Quark matter or QCD matter (see QCD) refers to any of a number of theorized phases of matter whose degrees of freedom include quarks and gluons. Quantum chromodynamics (abbreviated as QCD is a theory of the Strong interaction ( color force a Fundamental force describing the interactions of the In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. Gluons ( Glue and the suffix -on) are Elementary particles that cause Quarks to interact and are indirectly responsible for the These theoretical phases would 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 nucleons and nucleons bound into nuclei and surrounded by electrons, is completely disrupted, and the quarks roam freely in what is called a quark gluon plasma. In Physics a nucleon is a collective name for two Baryons the Neutron and the Proton. The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J A quark-gluon plasma (QGP is a phase of Quantum chromodynamics (QCD which exists at extremely high Temperature and/or Density. 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). The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles In particle physics the strong interaction, or strong force, or color force, holds Quarks and Gluons together to form Protons and Quantum chromodynamics (abbreviated as QCD is a theory of the Strong interaction ( color force a Fundamental force describing the interactions of the 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. Color confinement, often called just confinement, is the Physics phenomenon that Color charged particles (such as Quarks cannot be isolated singularly In Particle physics, a hadron ( from the ἁδρός hadrós, " stout, thick " ( The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International 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. In Thermodynamics and Chemistry, chemical potential, symbolized by μ, is a term introduced by the American engineer chemist and mathematical Such phases are called quark matter or QCD matter.
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Occurrence
Natural occurrence
- The early universe. In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties According to the theory of the big bang, at very early times, when the universe was only a few tens of microseconds old, the temperature was so high that all matter took the form of a hot phase of quark matter called the quark-gluon plasma (QGP). The Big Bang is the cosmological model of the Universe that is best supported by all lines of scientific evidence and Observation. A quark-gluon plasma (QGP is a phase of Quantum chromodynamics (QCD which exists at extremely high Temperature and/or Density.
- Compact stars (neutron stars). In Astronomy, the term compact star (sometimes compact object) is used to refer collectively to White dwarfs Neutron stars other exotic A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type A neutron star is much cooler than 1012 K, but it is compressed by its own weight to such high densities that it is reasonable to surmise that quark matter may exist in the interior. Compact stars composed mostly or entirely of quark matter are known as quark stars or strange stars. A quark star or strange star is a hypothetical type of Exotic star composed of Quark matter, or Strange matter. A quark star or strange star is a hypothetical type of Exotic star composed of Quark matter, or Strange matter. The nuclear matter component of strange stars, if there is one, is believed to consist only of a tiny surface crust.
- Strangelets. A strangelet is a hypothetical object consisting of a bound state of roughly equal numbers of up, down, and strange Quarks The size would be These are hypothetical lumps of strange matter that might populate interstellar space. For the physics concept see Strange matter. Strange Matter is a Children's book series created by Marty M They only exist if nuclear matter is metastable against decay into quark matter: this is generally regarded as a fairly radical hypothesis.
- Cosmic Ray Impacts. For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on High speed protons, now believed to have originated from "nearby" active galactic nuclei based on recent results of the Pierre Auger cosmic ray observatory routinely impact earth with center of momentum energies much greater than today's Particle colliders
Artificial occurrence
- Heavy-ion collisions. An active galactic nucleus ( AGN) is a compact region at the centre of a Galaxy which has a much higher than normal luminosity over some or all of the Electromagnetic Pierre Victor Auger ( May 14, 1899 &ndash December 25, 1993) was a French Physicist A center of momentum frame (or zero-momentum frame or COM frame of a system is any Inertial frame in which the Center of mass is at rest (has zero velocity Physicists can produce small short-lived regions of space whose energy density is comparable to that of the 20-microsecond-old universe. This is achieved by colliding heavy nuclei at high speeds. Extremely powerful accelerators are needed, such as RHIC at Brookhaven National Laboratory in the USA, or the future LHC at CERN in Switzerland/France. The Relativistic Heavy Ion Collider (RHIC pronounced like " Rick " ˈrɪk is a heavy- Ion Collider located at and operated by Brookhaven Brookhaven National Laboratory ( BNL) is a United States national laboratory located in Upton New York on Long Island, and was formally established The European Organization for Nuclear Research (Organisation Européenne pour la Recherche Nucléaire known as CERN There is good evidence that the quark-gluon plasma has been produced at RHIC [1].
