From R_AA via correlations to jets - the long road to tomography

TL;DR

This paper reviews the progress and challenges in using high-energy probes, especially correlations, to perform tomography of the quark-gluon plasma in heavy ion collisions, highlighting recent model constraints and future directions.

Contribution

It discusses advancements in modeling hard probes and correlations, emphasizing their potential for detailed medium tomography and identifying current limitations and prospects.

Findings

Correlations provide additional information beyond single hadron suppression.

Models of back-to-back correlations are well constrained by data.

Progress in understanding low P_T correlations like the ridge is ongoing.

Abstract

The main motivation to investigate hard probes in heavy ion collisions is to do tomography, i.e. to infer medium properties from the in-medium modification of hard processes. Yet while the suppression of high P_T hadrons has been measured for some time, solid tomographic information is slow to emerge. This can be traced back to theoretical uncertainties and ambiguities in modelling both medium evolution and parton-medium interaction. Ways to overcome these difficulties are to constrain models better and to focus on more differential observables. Correlations of high P_T hadrons offer non-trivial information beyond what can be deduced from single hadron suppression. They reflect not only the hard reaction being modified by the medium, but also the back reaction of the medium to the hard probe. Models for hard back-to-back correlations are now very well constrained by a wealth of data and allow insights into the nature of the parton-medium interaction as well as first true tomographic results. Models of full in-medium jet evolution are being actively developed, but have yet to make substantial contact with data. Progress is slower in the understanding of low P_T correlations, the ridge and the cone, although a qualitative understanding of the nature of the physics behind these correlations starts to emerge.