The ALICE experiment - A journey through QCD

The ALICE experiment was proposed in 1993, to study strongly interacting matter at extreme energy densities via a comprehensive investigation of nuclear collisions at the LHC. Its physics programme initially focused on the determination of the properties of the Quark-Gluon Plasma (QGP), a deconfined state of quarks and gluons and was extended along the years, covering a diverse ensemble of observables related to Quantum Chromodynamics (QCD), the theory of strong interactions. The experiment has studied Pb-Pb, Xe-Xe, p-Pb and pp collisions in the multi-TeV energy range, during the Run 1 and Run 2 data taking periods at the LHC (2009-2018). The aim of this review article is to gather and summarise a selection of ALICE physics results and to discuss their implications on the current understanding of the macroscopic and microscopic properties of strongly interacting matter at the highest temperature reached in the laboratory. It will be shown that it is possible to have a quantitative description of the properties of the QGP produced in Pb--Pb collisions. We also show that various features, commonly ascribed to QGP formation, are detected for a wide range of interacting system sizes. Precision measurements of QCD-related observables not directly connected to the study of the QGP will also be discussed. Prospects for future measurements with the ALICE detector and its foreseen upgrades will also be briefly described.


Accepted by: EPJC
e-Print: arXiv:2211.04384 | PDF | inSPIRE
Figure group

Figure 2

Pressure, energy density and entropy density normalised to the 4$^{\rm th}$ (3$^{\rm rd}$ for the latter) power of the temperature, from the Lattice QCD calculations of the HotQCD Collaboration, see Ref. . The dark lines show the prediction of the Hadron Resonance Gas model, the horizontal line corresponds to the ideal gas limit for the energy density. The vertical band indicates the cross-over transition region. Corresponding results from the Wuppertal-Budapest Collaboration can be found in Ref. .

Figure 4

The $xy$ distributions of the initial energy density (arbitrary units) from the MC-Glauber and IP-Glasma models for a heavy-ion collision .

Figure 5

A mapping of the energy density in the QGP phases vs time and space for a mid-central ($b=7.5$ fm) Pb-- \fivenn collision using the T$_{\rm{R}}$ENTo-VISHNU model chain .

Figure 6

A schematic representation of the QCD phase diagram. The green line and band shows the $\mu_{\rm B}$ region accessible to Lattice QCD calculations  . The line shows the pseudocritical temperature, whereas the band represents the half-width of the crossover transition i.e. the temperatures where the QGP and hadrons can co-exist. The open points show experimental results for the determination of the chemical freeze-out parameters . The location of atomic nuclei is also shown, as well as conjectured regions for the presence of a first order phase transition and of a critical point.

Figure 7

The ALICE detector. A short description of the various subdetectors, as well as information on their kinematic coverage, is given in the text.

Figure 8

(Top) The ${\rm d}E/{\rm d}x$ signal in the ALICE TPC as a function of magnetic rigidity. The expected curves for various particle species are also shown, with the inset panel showing the TOF mass measurement providing additional separation for helium isotopes when $p/Z>$2.3 GeV/$c$. (Bottom) The Time-of-Flight measured in the TOF system as a function of the particle momentum. Tracks are selected with standard cuts inside the pseudorapidity region $|\eta| < 0.5$.

Figure 9

ALICE particle identification and reconstruction capabilities, with the $p_{\rm T}$ coverage corresponding to the published measurements based on pp or Pb--Pb data samples.