Heavy Ion Collisions at RHIC and at the LHC: Theoretical Overview Urs Achim Wiedemann CERN PH-TH.

Heavy Ion Collisions at RHIC and at the LHC:

Theoretical OverviewUrs Achim Wiedemann

CERN PH-TH

From elementary interactions to collective phenomena

How do collective phenomena and macroscopic properties of matter emerge from fundamental interactions ?

1973: asymptotic freedom

QCD = quark model +gauge invariance

Today: mature theory with a precision frontier

• background in search for new physics• TH laboratory for non-abelian gauge theories

QCD much richer than QED:

• non-abelian theory• degrees of freedom change with

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Q2

Open questions• What is the QCD equation of state? How can we test it?

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Tc ≈175 MeV

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εc ≈ (3 − 5) εnuclear mattercold

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0q LqR + q RqL 0 ≈ (250 MeV )3

• What is the origin of mass in the universe?

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T

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μB

CFL2SCHadron Gas

Quark Gluon Plasma

… and more questions…

• Confinement: How does hadronization proceed dynamically?

How is it changed in dense QCD matter?

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LQCDC P ∝θ tr Gμν ˜ G μν[ ]

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θ ≈0

• Why is there no strong CP violation? Or is there at finite temperature?

• What are the properties of matter at the highest temperatures and densities?

Degrees of freedom? Viscosity? Heat Conductivity? Transport of conserved quantum numbers?

• What are the dominant microscopic mechanisms of QCD non-equilibrium dynamics and thermalization?

Parton energy loss? Plasma Instabilities, color chaos?

… and many more …

Question:Why do we need collider energies

to test properties of dense QCD matterwhich arise on typical scales €

sNN = 200GeV [RHIC]

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sNN = 5500GeV [LHC]

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T ≈150 MeV , Qs ≈1− 2 GeV ?

Answer 1: Large quantitative gains

Increasing the center of mass energy implies

Denser initial system, higher initial temperature

Longer lifetime

Bigger spatial extension

Stronger collective phenomena

A large body of experimental data from RHIC supports this argument.

Answer 2: Qualitatively novel access to properties of dense matter

For a detailed experimentation with dense QCD matter, one ideally wants to do DIS on the QGP.

… and we can by using auto-generated probes at high

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sNN

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Q2 >> T ≈150 MeV

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Q2Large allows us to embed well-controlled large- processes (hard probes) in dense nuclear matter.

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sNN

Q: How sensitive are hard probes?

Bjorken’s original estimate and its correction

Bjorken 1982: consider jet in p+p collision, hard parton interacts with underlying event collisional energy loss

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ΔE rad ≈ α sˆ q L2

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dE coll dL ≈10GeV fm

Bjorken conjectured monojet phenomenon in proton-proton

Today we know (th): radiative energy loss dominates

Baier Dokshitzer Mueller Peigne Schiff 1995

• p+p:

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L ≈ 0.5 fm, ΔE rad ≈100 MeV

• A+A:

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L ≈ 5 fm, ΔE rad ≈10 GeV

Negligible !

Monojet phenomenon!

observed at RHIC, see talk by Bill Zajc

(Bjorken realized later that this estimate was numerically erroneous.)

High pT Hadron Spectra

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RAA ( pT ,η ) =dN AA dpT dη

ncoll dN NN dpT dη

Centrality dependence:0-5% 70-90%

L large L small

Centrality dependence: Au+Au vs. d+Au● Final state suppression ● Initial state enhancement

partonic energy

loss

Leading hadron suppression at RHIC:

Abundant yield at collider energies (detailed differential study of experimental signal possible)

+ robust and large signal (medium effect much larger than theoretical uncertainties)

= Basis for controlled experimentation and controlled theoretical interpretation

The medium-modified Final State Parton Shower

Medium characterized bytransport coefficient:

Baier, Dokshitzer, Mueller, Peigne, Schiff (1996); Zakharov (1997); Wiedemann (2000); Gyulassy, Levai, Vitev (2000); Wang ...

