HOME

TheInfoList



OR:

Quark–gluon plasma (QGP or quark soup) is an interacting localized assembly of
quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...
s and
gluon A gluon ( ) is a type of Massless particle, massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a Spin (physi ...
s at thermal (local kinetic) and (close to) chemical (abundance) equilibrium. The word ''plasma'' signals that free
color charge Color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics (QCD). Like electric charge, it determines how quarks and gluons interact through the strong force; ho ...
s are allowed. In a 1987 summary, Léon Van Hove pointed out the equivalence of the three terms: quark gluon plasma, quark matter and a new state of matter. Since the temperature is above the
Hagedorn temperature The Hagedorn temperature, ''T''H, is the temperature in theoretical physics where hadronic matter (i.e. ordinary matter) is no longer stable, and must either "evaporate" or convert into quark matter; as such, it can be thought of as the "boiling p ...
—and thus above the scale of light u,d-quark mass—the pressure exhibits the relativistic Stefan–Boltzmann format governed by temperature to the fourth power ( T^) and many practically massless quark and gluon constituents. It can be said that QGP emerges to be the new phase of strongly interacting matter which manifests its physical properties in terms of nearly free dynamics of practically massless gluons and quarks. Both quarks and gluons must be present in conditions near chemical (yield) equilibrium with their
color charge Color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics (QCD). Like electric charge, it determines how quarks and gluons interact through the strong force; ho ...
''open'' for a new state of matter to be referred to as QGP. In the Big Bang theory, quark–gluon plasma filled the entire
Universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...
before matter as we know it was created. Theories predicting the existence of quark–gluon plasma were developed in the late 1970s and early 1980s. Discussions around heavy ion experimentation followed suit, and the first experiment proposals were put forward at
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...
and BNL in the following years. Quark–gluon plasma was detected for the first time in the laboratory at CERN in the year 2000.


General introduction

Quark–gluon plasma is a
state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. Different states are distinguished by the ways the ...
in which the elementary particles that make up the hadrons of
baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...
ic matter are freed of their
strong Strong may refer to: Education * The Strong, an educational institution in Rochester, New York, United States * Strong Hall (Lawrence, Kansas), an administrative hall of the University of Kansas * Strong School, New Haven, Connecticut, United ...
attraction for one another under extremely high energy densities. These particles are the
quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...
s and gluons that compose baryonic matter. In normal matter quarks are '' confined''; in the QGP quarks are '' deconfined''. In classical
quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...
(QCD), quarks are the
fermion In particle physics, a fermion is a subatomic particle that follows Fermi–Dirac statistics. Fermions have a half-integer spin (spin 1/2, spin , Spin (physics)#Higher spins, spin , etc.) and obey the Pauli exclusion principle. These particles i ...
ic components of
hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...
s (
meson In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...
s and baryons) while the
gluon A gluon ( ) is a type of Massless particle, massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a Spin (physi ...
s are considered the
boson In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0, 1, 2, ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have half odd-intege ...
ic components of such particles. The gluons are the force carriers, or bosons, of the QCD color force, while the quarks by themselves are their fermionic matter counterparts. Quark–gluon plasma is studied to recreate and understand the high energy density conditions prevailing in the Universe when matter formed from elementary degrees of freedom (quarks, gluons) at about 20 μs after the
Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...
. Experimental groups are probing over a 'large' distance the (de)confining
quantum vacuum In quantum field theory, the quantum vacuum state (also called the quantum vacuum or vacuum state) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. However, the quantum vacuum is not a simple ...
structure, which determines prevailing form of matter and laws of nature. The experiments give insight to the origin of matter and mass: the matter and antimatter is created when the quark–gluon plasma 'hadronizes' and the mass of matter originates in the confining vacuum structure.