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 leptons like electrons and neutrinos. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles In Particle physics, flavour or flavor (see spelling differences) is a Quantum number of Elementary particles related to their Leptons are a family of fundamental Subatomic particles comprising the Electron, the Muon, and the Tauon (or tau particle as well as their The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost These interact via the strong interaction, electromagnetism, and also the weak interaction which allows one flavor of quark to turn into another. In particle physics the strong interaction, or strong force, or color force, holds Quarks and Gluons together to form Protons and Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of The weak interaction (often called the weak force or sometimes the weak nuclear force) is one of the four Fundamental interactions of nature Electromagnetic interactions occur between particles that carry electrical charge; strong interactions occur between particles that carry color charge. In Particle physics, color charge is a property of Quarks and Gluons which are related to their Strong interactions in the context of Quantum
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. Baryons are the family of Subatomic particles with a Baryon number of 1 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 Mathematics and Physics, a phase space, introduced by Willard Gibbs in 1901 is a Space in which all possible states of a System
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 strangelet is a hypothetical object consisting of a bound state of roughly equal numbers of up, down, and strange Quarks The size would be 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. The impact parameter is defined as the perpendicular distance between the velocity vector of a projectile and the center of the object it is approaching (see diagram
Phase diagram
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 [2]. It is applicable to matter in a compact star, where the only relevant thermodynamic potentials are quark chemical potential μ and temperature T. In Thermodynamics and Chemistry, chemical potential, symbolized by μ, is a term introduced by the American engineer chemist and mathematical Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature For guidance it also shows the typical values of μ and T in heavy-ion collisions and in the early universe. For readers who are not familiar with the concept of a chemical potential, it is helpful to think of μ as a measure of the imbalance between quarks and antiquarks in the system. Higher μ means higher density of quarks.
Ordinary atomic matter as we know it is really a mixed phase, droplets of nuclear matter (nuclei) surrounded by vacuum, which exists at the low-temperature phase boundary between vacuum and nuclear matter, at μ=310MeV and T close to zero. If we increase the quark density (i. e. increase μ) keeping the temperature low, we move into a phase of more and more compressed nuclear matter. Following this path corresponds to burrowing more and more deeply into a neutron star. A neutron star is a type of remnant that can result from the Gravitational collapse of a massive Star during a Type II, Type Ib or Type 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. Color-flavor locking (CFL is a phenomenon that is expected to occur in ultra-high-density Quark matter. Color superconductivity is a phenomenon predicted to occur in quark matter if the Baryon density is sufficiently high (well above nuclear density and the temperature At intermediate densities we expect some other phases (labelled "non-CFL quark liquid" in the figure) whose nature is presently unknown. They might be other forms of color-superconducting quark matter, or something different.
Now, imagine starting at the bottom left corner of the phase diagram, in the vacuum where μ=T=0. If we heat up the system without introducing any preference 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). In Particle physics, pion (short for pi meson) is the collective name for three Subatomic particles, and. 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 travelling far back in time, 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). In Physical chemistry, Thermodynamics, Chemistry and Condensed matter physics, a critical point, also called a critical state Critical opalescence is a phenomenon which arises in the region of a continuous or second-order Phase transition.
Theoretical challenges: calculation techniques
The phase structure of quark matter remains mostly conjectural because it is difficult to perform calculations predicting the properties of quark matter. The reason is that QCD, the theory describing the dominant interaction between quarks, is strongly coupled at the densities and temperatures of greatest physical interest, and hence it is very hard to obtain any predictions from it. Here are brief descriptions of some of the standard approaches.
Lattice gauge theory
The only first-principles calculational tool currently available is lattice QCD, i. In Physics, lattice quantum chromodynamics (lattice QCD is a theory of Quarks and Gluons formulated on a space-time lattice. e. brute-force computer calculations. Because of a technical obstacle known as the fermion sign problem, this method can only be used at low density and high temperature (μ<T), and it predicts that the crossover to the quark-gluon plasma will occur around T=170 MeV [3] However, it cannot be used to investigate the interesting color-superconducting phase structure at high density and low temperature [4].