Salgado,Wiedemann PRD68:014008 (2003)

● energy loss of leading parton ● pt-broadening of shower €

ˆ q ≡μ 2

λ∝ ndensity

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ωc = ˆ q L2 2€

ωc ω = 32

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ˆ q L

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ωc ω =10

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ωc ω = 3.2

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ωc ω =1

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κ 2 ≡kT

2

ˆ q L

The fragility of leading hadrons

?

• The quenching is anomalously large (I.e. exceeds the perturbative estimate by ~ 5)

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ˆ q • Why is RAA = 0.2 natural ? Surface emission limits sensitivity to

Eskola, Honkanen, Salgado, WiedemannNPA747 (2005) 511, hep-ph/0406319

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ˆ q (τ =1 fm /c) ≥ 5GeV 2

fm≈ 5 ˆ q QCD

pert

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ˆ q How can we understand the size of ?

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ˆ q • defines short-distance behavior of expectation value of two light-like Wilson lines

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Tr W A +(x)W A (y)[ ] ≈ exp1

4ˆ q L (x − y)2 ⎡

⎣ ⎢ ⎤ ⎦ ⎥

o Well-defined but difficult problem in QCD.o Is this calculable from 1st principles in a thermal quantum field theory?

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ˆ q • Are there other classes of measurements sensitive to ?

o to test the microscopic dynamics of parton energy loss on which extraction of is based.o to confirm and further constrain the large value of .

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ˆ q

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ˆ q

How can we better gauge ‘hard probes’?

Where does thisassociated radiationgo to ?

How does this partonthermalize ?

What is the dependence on parton identity ?

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ΔEgluon > ΔEquark, m= 0 > ΔEquark, m>0

Parton energy loss depends on parton identity

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RD h = RAAD RAA

h

• Color charge dependence dominates

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RB h

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ΔEquark,m= 0 > ΔEquark,m>0

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ΔEgluon > ΔEquark

Massless “c,b”Armesto, Dainese, Salgado, Wiedemann, PRD71:054027, 2005

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1

kT2⇒

kT2

kT2 + m 2

E 2 ω2( )

2

• Vacuum and medium radiation is

suppressed due to quark mass Dokshitzer, Kharzeev, PLB 519 (2001) 199

Massive c,b

• Mass dependence dominates

• To test this at the LHC, exploit: light-flavored mesons - gluon parents D - mesons - quark parents (mc~0) B - mesons - quark parents (mb>0)

Jet modifications in dense QCD matter

Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 93 (2004) 242301

• Jets ‘blown with the wind’ Hard partons are not produced in the rest frame comoving with the medium

• ‘Longitudinal Jet heating’: The entire longitudinal jet multiplicity distribution softens due to medium effects. €

dN h dξ

Borghini,Wiedemann, hep-ph/0506218

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ξ =ln ETjet pT

h[ ]

Question:How to relate experimental data

to fundamental properties of dense QCD matter?

Approaches include:Perturbative QCD

Lattice QCDSaturation Physics

String theory…

Approach discussed here

String Theory Calculations of Properties of Matter• AdS/CFT correspondence relates Strong coupling problems Classical Problem in a curved in non-abelian QFT higher-dimensional space

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g2, Nc

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gs,α '= ls2 String coupling and

string tension

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λ ≡g2N T’Hooft coupling

Black hole horizon

Curvature radius

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R2

α '= λ = g2Nc

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TH =r0

π R2= T

• Translation into field theoretic quantities

• Finite T Lattice QCD is difficult to apply too problems involving real-time dynamics (moving QQbar pair, light-like Wilson loops, …)o hydrodynamic properties (Problem of analytical continuation to if lowest Matsubara frequency in imaginary time formalism is .)o ….

