# Partonic Energy Loss

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# Work Plan of the Hard Physics Working Group

The Hard Physics Working Group has decided to focus initially on the following problems:

# Purpose and Formulation of the QGP Brick Problem

In the recent past, several individual research groups arrived at apparently satisfactory descriptions of the nuclear modification factor by modeling jet quenching with one or only a few model parameters. Despite these individual successes, quantitative conclusions about the properties of the matter produced in heavy ion collisions have remained unclear so far. In particular, different groups arrived at phenomenologically successful descriptions by quoting values for the jet transport parameter, which differ by as much as an order of magnitude.

The different research groups in the Hard Physics working group have agreed to perform a set of simplified benchmark calculations. The aim of this exercise is to arrive at a common standard for comparing different models of jet quenching. This will clarify the origin of the large numerical discrepancies between different groups, thus providing a basis for quantitative conclusions.

The first problem is the “static QGP brick” problem, which will be completed within the coming months.

• Input Consider a brick of QGP without any time-evolution (static case). Consider a quark of 10 and a quark of 100 GeV energy, produced in the brick. The parton propagates over a length of either L=2 fm or L = 5 fm through this medium. If possible, choose a fixed coupling constant alpha_s = 0.3, to ease comparison. [If not possible, specify fully the choice of your running coupling constant.] The virtuality of the initial parton should be Q=E.
• Task 1 Plot in your model the probability distribution P(\Delta E) for the value of model parameters, for which the fraction of the average energy loss <\Delta E>/E is equal to 0.05, 0.1, 0.2, 0.4.
• Task 2 Specify for the above calculation the relation of your model parameters to the temperature of medium.
• Task 3 Plot for each probability distribution P(\Delta E) the corresponding single inclusive gluon energy distribution [or, alternatively, the primary distribution from which P(\Delta E) was calculated

Proposal (Berndt Mueller):

I propose that we expand the brick problem effort to permit parton cascade codes to participate.

There are currently at least 4 different codes in which the energy loss dE/dx in a thermal gluon bath can be calculated: MPC (Molnar), VNI/BMS (Bass et al), Ghi Shin's code, and the code written by Xu and Greiner. A first plot of dE/dx as a function of parton energy has been published by the last group in arXiv:0806.1169 .

# Brick Results

The results of different groups for the Brick Problem are given here:

• GLV
• WHDG
• GLV-WMC: (D)GLV to High Orders in Opacity, ${1\,\leq \,n\,\leq \,9}$ • AMY
• Higher-Twist
• YaJEM
• ASW

Output format for the computed energy loss distributions.

Comparison of the ASW/BDMPS and WHDG/GLV brick results

Proposed common definition of qhat, lambda and mu as a function of T.

# PCM Brick Results

The results of different groups on the Parton Cascade Model Brick result are given here:

APC = Andong Parton Cascade - nucl-th/0207041

# TECHQM Notes and Discussion Forum

The password-protected area for TECHQM Notes and discussion is here.

Public notes: