Pb-Pb collision in ALICE (click for animation)

Heavy Ion Collisions

Quark Gluon Plasma is a phase of quantum chromodynamics (QCD) which exists at extremely high temperature and density. It is believed to have existed during the first 20 or 30 microseconds after the universe came into existence in the Big Bang. Experiments at CERN's Super Proton Synchrotron first tried to create the QGP in the 1980s and 1990s. Currently, experiments at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) are continuing this effort. CERN's new experiment, ALICE, will start soon at the Large Hadron Collider (LHC). On the left, a reconstruction movie of one collision between two lead nuclei at a center of mass energy of 5.5 TeV per nucleon pair. It is believed that a QGP is created in the early phase such a collision. The particles produced after the freeze-out are flying through the various detectors.

The QCD phase diagram

QCD matter refers to any of a number of phases of matter whose degrees of freedom include quarks and gluons. These phases occur at extremely high temperatures and densities where hadronic matter is supposed to undergo a phase transition to a new state where quarks and gluons are no longer 'hidden' (or, more technically, confined) into nucleons.
By analogy with classical plasma this phase is called quark-gluon plasma - however, at a variance with most classical plasma, the QCD plasma is an extremely complicated phase with remaining interactions among the constituents, whose properties are under active scrutiny.

Under such extreme conditions, the familiar structure of matter, with quarks arranged into nucleons and nucleons bound into nuclei and surrounded by electrons, is completely disrupted, and the quarks roam freely. At ordinary temperatures or densities the nuclear force just confines the quarks into composite particles (hadrons) of size around 1 fm (corresponding to the QCD energy scale Λ_QCD≈200 MeV) and its effects are not noticeable at longer distances. However, when the temperature reaches the QCD energy scale (T of order 10^12K) or the density rises to the point where the average inter-quark separation is less than 1 fm (quark chemical potential μ around 400 MeV), the hadrons are melted into their constituent quarks, and the strong interaction becomes the dominant feature of the physics. Such phases are called quark matter or QCD matter.

The phase diagram of quark matter is not well known, either experimentally or theoretically. A commonly conjectured form of the phase diagram is shown in the figure. It is applicable to matter in a compact star, where the only relevant thermodynamic potentials are the quark chemical potential μ and temperature T. If we increase the quark density (i.e. increase μ) keeping the temperature low, we move into a phase of compressed nuclear matter. Eventually, at an unknown critical value of μ, there is a transition to quark matter. At ultra-high densities we expect to find the color-flavor-locked (CFL) phase of color-superconducting quark matter. At intermediate densities we expect some other phases (labelled "non-CFL quark liquid" in the figure) whose nature is presently unknown.

At the bottom left corner of the phase diagram, in the vacuum, we have μ=T=0. If we heat up the system without introducing any perference for quarks over antiquarks, this corresponds to moving vertically upwards along the T axis. At first, quarks are still confined and we create a gas of hadrons (pions, mostly). Then around T=170 MeV there is a crossover to the quark gluon plasma: thermal fluctuations break up the pions, and we find a gas of quarks, antiquarks, and gluons, as well as lighter particles such as photons, electrons, positrons, etc. Following this path corresponds to the state of the universe shortly after the big bang (where there was a very tiny preference for quarks over antiquarks).

The line that rises up from the nuclear/quark matter transition and then bends back towards the T axis, with its end marked by a star, is the conjectured boundary between confined and unconfined phases. Until recently it was also believed to be a boundary between phases where chiral symmetry is broken (low temperature and density) and phases where it is unbroken (high temperature and density). It is now known that the CFL phase exhibits chiral symmetry breaking, and other quark matter phases may also break chiral symmetry, so it is not clear whether this is really a chiral transition line. The line ends at the "chiral critical point", marked by a star in this figure, which is a special temperature and density at which striking physical phenomena (analogous to critical opalescence) are expected (see "open questions" below).