EPJ C Highlight - The ALICE experiment – A journey through QCD

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Published on 04 September 2024

The comprehensive review article “The ALICE experiment – A journey through QCD” has been recently published in the European Physical Journal C (EPJ C).

As of today, QCD is a gauge theory describing the interactions of quarks and gluons, the carriers of the strong force in analogy to the photon in electromagnetic processes. There is, however, a fundamental difference regarding the underling gauge symmetry, which is nonabelian in contrast to the one governing electromagnetic processes. Consequently, the strong force has the outstanding feature that its strength increases with increasing distance, resulting in a phenomenology of quark and gluon interactions depending upon the distance scale. In experiments where small distances are probed, the quarks and gluons behave like quasi-free elementary particles. Such processes can be described theoretically by perturbative methods; otherwise the quarks and gluons are confined, forming a small complex system inside a hadron, and lattice methods are required to describe the processes. In hindsight it becomes obvious why the searches for free quarks during the 1960s were doomed to fail.

With the advent of the Large Hadron Collider (LHC), the exploration of QCD proceeded at the unprecedented TeV energy scale. The ALICE collaboration built a detector dedicated to exploit this regime running with heavy-ion beams, such as lead-on-lead. This offers a unique way to generate large high-density and high-energy quark-gluon systems. They reveal a novel state of matter: quark-gluon plasma (QGP), which is a deconfined state of quarks and gluons. The main scope of this review is to analyze and discuss the properties of QGP, which opens a new window into the ‘colored world’ with a very rich phenomenology. The extreme conditions now reached in the lab provide conclusions about the very early universe.

The review is the result of a collaborative effort, outlined in nine chapters. The broad Introduction gives an overview of the experiment and the topics elaborated on in the subsequent chapters. It includes a description of the ALICE detector and its main features. Chapter 2, the most comprehensive one, is devoted to the centerpiece topic: quark-gluon plasma and its properties using a large number of observables. Chapter 3 deals with high-density QCD effects in pp and pN collisions. Chapter 4 describes the initial state of the collision. Chapter 5 addresses the nuclear physics program at the LHC. Chapter 6 presents QCD studies with high-Q2 processes in pp collisions. Chapter 7 outlines ALICE’s contributions beyond QCD physics, together with synergies between heavy-ion physics and other fields. A summary is presented in chapter 8. The review concludes with a fascinating outlook for the next two decades – especially the still young field of QGP holds the promise of a great future. A very rich bibliography equips the reader to pursue in-depth research on their own.

This extensive work manages to consistently strike a good balance between introductory material on one hand and research-level detail on the other, making it a valuable asset for experts and graduate students in the field, as well as interested readers from related fields of physics. It is currently the best available source of inspiration and information on matter in the deconfined state.