How the quark–gluon plasma fits into the general scheme of physics

QCD is one part of the modern theory of
particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...
called the
Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...
. Other parts of this theory deal with
electroweak interaction In particle physics, the electroweak interaction or electroweak force is the unified description of two of the fundamental interactions of nature: electromagnetism (electromagnetic interaction) and the weak interaction. Although these two force ...
s and
neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...
s. The theory of electrodynamics has been tested and found correct to a few parts in a billion. The theory of weak interactions has been tested and found correct to a few parts in a thousand. Perturbative forms of QCD have been tested to a few percent. Perturbative models assume relatively small changes from the ground state, i.e. relatively low temperatures and densities, which simplifies calculations at the cost of generality. In contrast, non-perturbative forms of QCD have barely been tested. The study of the QGP, which has both a high temperature and density, is part of this effort to consolidate the grand theory of particle physics. The study of the QGP is also a testing ground for finite temperature field theory, a branch of theoretical physics which seeks to understand particle physics under conditions of high temperature. Such studies are important to understand the early evolution of our universe: the first hundred microseconds or so. It is crucial to the physics goals of a new generation of observations of the universe (
WMAP The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic mic ...
and its successors). It is also of relevance to
Grand Unification Theories A Grand Unified Theory (GUT) is any Mathematical model, model in particle physics that merges the electromagnetism, electromagnetic, weak interaction, weak, and strong interaction, strong fundamental interaction, forces (the three gauge theory, ...
which seek to unify the three fundamental forces of nature (excluding gravity).


Reasons for studying the formation of quark–gluon plasma

The generally accepted model of the formation of the
Universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...
states that it happened as the result of the
Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...
. In this model, in the time interval of 10−10–10−6 s after the Big Bang, matter existed in the form of a quark–gluon plasma. It is possible to reproduce the density and temperature of matter existing of that time in laboratory conditions to study the characteristics of the very early Universe. So far, the only possibility is the collision of two heavy
atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford at the University of Manchester based on the 1909 Geiger–Marsden gold foil experiment. Aft ...
accelerated to energies of more than a hundred GeV. Using the result of a head-on collision in the volume approximately equal to the volume of the atomic nucleus, it is possible to model the density and temperature that existed in the first instants of the life of the Universe.


Relation to normal plasma

A plasma is matter in which charges are screened due to the presence of other mobile charges. For example:
Coulomb's Law Coulomb's inverse-square law, or simply Coulomb's law, is an experimental scientific law, law of physics that calculates the amount of force (physics), force between two electric charge, electrically charged particles at rest. This electric for ...
is suppressed by the screening to yield a distance-dependent charge, Q \rightarrow Q e^, i.e., the charge Q is reduced exponentially with the distance divided by a screening length α. In a QGP, the
color charge Color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics (QCD). Like electric charge, it determines how quarks and gluons interact through the strong force; ho ...
of the quarks and gluons is screened. The QGP has other analogies with a normal plasma. There are also dissimilarities because the color charge is non-abelian, whereas the
electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...
is abelian. Outside a finite volume of QGP the color-electric field is not screened, so that a volume of QGP must still be color-neutral. It will therefore, like a nucleus, have integer electric charge. Because of the extremely high energies involved, quark-antiquark pairs are produced by
pair production Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers ...
and thus QGP is a roughly equal mixture of quarks and antiquarks of various flavors, with only a slight excess of quarks. This property is not a general feature of conventional plasmas, which may be too cool for pair production (see however
pair instability supernova A pair-instability supernova is a type of supernova predicted to occur when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal radiatio ...
).


Theory

One consequence of this difference is that the color charge is too large for
perturbative In quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one. The idea is to start with a simple system for which ...
computations which are the mainstay of QED. As a result, the main theoretical tools to explore the theory of the QGP is lattice gauge theory. The transition temperature (approximately ) was first predicted by lattice gauge theory. Since then lattice gauge theory has been used to predict many other properties of this kind of matter. The
AdS/CFT correspondence In theoretical physics, the anti-de Sitter/conformal field theory correspondence (frequently abbreviated as AdS/CFT) is a conjectured relationship between two kinds of physical theories. On one side are anti-de Sitter spaces (AdS) that are used ...
conjecture may provide insights in QGP, moreover the ultimate goal of the fluid/gravity correspondence is to understand QGP. The QGP is believed to be a phase of QCD which is completely locally thermalized and thus suitable for an effective fluid dynamic description.