Weak coupling theory
Because QCD is asymptotically free it becomes weakly coupled at unrealistically high densities, and diagrammatic methods can be used [5]. In Physics, asymptotic freedom is the property of some gauge theories in which the interaction between the particles such as Quarks, becomes arbitrarily Such methods show that the CFL phase occurs at very high density. At high temperatures, however, diagrammatic methods are still not under full control.
Models
To obtain a rough idea of what phases might occur, one can use a model that has some of the same properties as QCD, but is easier to manipulate. Many physicists use Nambu-Jona-Lasinio models, which contain no gluons, and replace the strong interaction with a four-fermion interaction. In Quantum field theory, the Nambu-Jona-Lasinio model (named for its developers Yoichiro Nambu and Giovanni Jona-Lasinio) is a theory of interacting Mean-field methods are commonly used to analyse the phases. Another approach is the bag model, in which the effects of confinement are simulated by an additive energy density that penalizes unconfined quark matter. In Physics a nucleon is a collective name for two Baryons the Neutron and the Proton.
Effective theories
Many physicists simply give up on a microscopic approach, and make informed guesses of the expected phases (perhaps based on NJL model results). For each phase, they then write down an effective theory for the low-energy excitations, in terms of a small number of parameters, and use it to make predictions that could allow those parameters to be fixed by experimental observations [6].
Other approaches
There are other methods that are sometimes used to shed light on QCD, but for various reasons turn out not to be particularly useful in studying quark matter.
- 1/N expansion. Treat the number of colors N, which is actually 3, as a large number, and expand in powers of 1/N. It turns out that at high density the higher-order corrections are large, and the expansion gives misleading results.
- Supersymmetry. In Particle physics, supersymmetry (often abbreviated SUSY) is a Symmetry that relates elementary particles of one spin to another particle that Adding scalar quarks (squarks) and fermionic gluons (gluinos) to the theory makes it more tractable, but the thermodynamics of quark matter depends crucially on the fact that only fermions can carry quark number, and on the number of degrees of freedom in general.
Experimental challenges
Experimentally, it is hard to map the phase diagram of quark matter because it is impossible to achieve high enough temperatures and densities in the laboratory. Heavy-ion collisions provide information about the crossover from hadronic matter to QGP. Observations of compact stars may provide information about the high-density low-temperature region. Studies of the cooling, spin-down, and precession of these stars have already given information about the properties of their interior. As observations become more precise we hope to learn more.
One of the natural subjects for future research is the exact location of the chiral critical point. Some ambitious lattice QCD calculations may have found evidence for it, and future calculations will clarify the situation. Heavy-ion collisions might be able to measure its position experimentally, but this will require scanning across a range of values of μ and T [7], a project that may be undertaken in future experiments.
See also
Further reading
- S. Quantum chromodynamics (abbreviated as QCD is a theory of the Strong interaction ( color force a Fundamental force describing the interactions of the A quark-gluon plasma (QGP is a phase of Quantum chromodynamics (QCD which exists at extremely high Temperature and/or Density. In Physics, lattice quantum chromodynamics (lattice QCD is a theory of Quarks and Gluons formulated on a space-time lattice. For the physics concept see Strange matter. Strange Matter is a Children's book series created by Marty M A quark star or strange star is a hypothetical type of Exotic star composed of Quark matter, or Strange matter. Hands, "The phase diagram of QCD" arXiv:physics/0105022 Contemp. Phys. 42, 209 (2001).
References
- ^ B. Müller "Quark Matter 2005 -- Theoretical Summary", arxiv.org:nucl-th/0508062
- ^ M. Alford, K. Rajagopal, T. Schäfer, A. Schmitt, "Color superconductivity in dense quark matter", arXiv:0709.463, Reviews of Modern Physics (to be published)
- ^ U. Heller, "Recent progress in finite temperature lattice QCD", PoS (LAT2006) 011
- ^ C. Schmidt, "Lattice QCD at Finite Density", PoS (LAT2006) 021
- ^ D. Rischke, "The quark-gluon plasma in equilibrium", Prog. Part. Nucl. Phys. 52, 197 (2004)
- ^ T. Schäfer, "Quark matter", arxiv.org:hep-ph/0304281
- ^ K. Rajagopal, "Mapping the QCD Phase Diagram", Nucl.Phys. A661 (1999) 150-161
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