Hard Probes 2006

Urs Achim WiedemannSUNY Stony Brook and RIKEN BNL

The “Not a Theory Summary”-Talk

Asilomar, 15 June 2006

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

Hard Probes: one promising method to address this question - experimentally - theoretically

Q: What makes a hard process a hard probe of soft matter?

DIS - the most successful hard probe so far• Deep inelastic scattering (DIS) is a hard probe of cold nuclear matter.

• What makes DIS so successful?

- benchmark: perturbatively controlled high - = ‘hard’ process

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Q2

- properties of matter (=pdfs) are field-theoretically well-defined [OPE]

- interaction between probe and medium (where probe ends and medium starts) described in controlled dynamical framework [DGLAP]

Lesson: success of hard probe depends on theoretical control of both 1. the hard probe 2. the properties of matter, which are probed.

Jet Quenching = DIS on the QGP?

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Q >> T ≈150 MeV

Qualitative idea:

Abundant yield of hard probes at collider energies + robust signal (medium sensitivity >> th. uncertainties)

= Basis for detailed investigation of dense QCD matter

Accuracy of jet-quenching benchmarks

pT [GeV]5 10

• impressive for pions at midrapidity (but don’t forget uncertainties at RHIC forward rapidity)

• more problematic for heavy quarks (where TH baseline is most needed to disentangle contributions to single electron spectra)

Matteo Cacciari, Carlos Salgado,Jian-wei Qiu

mainly an EXP challenge

Modeling the medium seen by jets• assumptions in all current jet quenching calculations are simplistic

K. Rajagopal

+ +

Vacuum DGLAP type

+

Are static scatteringcenters sufficiently realistic? What do they stand for?

• Is there a rigorous field-theoretic definition of the properties of ultra-dense QCD matter tested by jet quenching?

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Tr W A +(x)W A (y)[ ]med

= exp − 14

ˆ q L− (x − y)2[ ]

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W x( ) = P exp i dz−T a Aa+ x,z−( )

0

L−

∫ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥

• The role of collisional energy loss is an open question: - numerically:

- conceptually: Does the ratio of radiative vs. collisional energy loss depend on what constitutes the medium? Can one really separate radiative and collisional contributions?

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ΔE rad /ΔE coll ~ O(10)

X.N. Wang

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ΔE rad /ΔEcoll ~ O(1)

M. Djordjevic

Radiative or Collisional?

Modeling the medium seen by jets - flow

• Discussion of interplay between jets and flow illustrates: One of the weakest links between theory and data is the modeling of the spatio-temporal evolution of the produced matter.

Could make a factor 5 difference in

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ˆ q

N. Armesto, T. Renk

Quarkonium in the QGP

Qualitative idea:

= Thermometer of dense QCD matter H. Satz

Quarkonium in matter• Benchmark J.W. Qiu

• Lattice QCD describes quarkonium dissociation at rest in thermal environment.

Weakest link between theory and data: collective dynamics

F. Karsch

• Exogamous production of quarkonium is an interesting medium effect, but somewhat obscures the gauging of the quarkonium thermometer. How can one disentangle recombination and dissociation processes?

B. Thews

Unlike HQ-production, transition from HQ-pair to quarkonium is sensitive to soft physics.

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M 2 − 4mQ2

4mQ2

<<1

Can the medium of HICs serve as a tool to disentangle different quarkonium production mechanisms?

Elmag. Probes

Qualitative idea:

Sufficient yield of elmag probes

+ particularly clean signal (no final state effects)

= Basis for detailed investigation of dense QCD matter

Elmag. Probes

J. Kapusta, C. Gale,J. Ruppert

• Weakest link between theory and data: - collective dynamics - control over large number of sources

• Property of medium arguably well-defined

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ω d3R

d3k= −

gμν

2π( )3 ImΠ μν

R (k)1

eβω −1

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E+E−

d6R

d3 p+ d3 p−

=2e2

2π( )6

1

k 4Lμν ImΠ μν

R (k)1

eβω −1

photons

dileptons

Elmag spectra probe directly elmag current-current correlation function in mediumIF dynamical evolution scenario is known.

• Spectral densities from: - Lattice QCD - AdS/CFT - sum rules

S. Gupta, P. Kovtun

AdS/CFT• a novel and at present: unique testing ground for those theoretical techniques, which are required by the most basic insights from RHIC (namely non-perturbatively strong coupling and strong collective dynamics), but which are not (yet) available in QCD.

- AdS/CFT cousins of QCD exhibit much of what we are interested in: confinement, mass gap, global symmetries, chiral symmetry breaking - can be studied in weak and strong coupling limit, thus allowing for comparison of perturbative and non-perturbative techniques

• non-abelian thermal gauge theories share important generic features - energy density in strong coupling = 3/4 of energy density of free gas - universal viscosity to entropy density ratio - …

K. Rajagopal, P. Kovtun

Parton energy loss from AdS/CFT

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

Extra dimension r, the bulk withAdS black hole metric.

horizon

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r0

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r = ∞

Surface with boundary C in our world

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S(C)

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Tr W (C)[ ]med

= exp − 14

ˆ q L− (x − y)2[ ]• First strong-coupling calculation of

non-perturbative medium property entering radiative energy loss calculations

• Translation into field theoretic quantities:

Hawking temperature is QGP temperature

String tension determines t’Hooft coupling

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1 4πα '

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TH =r0

π R2= T

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R2

α '= λ

K. Rajagopal

• Insights:

See also P. Kovtun

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ˆ q SYM =18.87 α SYM Nc T 3 / ∝ ndensity

Punchline of this talk

• The connection between theory and data is only as strong as its weakest link. There are numerous examples, that for the sector of Hard Probes, the weakest link is currently the modeling of the produced matter.

• Many properties of the produced matter can be calculated in well-defined settings

- Lattice QCD (+ strong coupling, - no real time dynamics)- AdS/CFT (+ strong coupling, + real time dynamics, - not QCD)- HTL (- weak coupling, + real time dynamics)

• Embedding hard probes in a realistic geometrical and dynamical setting (hydrodynamics, dissipative hydrodynamics) is a prerequisite for: - determining numerically sensible values for medium properties tested by hard probes - characterizing at least qualitatively many of the collective effects present in dense matter (I.e Mach cones)