Production

Production of QGP in the laboratory is achieved by colliding heavy atomic nuclei (called heavy ions as in an accelerator atoms are ionized) at relativistic energy in which matter is heated well above the Hagedorn temperature ''T''H = 150 MeV per particle, which amounts to a temperature exceeding 1.66 trillion K. This can be accomplished by colliding two large nuclei at high energy (note that is not the energy of the colliding beam).
Lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
and
gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
nuclei have been used for such collisions at
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...
SPS and BNL RHIC, respectively. The nuclei are accelerated to ultrarelativistic speeds ( contracting their length) and directed towards each other, creating a "fireball", in the rare event of a collision. Hydrodynamic simulation predicts this fireball will expand under its own
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
, and cool while expanding. By carefully studying the spherical and elliptic flow, experimentalists put the theory to test.


Diagnostic tools

There is overwhelming evidence for production of quark–gluon plasma in relativistic heavy ion collisions. The important classes of experimental observations are * Strangeness production * Elliptic flow * Jet quenching * J/ψ melting *
Hanbury Brown and Twiss effect In physics, the Hanbury Brown and Twiss (HBT) effect is any of a variety of correlation and anti-correlation effects in the intensity (physics), intensities received by two detectors from a beam of particles. HBT effects can generally be attribut ...
and
Bose–Einstein correlations In astronomy, optics and particle physics, the Bose–Einstein correlations refer to correlations between identical bosons (like the photon, the quanta of light). Description The interference between two (or more) waves establishes a corre ...
* Single particle spectra (thermal
photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that ...
and thermal dileptons)


Expected properties


Thermodynamics

The cross-over temperature from the normal hadronic to the QGP phase is about . This "crossover" may actually ''not'' be only a qualitative feature, but instead one may have to do with a true (second order)
phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...
, e.g. of the universality class of the three-dimensional Ising model. The phenomena involved correspond to an energy density of a little less than . For relativistic matter, pressure and temperature are not independent variables, so the
equation of state In physics and chemistry, an equation of state is a thermodynamic equation relating state variables, which describe the state of matter under a given set of physical conditions, such as pressure, volume, temperature, or internal energy. Most mo ...
is a relation between the energy density and the pressure. This has been found through lattice computations, and compared to both
perturbation theory In mathematics and applied mathematics, perturbation theory comprises methods for finding an approximate solution to a problem, by starting from the exact solution of a related, simpler problem. A critical feature of the technique is a middle ...
and
string theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and intera ...
. This is still a matter of active research. Response functions such as the
specific heat In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...
and various quark number susceptibilities are currently being computed.


