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Phases of matter whose degrees of freedom include Quark s and Gluon s. These phases occur at extremely high temperatures and densities, billions of times higher than can be produced in equilibrium in laboratories. Under such extreme conditions, the familiar structure of matter, with quarks arranged into Nucleon s and nucleons bound into Nuclei and surrounded by Electrons , is completely disrupted, and the quarks roam freely. This is analogous to the way that the crystal structure of ice is disrupted by heating or compression, and melts into a liquid of more elementary constituents (water molecules). In the Standard Model of particle physics, the strongest force is the Strong Interaction , which is described by the theory of Quantum Chromodynamics (QCD). At ordinary temperatures or densities this force just Confines the quarks into composite particles ( Hadrons ) of size around 10-15m = 1 Femtometer = 1 fm (corresponding to the QCD energy scale ΛQCD≈200 MeV) and its effects are not noticeable at longer distances. However, when the temperature reaches the QCD energy scale (T of order 1012K) 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.
OCCURRENCE IN NATURE
THERMODYNAMICS The context for understanding the thermodynamics of quark matter is the Standard Model of particle physics, which contains six different Flavors of quarks, as well as Lepton s like Electron s and Neutrino s. These interact via the Strong Interaction , Electromagnetism , and also the Weak Interaction which allows one flavor of quark to turn into another. Electromagnetic interactions occur between particles that carry electrical charge; strong interactions occur between particles that carry Color Charge . The correct thermodynamic treatment of quark matter depends on the physical context. For large quantities that exist for long periods of time (the "thermodynamic limit"), we must take into account the fact that the only conserved charges in the standard model are quark number (equivalent to Baryon number), electric charge, the eight color charges, and lepton number. Each of these can have an associated chemical potential. However, large volumes of matter must be electrically and color-neutral, which determines the electric and color charge chemical potentials. This leaves a three-dimensional phase space, parameterized by quark chemical potential, lepton chemical potential, and temperature. In compact stars quark matter would occupy cubic kilometers and exist for millions of years, so the thermodynamic limit is appropriate. However, the neutrinos escape, violating lepton number, so the phase space for quark matter in compact stars only has two dimensions, temperature (T) and quark number chemical potential μ (see next section). A strangelet is not in the thermodynamic limit of large volume, so it is like an exotic nucleus: it may carry electric charge. A heavy-ion collision is in neither the thermodynamic limit of large volumes nor long times. Putting aside questions of whether it is sufficiently equilibrated for thermodynamics to be applicable, there is certainly not enough time for weak interactions to occur, so flavor is conserved, and there are independent chemical potentials for all six quark flavors. The initial conditions (the Impact Parameter of the collision, the number of up and down quarks in the colliding nuclei, and the fact that they contain no quarks of other flavors) determine the chemical potentials. PHASE DIAGRAM |
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