Relativistic heavy-ion collisions

Relativistic heavy-ion collisions

Collisions between heavy atomic nuclei at relative velocities close to the speed of light. These high-energy nuclear collisions are usually divided into two different domains, relativistic and ultrarelativistic collisions, depending on whether the kinetic energy per nucleon (the generic name for protons and neutrons) is either close to the rest mass of the nucleon (relativistic collisions) or much larger than the nucleon rest mass (ultrarelativistic collisions).

By utilizing high-energy nuclear collisions, it is possible to study nuclear matter under conditions of very high temperatures and densities. The most common form of nuclear matter, at least under terrestrial conditions, is found in the atomic nucleus, which consists of protons and neutrons bound together by the strong nuclear force. If nuclear matter is heated up to temperatures comparable to the rest mass of the pion, it becomes a mixture of nucleons, pions, and various other particles, collectively denoted hadrons. Under these circumstances, nuclear matter is referred to as hadronic matter. See Hadron, Neutron, Nuclear structure, Proton

According to the quantum chromodynamics (QCD) theory, all hadrons are bound states of a set of more fundamental entities called quarks. The quarks are confined within the hadrons by the exchange of gluons. Quantum chromodynamics calculations using the most powerful computers available show that if hadronic matter is further heated or compressed to very high densities it will undergo a phase transition into a new phase of matter, called the quark-gluon plasma. In this phase the hadrons will lose their identity, and the quarks and gluons will be deconfined within volumes much larger than the typical hadron volume of 0.1–0.5 cubic femtometer. Quantum chromodynamics calculations indicate that the phase transition will occur at a critical density around 5–10 times the normal nuclear matter density of approximately 0.2 nucleon/fm³, or at a critical temperature around 150 MeV. See Gluons, Quantum chromodynamics, Quark-gluon plasma, Quarks

When two nuclei collide at high energies, some of the nucleons in each nucleus, called spectators, will continue their motion unaffected, while other nucleons, called participants, will strike one or several nucleons in the other nucleus. In the overlap volume a hot and dense fireball will develop. If the temperature or density of the fireball becomes larger than the critical values, a quark-gluon plasma will be created with an estimated lifetime of 1–5 × 10^-23 s. The fireball will start to expand and cool, and the quarks in the plasma will eventually be reconfined into a large number of hadrons (hadronization). After further expansion the hadrons will cease interacting with each other (freeze out) and leave the collision zone without further mutual interactions.

In the search for the quark-gluon plasma, a fundamental problem is that even if the plasma is created in the early phases of the collisions, the subsequent hadronization and scattering of the hadrons before freeze-out might mask any traces of the plasma. In order to circumvent this problem, many plasma signatures have been proposed.