A comparison of various measures for the average transport coefficient qhat
Abstract
Jet quenching, i.e. the suppression of high transverse momentum PT 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.