Heavy-ion collision

A high-energy collision between atomic nuclei that can create extremely hot, dense matter. Experiments use the resulting particles to study quantum chromodynamics and the strong force.

Relativistic heavy-ion collision: A collision of heavy nuclei at speeds near light speed, studied to create and probe hot nuclear matter. This is the high-energy regime in which nuclear matter can become a quark–gluon plasma.
Quantum chromodynamics: The quantum field theory describing quarks, gluons, and their strong interaction. It governs the matter and forces probed in heavy-ion collisions.
Large Hadron Collider: The CERN particle accelerator that collides protons and ions at the world’s highest collider energies. Its lead-ion runs produce especially energetic samples of hot QCD matter.
Proton–proton collision: A collision between two protons, used to study particle interactions and provide a baseline for nuclear collisions. It lacks the large nuclear medium that makes heavy-ion collisions distinctive.
QCD phase diagram: A map of possible states of strongly interacting matter as temperature and baryon density vary. Changing collision energy samples different regions of this phase diagram.
Quark–gluon plasma: A state of matter in which quarks and gluons are no longer confined inside individual hadrons. It can form briefly after the nuclei deposit enough energy into a small volume.
Nucleus: The compact, positively charged center of an atom, composed of protons and neutrons. Heavy-ion experiments collide nuclei containing many strongly interacting nucleons.
ALICE experiment: A CERN experiment designed to study heavy-ion collisions and the quark–gluon plasma. ALICE measures the diverse particles emerging from lead-ion collisions.
Proton–nucleus collision: A collision between a proton and a nucleus, probing nuclear effects with a smaller collision system. It helps distinguish effects of the initial nucleus from those of a large hot medium.
QCD critical point: A hypothesized endpoint of a first-order phase transition in the QCD phase diagram. Fluctuations in collision data may reveal whether this point exists.

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