

Scaled transport coefficientslightly smaller at LHC

Increase of η/s and decrease of q/T3 with collision energy!
are probably due to a common origin, e.g. running 𝛼SResults agree reasonably well with expectation:
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Conclusion
High-pT particles are a ‘hard probe’ of the Quark
Gluon Plasma!
• Use these to find the transport coefficient of the QGP:!
•••RHIC:!
LHC:!

Increase from RHIC to LHC slightly smaller thanexpected!
• Similar effect observed in viscosity η!
• Probably common origin, e.g. running 𝛼S•

Next step: use other observables, e.g. jets,
to test and improve energy loss models
18Extra slides

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 modeling20

A simplified approach
Parton spectrum Energy loss distributiondN=
dpT hadrdN
dE jets
known
pQCDxPDF

Fragmentation (function)⊗ 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
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RHIC and LHC
10.8

RHIC

q =0.8, 1.0, 1.2, 1.4, 1.7 GeV 2/fm
00.6

RAA

0.4

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

0.8

RHIC

0.6

0.4
0.2

0.2

0.8

0CUJET2.0(αmax,fE,fM)

68

10 12 14pT (GeV/c)

0.6 18160.40.2

LHC20810121416

p (GeV)

1820

1200.8

06T4060
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
00.6RAA0RAA(pT)

RAA(pT)CUJET2.0(αmax,fE,fM)

0.4

LHC

0.2100Burke 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)

01020

30

40

5060

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 well
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RHIC and LHC
CUJET 2.0HT-BW
PHENIX 08+124CMS+ALICEχ2/d.o.f2

χ /d.o.f.(pT>8)8643212000.2

0.22 0.24 0.26 0.28
αmax

0.30.32CUJET: 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+12CMS+ALICE

10q0 GeV /fm (LHC)

HT: transport coeff is parameter!
Higher at LHC
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Nuclear geometry: Npart, Ncollb

Two limiting possibilities:- Each nucleon only interacts once, ‘wounded nucleons’Npart = nA + nB (ex: 4 + 5 = 9 + …)
Relevant for soft production; long timescales: σ ∝ Npart- Nucleons interact with all nucleons they encounter
Ncoll = nA x nB (ex: 4 x 5 = 20 + …)
Relevant for hard processes; short timescales: σ ∝ Nbin

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‘Energy loss’

1/Nbin d2N/d2pT

Nuclear modification factor RAA‘Absorption’p+p

Downward shift

Shift spectrum to left

A+ApT

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)

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Nuclear modification factor
RAA =dN / dpT Pb + Pb
N coll dN / dpT p + pSuppression factor 2-6
Significant pT-dependence
Similar at RHIC and LHC?
So what does it mean?
26