Flow

The discovery of the perfect liquid was a turning point in physics. Experiments at RHIC have revealed a wealth of information about this remarkable substance, which we now know to be a QGP. Nuclear matter at "room temperature" is known to behave like a
superfluid Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortex, vortices that continue to rotate indefinitely. Superfluidity occurs ...
. When heated the nuclear fluid evaporates and turns into a dilute gas of nucleons and, upon further heating, a gas of baryons and mesons (hadrons). At the critical temperature, ''T''H, the hadrons melt and the gas turns back into a liquid. RHIC experiments have shown that this is the most perfect liquid ever observed in any laboratory experiment at any scale. The new phase of matter, consisting of dissolved hadrons, exhibits less resistance to flow than any other known substance. The experiments at RHIC have, already in 2005, shown that the Universe at its beginning was uniformly filled with this type of material—a super-liquid—which once the Universe cooled below ''T''H evaporated into a gas of hadrons. Detailed measurements show that this liquid is a quark–gluon plasma where quarks, antiquarks and gluons flow independently. In short, a quark–gluon plasma flows like a splat of liquid, and because it is not "transparent" with respect to quarks, it can attenuate jets emitted by collisions. Furthermore, once formed, a ball of quark–gluon plasma, like any hot object, transfers heat internally by radiation. However, unlike in everyday objects, there is enough energy available so that gluons (particles mediating the
strong force In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interactions. It confines quarks into protons, neutrons, and other hadron particles, an ...
) collide and produce an excess of the heavy (i.e., high-energy) strange quarks. Whereas, if the QGP did not exist and there was a pure collision, the same energy would be converted into a non-equilibrium mixture containing even heavier quarks such as
charm quark The charm quark, charmed quark, or c quark is an elementary particle found in composite subatomic particles called hadrons such as the J/psi meson and the charmed baryons created in particle accelerator collisions. Several bosons, including th ...
s or
bottom quark The bottom quark, beauty quark, or b quark, is an elementary particle of the third generation. It is a heavy quark with a charge of −  ''e''. All quarks are described in a similar way by electroweak interaction and quantum chromodynamic ...
s. The equation of state is an important input into the flow equations. The
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. More simply, the speed of sound is how fast vibrations travel. At , the speed of sound in a ...
(speed of QGP-density oscillations) is currently under investigation in lattice computations. The
mean free path In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
of quarks and gluons has been computed using
perturbation theory In mathematics and applied mathematics, perturbation theory comprises methods for finding an approximate solution to a problem, by starting from the exact solution of a related, simpler problem. A critical feature of the technique is a middle ...
as well as
string theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and intera ...
. Lattice computations have been slower here, although the first computations of transport coefficients have been concluded. These indicate that the mean free time of quarks and gluons in the QGP may be comparable to the average interparticle spacing: hence the QGP is a liquid as far as its flow properties go. This is very much an active field of research, and these conclusions may evolve rapidly. The incorporation of dissipative phenomena into hydrodynamics is another active research area.


Jet quenching effect

Detailed predictions were made in the late 1970s for the production of jets at the CERN Super Proton–Antiproton Synchrotron. UA2 observed the first evidence for jet production in hadron collisions in 1981, which shortly after was confirmed by UA1. The subject was later revived at RHIC. One of the most striking physical effects obtained at RHIC energies is the effect of quenching jets. At the first stage of interaction of colliding relativistic nuclei, partons of the colliding nuclei give rise to the secondary partons with a large transverse impulse ≥ 3–6 GeV/s. Passing through a highly heated compressed plasma, partons lose energy. The magnitude of the energy loss by the parton depends on the properties of the quark–gluon plasma (temperature, density). In addition, it is also necessary to take into account the fact that colored quarks and gluons are the elementary objects of the plasma, which differs from the energy loss by a parton in a medium consisting of colorless hadrons. Under the conditions of a quark–gluon plasma, the energy losses resulting from the RHIC energies by partons are estimated as . This conclusion is confirmed by comparing the relative yield of hadrons with a large transverse impulse in nucleon-nucleon and nucleus-nucleus collisions at the same collision energy. The energy loss by partons with a large transverse impulse in nucleon-nucleon collisions is much smaller than in nucleus-nucleus collisions, which leads to a decrease in the yield of high-energy hadrons in nucleus-nucleus collisions. This result suggests that nuclear collisions cannot be regarded as a simple superposition of nucleon-nucleon collisions. For a short time, ~1 μs, and in the final volume, quarks and gluons form some ideal liquid. The collective properties of this fluid are manifested during its movement as a whole. Therefore, when moving partons in this medium, it is necessary to take into account some collective properties of this quark–gluon liquid. Energy losses depend on the properties of the quark–gluon medium, on the parton density in the resulting fireball, and on the dynamics of its expansion. Losses of energy by light and heavy quarks during the passage of a fireball turn out to be approximately the same. In November 2010, CERN announced the first direct observation of jet quenching, based on experiments with heavy-ion collisions.


