Studying Hot and Dense QCD Matter

in the LHC era

Urs Achim WiedemannCERN, PH-TH Department

Quark Matter 200930 MarchKnoxville

<<Many in the RHIC community … are interested in the LHC heavy-ion program, but have several questions …

What can we learn at the LHC that is qualitatively new?

Are collisions at LHC similar to RHIC ones, just with a somewhat hotter/denser initial state? If not, why not?

… These questions are asked in good faith.>>

Glenn Young, 25 January e-mail inviting me for a plenary talk

• Large quantitative gains at LHC? YES - bigger - longer - denser

Testing QCD thermodynamics at RHIC and LHCSPS RHIC LHC

• Qualitatively the same system @ LHC “just hotter and denser”? No

QCD thermodynamics indicates characteristic qualitative differences in going from ~1.5 Tc to ~5 Tc.

Which ones? p.t.o.

Karsch, Laermann, Peikert,NPB605 (2001) 579

RBC-Bielefeld,PoS LAT2007:217,2007.

The renormalized Polyakov loop• Trace of Wilson line

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Tr Ωr x ( ) ≡ Tr P exp i dτ A0 τ ,

r x ( )

0

β

∫ ⎡

⎣ ⎢

⎤

⎦ ⎥

⎧ ⎨ ⎪

⎩ ⎪

⎫ ⎬ ⎪

⎭ ⎪

RBC-Bielefeld,PoS LAT2007:217,2007.

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Tr Ωr x ( )⇒ z TrΩ

r x ( )

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Tr Ωr x ( ) = exp −β ΔFq

r x ( )[ ]

- transforms under Z(3)

- interpretation

• Good order parameter for pure SU(3)

A.Kurkela

Limit reached above 3-5 Tc

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Tr Ωr x ( )

Relation to the validity of quasi-particle pictures

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Re A0[ ]

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Im A0[ ]

• The effective potential at T>>Tc

- height of potential barrier

- fluctuations between minima negligible only above 3-5 Tc

Gross, Pisarski, YaffeRev.Mod.Phys.53:43,1981

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Veff

MAX∝T 4

• Quasi-particle models - generally rely on expanding Veff around A0=(0,0)

- this breaks Z(3) invariance

- the complex minima lie outside domain of validity of description

- no sound basis for T< 2-3 Tc

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Veff Tr Ωr x ( )[ ]

Pressure/T4

a quasi-particle model

Lattice data

SB

J.P.Blaizot, QM06

• Quasi-particle models vs. Lattice - significant deviations/uncertainties below

• Strong fluctuations between different Z(3) minima deviates from asymptotic value above

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T < 2 − 3 Tc

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Tr Ωr x ( )

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Tr Ωr x ( )

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T < 2 − 3 Tc

Indicative of qualitative novel physics above 2-3 Tc

QCD thermodynamics is qualitatively different above 2-3 Tc

• Beware: - Lattice shows - naïve quasi-particle models

- LHC reaches well beyond T=400 MeV, where QCD thermodynamics is different

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ε−3p∝ΛQCD2 T 2

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ε−3p∝T 4

QCD vs. AdS/CFT

• conformal

• asymptotic freedom

• superymmetric

• chiral condensate

N=4 SYM in vacuum

Physics near vacuum and at high energy is very different

• confinement

QCD in vacuum

N=4 SYM finite T

QCD at finite T

• degrees of freedom

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ε−3p

• quasi-particles

YESNO

NO YES

NO YES

NOYES

NO melted

NOlessimportant

NO

NObroken

deconfined

YES NO YES approx at T >>Tc

•

Very different, but may be taken care of by normalization

NO

Zero Small for T>2-3 Tc only

Conceivable for T>2-3 Tc only

EXP: Conceptual questions in reach of LHC

TH: Which field theoretical tools are best suited?

Elliptic flowHydro curves atCERN SPS

• To establish hydrodynamic behavior, reliable theoretical baseline needed

- close to perfect liquid ?

- sensitivity to ? - sensitivity to dissipative properties such as ?

- is the prefect liquid more like ketchup or custard?

TECHQM – Collaboration https://wiki.bnl.gov/TECHQM/index.php/Main_Page

NA45: PRL92 (2004) 032301

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ε−3p

€

η /s

€

ηdu flow

dx

⎛

⎝ ⎜

⎞

⎠ ⎟

Shear thickening? orShear thickening?

“True” Jets

Et

€

ΔΦ

€

Δη

Talks byS. SalurT. Renk,K. Zapp,I. Vitev…

Leading hadron suppression “True” jet quenching

- sufficient for leading fragment?

