ALICE is designed to study the physics of strongly interacting matter at extreme conditions of high temperature and energy density, where a phase of matter called the quark-gluon plasma (QGP) forms. The characterization of the QGP matter require creating a sufficiently large volume of hot and dense matter and is therefore pursued in collisions of heavy nuclei at the highest possible energies at the CERN LHC.
Under normal conditions, like those currently prevailing in the Universe, the quarks are bound (’confined’) into composite objects called baryons (bound states of three quarks, with the proton and neutron being the most prominent examples) and mesons (bound states of a quark and antiquark pair). Quantum Chromodynamics (QCD), the theory of the strong interaction, predicts that at high energy density a transition occurs from ordinary nuclear matter to a plasma of free (’deconfined’) quarks and gluons – a state of matter that existed in the early Universe and that might still be found today in the core of neutron stars.
Recreating this primordial state of matter, the QGP, in the laboratory and studying its evolution opens the possibility to study how matter is organized and the mechanisms that confine quarks and gluons. The focus on examining how collective expansion and macroscopic properties of a many-body system like the QGP emerge from the microscopic laws of elementary-particle physics. Collisions of protons or protons and nuclei have recently revealed collective features that are similar to those seen in collisions of nuclei, and are thus use to explore the limits of QGP formation.
ALICE also studies several other aspects of the strong interaction, that have connections to nuclear and hadronic physics, and to astrophysics. These include the strong interaction between pairs of hadrons, which is analogue to the interaction within atomic nuclei, and the formation and annihilation of light nuclear states, which is relevant for processes that occur in outer space.