Direct photons and dileptons

Direct photons and dileptons are arguably most penetrating tools to study relativistic heavy ion collisions. They are produced, by various mechanisms spanning the space-time evolution of the strongly interacting fireball. They provide in principle a snapshot on the initial stage as well. They are hard to decipher and interpret as most of the signal is originating from hadron decays long after the QGP fireball has disintegrated.


Glasma hypothesis

Since 2008, there is a discussion about a hypothetical precursor state of the quark–gluon plasma, the so-called "Glasma", where the
dressed particle In theoretical physics, the term dressed particle refers to a bare particle together with some excitations of other quantum fields that are physically inseparable from the bare particle. For example, a dressed electron includes the cloud of virt ...
s are condensed into some kind of glassy (or amorphous) state, below the genuine transition between the confined state and the plasma liquid. This would be analogous to the formation of metallic glasses, or amorphous alloys of them, below the genuine onset of the liquid metallic state. Although the experimental high temperatures and densities predicted as producing a quark–gluon plasma have been realized in the laboratory, the resulting matter does ''not'' behave as a quasi-ideal state of free quarks and gluons, but, rather, as an almost perfect dense fluid. Actually, the fact that the quark–gluon plasma will not yet be "free" at temperatures realized at present accelerators was predicted in 1984, as a consequence of the remnant effects of confinement.


Neutron stars

It has been hypothesized that the core of some massive
neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
s may be a quark–gluon plasma.