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E >>ω >> kT ,qTmed >> ΛQCD

Had

ron

s

• branching of leading parton based on

BDMPS-Z-ASW-GLV-WDHG-etc

TECHQM – Collaboration https://wiki.bnl.gov/TECHQM/index.php/Main_Page

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E ≥ ω ≥ kT ,qTmed ≥ ΛQCD

• Exact energy conservation indispensable

=> Monte Carlo needed

• branching of subleading partons not needed

• leading and subleading branchings must be treated on equal footing

=> Monte Carlo

• perturbatrive (vacuum) baseline analytical calculation or MC

• perturbatrive (vacuum) baseline => Monte Carlo needed

Constraining Models of Jet Quenching

Talk byK. Zapp

Monte Carlo Model of Jet Quenching

Requirement: reproduce all analytically known limits

Vacuum Parton Shower

Collisional Energy Loss

Radiative Energy Loss

Perturbative Baseline+ Hadronization Model

Elastic scatteringlimit

BDMPS-limit

Monte Carlo including LPM-effectNeeded: probabilistic implementation of quantum interference

Solution: in vacuum => angular ordering only in medium => formation time constraint only (follows analytically from BDMPS-ASW)

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dI

dω∝

1 ω3 / 2 for ω << ωc

1 ω3 for ω >> ωc

⎧ ⎨ ⎩

⎫ ⎬ ⎭

MC reproduces BDMPS-limits

K. Zapp, J. Stachel, UAWarXiv:0812.3888

Talk byK. Zapp

From limiting cases to full MC• In the BDMPS-limit:

Talk byK. Zapp

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dσ el

dqT2

Θ μ cut2 − qT

2( )⇒

dσ el

dqT2∝

1

qT2 + μ 2

( )2€

ΔE = ωdI

dω0

ωc∫ ∝ qL2

• Relax soft scattering approximation

• Exact E-p-conservation

• Realistic cross sections

H

Extrem

e incohere

nt

K. Zapp, J. Stachel, UAW in prep (lines denote MC results)

Main messages:- more realistic => more incoherent => larger e-loss- quadratic L2 supression due to coherence- linear L-enhancement due to incoherence

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E =100GeV

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Lc ∝1 ˆ q [ ]

Jet shapes, energy flows• Thrust – baseline

• Thrust – medium above baseline

• Jet multiplicity distributions

Recent jet quenching MCs:

JEWEL (K.Zapp et al.)Q-Pythia (Santiago group)YaJEM (T.Renk)

Talks by C.Salgado, T.Renk

Jet Finding Algorithms• Tremendous recent progress on jet finding algorithms - novel class of IR and collinear safe algorithms satisfying SNOWMASS accords kt(FastJet) anti-kt(FastJet) SISCone - new standard for p+p@LHC - fast algorithms, suitable for heavy ions!

M. Cacciari, G. Salam, G. Soyez, JHEP 0804:005,2008

Event multiplicity

Run

time

[sec

]

• Catchment area of a jet - novel tools for separating soft fluctuations from jet remnants

- interplay with MCs of jet quenching needed

60,000 MB eventsdNch/dη ~ 50 !Abundant rate of p+p events with

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dN ch dη ≅ 50 −100- comparable to semi-peripheral Cu+Cu @ RHIC- includes events not dominated by hard jets

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εBj τ 0 =1 fm( ) ≈ 5 −10GeV / fm3

Does multiplicity drive collectivity?

M.A.Lisa et al., Ann.Rev.Nucl.Part.Sci.55:357-402,2005

J. Schukraft, QM06

Jürgen’s mini-serving of the QGP: light ion physics with protons @ LHC?

@ LHC, even proton wave functions are dense

Central issue since Saturation Unitarization of hadronic cross section

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≠

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σ d4 j = σ s

2 j( )

2σ eff

• Double hard cross section

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σ eff

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σ eff ≈1

3σ inel

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σ d4 j ET , jet

min ≥ 20GeV( ) ≅10μb

- scale factor at Tevatron for

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ET , jetmin ≥ 20GeV

- hard partons closer localized/ correlated than average soft ones

- at LHC

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ET , jetmin

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σ d4 j ET , jet

min ≥100GeV( ) ≅ 50 pb

LHC is first collider to give access to scale dependence of scale factor(transverse growth of hard components in hadron wave functions)

Kovner, UAW Phys.Rev.D66:051502,2002.

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s

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5TeV€

10TeV

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14TeV

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50GeV

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100GeV

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σ eff

S.Domdey, H.J.Pirner, UAW, in prep

The LHeC-project

http://www.ep.ph.bham.ac.uk/exp/LHeC/

• 70 GeV electron on 7 TeV proton or 2.75 TeV Pb • Ring-ring or linac-ring

• LHeC physics program

- discovery machine

- precision in the era of sLHC * pdfs at large x and Q2

separating all flavors, valence and sea

* strong coupling at per-mille accuracy to test grand unification * …

- QCD at extreme parton densities

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se− parton ≤1TeV

Summary

LHC heavy ion program is a giant leap into the unexplored regime of high temperature QCD.

We have all reasons to expect that this will allow for major advances on fundamental open questions in QCD.