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ω → 0

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2πT

Maldacena, 1997

AdS/CFT Calculation of Quenching Parameter

Maldacena (1998)Rey and Lee (1998)

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W F (C)T

≡ exp iS(C)( )

Our (3+1)-dim worldWilson loop C in our world

horizon

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r = r0

: area of string world sheet with boundary C.

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S(C)

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r = Λ

→ ∞

• Result for the quenching parameter

• AdS/CFT Recipe

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W A Clight−like( ) = exp −1

4ˆ q

L−

2L2

⎡

⎣ ⎢

⎤

⎦ ⎥= exp i2S Clight−like( )[ ]

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ˆ q SYM =π 3 / 2Γ 3

4( )Γ 5

4( )λ T 3 ≈ 26.68 α SYM Nc T 3 Liu, Rajagopal, Wiedemann,

Phys. Rev. Lett. 97:182301, 2006

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Nc = 3

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αSYM =1 2

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ˆ q SYM = 4.4 GeV 2

fm for T = 300 MeV

• “Numerology”: relate N=4 SYM to QCD by fixing

Is this comparison meaningful ?

Comment on: Is comparions meaningful?

• conformal

• no asmptotic freedom no confinement

• supersymmetric

• no chiral condensate

• no dynamical quarks, 6 scalar and 4 Weyl fermionic fields in adjoint representation

N=4 SYM theory

Physics near vacuum and at very high energy is very different from that of QCD

At finite temperature: Is comparions meaningful?

• conformal

• no asymptotic freedom no confinement

• supersymmetric (badly broken)

• no chiral condensate

• no dynamical quarks, 6 scalar and 4 Weyl fermionic fields in adjoint representation

N=4 SYM theory at finite T QCD at T ~ few x Tc

• near conformal (lattice)

• not intrinsic properties of QGP at strong coupling

• not present

• not present

• may be taken care of by proper normalization

In solid state physics, materials of different microscopic composition and interaction show similar thermal properties.Here: non-abelian gauge theories of different particle content and symmetries show similar thermal properties above Tc.

Is there a new form of “universality class” at high temperature?

QCD Saturation Physics:QCD at the highest parton densities

Venugopalan McLerran; Jalilian-Marian,Kovner,Leonidov,Weigert; Balitsky; Kovchegov;…

This requires that the action is large

Need weak coupling and strong fields, satisfied at sufficiently small Bjorken x,where hard processes develop over long distance

At highest , there is a qualitatively novel regime of QCD, in which

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sNN

• Parton densities are maximal up to large scales

• Coupling constant is small

• Semi-classical methods apply€

Q2 < Qsat2 ~ (2 − 3GeV )2

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ρ ~ 1 α s

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α s(Qsat2 >> ΛQCD )<<1

Can we test this novel QCD regime in the laboratory?

The kinematical range accessible

• Small x higher initial parton density qualitatively different matter produced at LHC mid-rapidity? tests of saturation phenomena? - bulk observables - pt-spectra in scaling regime - rapidity vs. dependence - …

• Large abundant yield of hard probes precise tests of properties of produced matter - color field strength - collective flow - viscosity - …

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Q2

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sNN

The Next-to-last Slide• This presentation was not comprehensive. I missed to mention: and how they relate to first principle calculations in a QFT:

- collective flow - transport coefficients, relaxation times - electromagnetic probes - spectral functions - rapidity and -dependence - saturated non-linear QCD evolution

• Instead of being comprehensive, I emphasized - how controlled experimentation with dense QCD matter is possible

- how the field makes progess in relating measurements to fundamental properties of matter calculable in QCD.

Abundant yield + robust signal + theory = understanding of hard probes (e.g. jet quenching >> uncertainties)

The RHICness of the LHC

Abundant yield of hard probes + robust signal (medium sensitivity

>> uncertainties)

= detailed understanding of dense QCD matter

• Jets• identified hadron specta• D-,B-mesons• Quarkonia• Photons• Z-boson tags

The probes:

The wide kin. range:

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Q2 ,x, A, luminosity QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.