In-laboratory formation of deconfined matter

A quark–gluon plasma (QGP) or quark soup is a
state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. Different states are distinguished by the ways the ...
in quantum chromodynamics (QCD) which exists at extremely high
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
and/or
density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
. This state is thought to consist of asymptotically free strong-interacting quarks and gluons, which are ordinarily confined by
color confinement In quantum chromodynamics (QCD), color confinement, often simply called confinement, is the phenomenon that color-charged particles (such as quarks and gluons) cannot be isolated, and therefore cannot be directly observed in normal conditions b ...
inside atomic nuclei or other hadrons. This is in analogy with the conventional plasma where nuclei and electrons, confined inside
atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s by electrostatic forces at ambient conditions, can move freely. Experiments to create artificial quark matter started at CERN in 1986/87, resulting in first claims that were published in 1991. It took several years before the idea became accepted in the community of particle and nuclear physicists. Formation of a new state of matter in Pb–Pb collisions was officially announced at CERN in view of the convincing experimental results presented by the CERN SPS WA97 experiment in 1999, and later elaborated by Brookhaven National Laboratory's
Relativistic Heavy Ion Collider The Relativistic Heavy Ion Collider (RHIC ) is the first and one of only two operating heavy- ion colliders, and the only spin-polarized proton collider ever built. Located at Brookhaven National Laboratory (BNL) in Upton, New York, and used ...
. Quark matter can only be produced in minute quantities and is unstable and impossible to contain, and will radioactively decay within a fraction of a second into stable particles through
hadronization Hadronization (or hadronisation) is the process of the formation of hadrons out of quarks and gluons. There are two main branches of hadronization: quark-gluon plasma (QGP) transformation and colour string decay into hadrons. The transformation o ...
; the produced hadrons or their decay products and
gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
s can then be detected. In the
quark matter Quark matter or QCD matter ( quantum chromodynamic) refers to any of a number of hypothetical phases of matter whose degrees of freedom include quarks and gluons, of which the prominent example is quark-gluon plasma. Several series of conferences ...
phase diagram, QGP is placed in the high-temperature, high-density regime, whereas ordinary matter is a cold and rarefied mixture of nuclei and vacuum, and the hypothetical quark stars would consist of relatively cold, but dense quark matter. It is believed that up to a few microseconds (10−12 to 10−6 seconds) after the Big Bang, known as the
quark epoch In physical cosmology, the quark epoch was the period in the evolution of the early universe when the fundamental interactions of gravitation, electromagnetism, the strong interaction and the weak interaction had taken their present forms, but ...
, the Universe was in a quark–gluon plasma state. The strength of the
color force Color Force is an American independent film and television production company founded in 2007 by producer and film executive Nina Jacobson after her 2006 termination as president of Disney's Walt Disney Studios (division), Buena Vista Motion P ...
means that unlike the gas-like plasma, quark–gluon plasma behaves as a near-ideal
Fermi liquid Fermi liquid theory (also known as Landau's Fermi-liquid theory) is a theoretical model of interacting fermions that describes the normal state of the conduction electrons in most metals at sufficiently low temperatures. The theory describes the ...
, although research on flow characteristics is ongoing. Liquid or even near-perfect liquid flow with almost no frictional resistance or viscosity was claimed by research teams at RHIC and LHC's
Compact Muon Solenoid The Compact Muon Solenoid (CMS) experiment is one of two large general-purpose particle physics detectors built on the Large Hadron Collider (LHC) at CERN in Switzerland and France. The goal of the CMS experiment is to investigate a wide range ...
detector. QGP differs from a "free" collision event by several features; for example, its particle content is indicative of a temporary
chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...
producing an excess of middle-energy strange quarks vs. a nonequilibrium distribution mixing light and heavy quarks ("strangeness production"), and it does not allow particle jets to pass through ("jet quenching"). Experiments at CERN's Super Proton Synchrotron (SPS) begun experiments to create QGP in the 1980s and 1990s: the results led CERN to announce evidence for a "new state of matter" in 2000. Scientists at Brookhaven National Laboratory's Relativistic Heavy Ion Collider announced they had created quark–gluon plasma by colliding gold ions at nearly the speed of light, reaching temperatures of 4 trillion degrees Celsius. Current experiments (2017) at the
Brookhaven National Laboratory Brookhaven National Laboratory (BNL) is a United States Department of Energy national laboratories, United States Department of Energy national laboratory located in Upton, New York, a hamlet of the Brookhaven, New York, Town of Brookhaven. It w ...
's Relativistic Heavy Ion Collider (RHIC) on Long Island (New York, USA) and at CERN's recent
Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It was built by the CERN, European Organization for Nuclear Research (CERN) between 1998 and 2008, in collaboration with over 10,000 scientists, ...
near Geneva (Switzerland) are continuing this effort,Perfect' Liquid Hot Enough to be Quark Soup"
.
by colliding relativistically accelerated gold and other ion species (at RHIC) or lead (at LHC) with each other or with protons. Three experiments running on CERN's Large Hadron Collider (LHC), on the spectrometers
ALICE Alice may refer to: * Alice (name), most often a feminine given name, but also used as a surname Literature * Alice (''Alice's Adventures in Wonderland''), a character in books by Lewis Carroll * ''Alice'' series, children's and teen books by ...
,
ATLAS An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...
and CMS, have continued studying the properties of QGP. CERN temporarily ceased colliding
protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' ( elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an electron (the pro ...
, and began colliding
lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
ions for the ALICE experiment in 2011, in order to create a QGP. A new record breaking temperature was set by ALICE: A Large Ion Collider Experiment at CERN in August 2012 of about 5.5 trillion () kelvin as claimed in their Nature PR. The formation of a quark–gluon plasma occurs as a result of a
strong interaction In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interaction, fundamental interactions. It confines Quark, quarks into proton, protons, n ...
between the partons (quarks, gluons) that make up the nucleons of the colliding heavy nuclei called heavy ions. Therefore, experiments are referred to as relativistic heavy ion collision experiments. Theoretical and experimental works show that the formation of a quark–gluon plasma occurs at the temperature of T ≈ 150–160 MeV, the Hagedorn temperature, and an energy density of ≈ 0.4–1 GeV / fm3. While at first a phase transition was expected, present day theoretical interpretations propose a phase transformation similar to the process of ionisation of normal matter into ionic and electron plasma.


