The heavy ion experiment at the Large Hadron Collider
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.
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Physics
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.
Experiment
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…
Collaboration
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.
Results
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.
