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Heavy Ion Collisions at RHIC and at the LHC: Theoretical Overview. Urs Achim Wiedemann CERN PH-TH. 1973: asymptotic freedom . QCD = quark model +gauge invariance. Today: mature theory with a precision frontier. background in search for new physics

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heavy ion collisions at rhic and at the lhc theoretical overview
Heavy Ion Collisions at RHIC and at the LHC: Theoretical Overview

Urs Achim Wiedemann


from elementary interactions to collective phenomena

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
From elementary interactions to collective phenomena

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

open questions

Quark Gluon Plasma

Hadron Gas



  • What is the QCD equation of state? How can we test it?
Open questions
  • What is the origin of mass in the universe?
and more questions

Confinement: How does hadronization proceed dynamically?

How is it changed in dense QCD matter?

  • 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 more questions…

… and many more …



Why do we need collider energies

to test properties of dense QCD matter

which arise on typical scales


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 1: Large quantitative gains
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

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

Q: How sensitive are hard probes?

Answer 2: Qualitatively novel access to properties of dense matter
bjorken s original estimate and its correction
Bjorken’s original estimate and its correction

Bjorken 1982: consider jet in p+p collision, hard parton interacts with

underlying event collisional energy loss

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

Bjorken conjectured monojet phenomenon in proton-proton

Today we know (th): radiative energy loss dominates

Baier Dokshitzer Mueller Peigne Schiff 1995

  • p+p:

Negligible !

Monojet phenomenon!

  • A+A:

observed at RHIC, see talk by Bill Zajc

high p t hadron spectra



Centrality dependence:

L large

L small

High pT Hadron Spectra
centrality dependence au au vs d au

partonic energy loss

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

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

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

The medium-modified Final State Parton Shower

Medium characterized by

transport coefficient:

  • pt-broadening of shower
  • energy loss of leading parton

Salgado,Wiedemann PRD68:014008 (2003)

the fragility of leading hadrons

Why is RAA = 0.2 natural ?

  • Surface emission limits sensitivity to


The fragility of leading hadrons

Eskola, Honkanen, Salgado, Wiedemann

NPA747 (2005) 511, hep-ph/0406319

  • The quenching is anomalously large

(I.e. exceeds the perturbative estimate by ~ 5)


How can we understand the size of ?

  • Are there other classes of measurements sensitive to ?
  • to test the microscopic dynamics of parton energy loss
  • on which extraction of is based.
  • to confirm and further constrain the large value of .
  • defines short-distance behavior of expectation value of

two light-like Wilson lines

  • Well-defined but difficult problem in QCD.
  • Is this calculable from 1st principles
  • in a thermal quantum field theory?
how can we better gauge hard probes

Where does this

associated radiation

go to ?

How does this parton

thermalize ?

What is the dependence on parton identity ?

How can we better gauge ‘hard probes’?
parton energy loss depends on parton identity

Vacuum and medium radiation is

suppressed due to quark mass

  • To test this at the LHC, exploit:

light-flavored mesons - gluon parents

D - mesons - quark parents (mc~0)

B - mesons - quark parents (mb>0)

Dokshitzer, Kharzeev, PLB 519 (2001) 199

Armesto, Dainese, Salgado, Wiedemann, PRD71:054027, 2005

Massless “c,b”

Massive c,b

  • Color charge dependence


  • Mass dependence


Parton energy loss depends on parton identity
jet modifications in dense qcd matter
Jet modifications in dense QCD matter
  • ‘Longitudinal Jet heating’:
  • The entire longitudinal jet
  • multiplicity distribution softens
  • due to medium effects.

Borghini,Wiedemann, hep-ph/0506218

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

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



How to relate experimental data

to fundamental properties

of dense QCD matter?

Approaches include:

Perturbative QCD

Lattice QCD

Saturation 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

String Theory Calculations of Properties of Matter

Maldacena, 1997

String coupling and string tension

T’Hooft coupling

  • Translation into field theoretic quantities

Black hole horizon

Curvature radius

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

Wilson loop C in our world

Our (3+1)-dim world

: area of string world sheet with boundary C.


AdS/CFT Calculation of Quenching Parameter
  • AdS/CFT Recipe

Maldacena (1998)

Rey and Lee (1998)

  • Result for the quenching parameter

Liu, Rajagopal, Wiedemann,

Phys. Rev. Lett. 97:182301, 2006

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

for T = 300 MeV

Is this comparison meaningful ?

comment on is comparions meaningful
Comment on: Is comparions meaningful?

N=4 SYM theory

  • conformal

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

  • no asmptotic freedom

no confinement

  • supersymmetric
  • no chiral condensate
  • no dynamical quarks, 6 scalar

and 4 Weyl fermionic fields in

adjoint representation

at finite temperature is comparions meaningful
At finite temperature: Is comparions meaningful?

N=4 SYM theory at finite T

QCD at T ~ few x Tc

  • conformal
  • near conformal (lattice)
  • no asymptotic freedom

no confinement

  • not intrinsic properties of

QGP at strong coupling

  • supersymmetric (badly broken)
  • not present
  • no chiral condensate
  • not present
  • no dynamical quarks, 6 scalar

and 4 Weyl fermionic fields in

adjoint representation

  • may be taken care of by

proper normalization

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

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.

qcd saturation physics qcd at the highest parton densities

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

  • Parton densities are maximal

up to large scales

  • Coupling constant is small
  • Semi-classical methods apply

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

QCD Saturation Physics:QCD at the highest parton densities

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

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

- …

The kinematical range accessible
the next to last slide

Abundant yield + robust signal + theory = understanding

of hard probes (e.g. jet quenching >> uncertainties)

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.
the rhicness of the lhc

The probes:

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

The wide kin. range:

,x, A, luminosity

Abundant yield

of hard probes

+ robust signal

(medium sensitivity >> uncertainties)

= detailed understanding

of dense QCD matter

The RHICness of the LHC