Quark–gluon plasma and the onset of deconfinement

The central issue of the formation of a quark–gluon plasma is the research for the onset of deconfinement. From the beginning of the research on formation of QGP, the issue was whether
energy density In physics, energy density is the quotient between the amount of energy stored in a given system or contained in a given region of space and the volume of the system or region considered. Often only the ''useful'' or extractable energy is measure ...
can be achieved in nucleus-nucleus collisions. This depends on how much energy each nucleon loses. An influential reaction picture was the scaling solution presented by Bjorken. This model applies to ultra-high energy collisions. In experiments carried out at CERN SPS and BNL RHIC more complex situation arose, usually divided into three stages: * Primary parton collisions and baryon stopping at the time of complete overlapping of the colliding nuclei. * Redistribution of particle energy and new particles born in the QGP fireball. * The fireball of QGP matter equilibrates and expands before hadronizing. More and more experimental evidence points to the strength of QGP formation mechanisms—operating even in LHC-energy scale proton-proton collisions.


Further reading


Books

* * * * * * * * *


Review articles with a historical perspective of the field

* * * * * * * *


See also

*
Color confinement In quantum chromodynamics (QCD), color confinement, often simply called confinement, is the phenomenon that color-charged particles (such as quarks and gluons) cannot be isolated, and therefore cannot be directly observed in normal conditions b ...
* Color-glass condensate *
Hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...
s (that is
meson In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...
s and
baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...
s) *
Hadronization Hadronization (or hadronisation) is the process of the formation of hadrons out of quarks and gluons. There are two main branches of hadronization: quark-gluon plasma (QGP) transformation and colour string decay into hadrons. The transformation o ...
*
Hagedorn temperature The Hagedorn temperature, ''T''H, is the temperature in theoretical physics where hadronic matter (i.e. ordinary matter) is no longer stable, and must either "evaporate" or convert into quark matter; as such, it can be thought of as the "boiling p ...
*
Neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
*
Plasma physics Plasma () is a state of matter characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form of ordinary matter in the universe, mostly in stars (including th ...
*
QCD matter Quark matter or QCD matter ( quantum chromodynamic) refers to any of a number of hypothetical phases of matter whose degrees of freedom include quarks and gluons, of which the prominent example is quark-gluon plasma. Several series of conferences ...
*
Quantum electrodynamics In particle physics, quantum electrodynamics (QED) is the Theory of relativity, relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quant ...
*
Quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...
*
Quantum hydrodynamics In condensed matter physics, quantum hydrodynamics (QHD) is most generally the study of hydrodynamic-like systems which demonstrate quantum mechanical behavior. They arise in semiclassical mechanics in the study of metal and semiconductor device ...
* Relativistic plasma * Relativistic nuclear collision * Strangeness production *
Strange matter Strange matter (or strange quark matter) is quark matter containing strange quarks. In extreme environments, strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in siz ...
*
List of unsolved problems in physics The following is a list of notable unsolved problems grouped into broad areas of physics. Some of the major unsolved problems in physics are theoretical, meaning that existing theories seem incapable of explaining a certain observed phenomenon ...


References


External links

*
The Relativistic Heavy Ion Collider
a
Brookhaven National Laboratory

The Alice Experiment
a
CERN

The Indian Lattice Gauge Theory Initiative
* Quark matter reviews
2004 theory2004 experiment
* Quark–Gluon Plasma reviews
2011 theory
* Lattice reviews
20032005

BBC article mentioning Brookhaven results (2005)


* "Hadrons and Quark–Gluon Plasma" by Jean Letessier and Johann Rafelski Cambridge University Press (2002) , Cambridge, UK; {{DEFAULTSORT:Quark-gluon plasma Quark matter Phases of matter Exotic matter Gluons