A comparison of various measures for the average transport coefficient qhat

TL;DR

This paper critically examines the methods used to extract and define the average transport coefficient <qhat> in jet quenching studies, revealing significant uncertainties and questioning the validity of using a single average parameter.

Contribution

It demonstrates that different definitions of the average transport coefficient lead to substantial variations, challenging the practice of summarizing jet-medium interactions with one parameter.

Findings

Different average definitions yield up to fourfold differences in <qhat> values.

The choice of average affects the observed centrality dependence of <qhat>.

Using a single <qhat> parameter may oversimplify complex jet-medium interactions.

Abstract

Jet quenching, i.e. the suppression of high transverse momentum P_T hadron production in ultrarelativistic heavy-ion collisions is among the most striking experimental signatures of bulk medium formation. Efforts with the aim of extracting quantitative information about the bulk medium from the measured suppression mainly focus on the extraction of an averaged transport coefficient <qhat> as a measure of the medium jet quenching power, with the underlying assumption that <qhat> is a meaningful quantity to make comparisons both among different models and between models and data. In this note, the main uncertainties associated with the extraction of <qhat> from model fits to data are briefly reviewed before the notion of an average transport coefficient itself is investigated. It is shown in a case study that the choice of a meaningful average is far from unique and that different well-motivated definitions of averages lead not only to differences in the outcome of about a factor four, but also in a different change of <qhat> with collision centrality. This casts some doubt on the idea that condensing the information about jet-medium interactions into a single average parameter is a meaningful procedure.