ALICE is the acronym for A Large Ion Collider Experiment, one of the largest experiments in the world devoted to research in the physics of matter at an infinitely small scale. Hosted at CERN, the European Laboratory for Particle Physics, the ALICE experiment is located at the territory of the community of Sergy (Ain, France) and is installed 60 metres below ground. The main goal of the ALICE collaboration is to characterize the physical properties of the Quark-Gluon Plasma (QGP), a state of matter created under the extreme conditions of temperature and energy density created in nuclear collisions, at the Large Hadron Collider (LHC), the world's largest accelerator.
ALICE is optimized to study collisions of nuclei at the ultra-relativistic energies provided by the LHC. These collisions offer the best experimental conditions to produce the quark–gluon plasma. Such conditions are believed to have existed up to a few millionths of a second after the Big Bang before quarks and gluons were bound together to form protons and neutrons. Recreating this primordial state of matter in the laboratory and understanding how it evolves will allow us to shed light on questions about how matter is organized and the mechanisms that confine quarks and gluons.
Ordinary matter is made up of atoms, each of which consists of a nucleus surrounded by a cloud of electrons. Nuclei consist of hadrons (such as protons and neutrons), which in turn are made of quarks. Quarks are bound together in hadrons by the strong interaction mediated by gluons. The strong interaction is one of the four fundamental forces.
To recreate conditions similar to those of the early universe, powerful accelerators deliver head-on collisions between massive ions, such as gold or lead nuclei. In these heavy-ion collisions the hundreds of protons and neutrons from these nuclei smash into one another at energies of upwards of a few trillion electronvolts each. The extreme energy density that is then reached causes the formation of the quark-gluon plasma. The QGP quickly cools until the individual quarks and gluons recombine into a blizzard of ordinary matter that speeds away in all directions. The ALICE detectors are designed to efficiently detect the particles produced in such lead-lead collisions at the LHC.
No isolated quark has ever been observed: quarks, as well as gluons, seem to be bound permanently together and confined inside hadrons. This phenomenon is known as confinement. It relates to the fact that, in addition to an electric charge (which is a fraction of the electron's one), each quark has a colour charge, which can only take 3 values (red, blue or green). For normal conditions of temperature and density, quarks only exist within hadrons - aggregates of two or three quarks that are colour neutral (i.e. white). Confinement is also the fundamental property that gives each hadron a mass much larger than the sum of the masses of its constituents: in fact, confinement generates about 99% of the nuclear mass.
The QGP is a state of matter wherein quarks and gluons are no longer confined in hadrons. Such a state is predicted by the current theory of the strong interaction (called quantum chromodynamics, QCD) for very high temperatures and very high densities. The transition between confined matter and the QGP would occur when the temperature exceeds a critical value estimated to be around 2000 billion degrees (about 100 000 times the temperature of the core of the Sun). Such extreme temperatures have not existed in Nature since the birth of the Universe: it is believed that for a few millionths of a second after the Big Bang the temperature was above the critical value, and the entire Universe was in a quark-gluon plasma state.
ALICE is a 10,000-tonnes detector – 26 m long, 16 m high, and 16 m wide. It sits in a vast cavern 56 m below ground in the territory of Sergy on the border with St Genis-Pouilly in France. The detector is designed to measure, in the most complete way possible, the particles produced in the collisions which take place at its centre, so that the evolution of the system produced during these collisions can be reconstructed and studied. To do so, many different subdetectors have to be used, each providing a different piece of information. To understand such a complex system, one needs to observe it from different points of view, using different instruments at the same time, in the same way that a satellite looks at the earth combining detectors sensitive to different wavelengths, allowing us to see forests or clouds or archeological sites…
The idea of building a dedicated heavy-ion detector for the LHC was first aired at the historic Evian meeting "Towards the LHC experimental programme" in March 1992. From the ideas presented there, the ALICE collaboration was formed in 1993. The wealth of published scientific results and the upgrade programme of ALICE have attracted numerous institutes and scientists from all over the world.
ALICE recorded the very first proton collisions provided by the LHC in November 2009. During the LHC Run 1 (2009-2013), ALICE participated in all data-taking campaigns, recording proton-proton collisions at centre-of-mass energies of 0.9, 2.76, 7 and 8 TeV, proton-lead collisions at 5 TeV and lead-lead collisions at 2.76 TeV. In the case of proton-lead and lead-lead collisions, this is the energy for each collision between two nucleons. In Run 2 (2015-2018) the centre of mass energy was increased to 13 TeV for proton-proton, 8 for proton-lead and 5 for lead-lead. ALICE also recorded comparison data in proton-proton and proton-lead at the same energy as Pb-Pb, 5 TeV. In addition, a short data-taking campaign with xenon-xenon collisions was carried out. From the data collected so far a wealth of interesting results has been produced, spanning from the characterization of the high-density system formed in Pb-Pb collisions and the study of Quark-Gluon Plasma manifestations, to the surprising finding that also in proton-proton and proton-lead collisions some features of particle production resembles those seen in lead-lead.