# Presentation: 149102

## Hard Probes 2006

- Urs Achim Wiedemann
- SUNY Stony Brook and RIKEN BNL

- The “Not a Theory Summary”-Talk

*Asilomar, 15 June 2006*

## From elementary interactions to collective phenomena

- _From elementary interactions to collective phenomena_

## 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?

-  _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.

**Notes:**
Let me start with what is arguable the most successful hard probe explored so far: DIS.
This field is what our field strives to become: it is an undisputable chapter of a QCD textbook

It may be instructive to contemplate shortly, what makes DIS so successful

## Jet Quenching = DIS on the QGP?

## Accuracy of jet-quenching benchmarks

## Modeling the medium seen by jets

-  assumptions in _all_ current jet quenching calculations are simplistic

**K. Rajagopal**

- +

- +

- +

**Are static scattering**
**centers 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?

-  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?

**X.N. Wang**

**M. Djordjevic**

- Radiative or
- Collisional?

## Modeling the medium seen by jets - flow

## Quarkonium in the QGP

- _Qualitative idea:_
- = _Thermometer_ of dense QCD matter

**H. Satz**

## Quarkonium in matter

## 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

## 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

## 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)