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In particle physics, the parton model is a model of hadrons, such as protons and neutrons, proposed by Richard Feynman. It is useful for interpreting the cascades of radiation (a parton shower) produced from quantum chromodynamics (QCD) processes and interactions in high-energy particle collisions.
The parton model was proposed by Richard Feynman in 1969, used originally for analysis of high-energy hadron collisions. [1] It was applied to electron-proton deep inelastic scattering by James Bjorken and Emmanuel Anthony Paschos. [2] Later, with the experimental observation of Bjorken scaling, the validation of the quark model, and the confirmation of asymptotic freedom in quantum chromodynamics, partons were matched to quarks and gluons. The parton model remains a justifiable approximation at high energies, and others[ who? ] have extended the theory[ how? ] over the years.
Murray Gell-Mann preferred to use the term "put-ons" to refer to partons. [3]
In 1994, partons were used by Leonard Susskind to model holography. [4]
Any hadron (for example, a proton) can be considered as a composition of a number of point-like constituents, termed "partons".
Just as accelerated electric charges emit QED radiation (photons), the accelerated coloured partons will emit QCD radiation in the form of gluons. Unlike the uncharged photons, the gluons themselves carry colour charges and can therefore emit further radiation, leading to parton showers. [5] [6] [7]
The hadron is defined in a reference frame where it has infinite momentum – a valid approximation at high energies. Thus, parton motion is slowed by time dilation, and the hadron charge distribution is Lorentz-contracted, so incoming particles will be scattered "instantaneously and incoherently".[ citation needed ]
Partons are defined with respect to a physical scale (as probed by the inverse of the momentum transfer).[ clarification needed ] For instance, a quark parton at one length scale can turn out to be a superposition of a quark parton state with a quark parton and a gluon parton state together with other states with more partons at a smaller length scale. Similarly, a gluon parton at one scale can resolve into a superposition of a gluon parton state, a gluon parton and quark-antiquark partons state and other multiparton states. Because of this, the number of partons in a hadron actually goes up with momentum transfer. [8] At low energies (i.e. large length scales), a baryon contains three valence partons (quarks) and a meson contains two valence partons (a quark and an antiquark parton). At higher energies, however, observations show sea partons (nonvalence partons) in addition to valence partons. [9]
A parton distribution function (PDF) within so called collinear factorization is defined as the probability density for finding a particle with a certain longitudinal momentum fraction x at resolution scale Q2. Because of the inherent non-perturbative nature of partons which cannot be observed as free particles, parton densities cannot be calculated using perturbative QCD. Within QCD one can, however, study variation of parton density with resolution scale provided by external probe. Such a scale is for instance provided by a virtual photon with virtuality Q2 or by a jet. The scale can be calculated from the energy and the momentum of the virtual photon or jet; the larger the momentum and energy, the smaller the resolution scale—this is a consequence of Heisenberg's uncertainty principle. The variation of parton density with resolution scale has been found to agree well with experiment; [10] this is an important test of QCD.
Parton distribution functions are obtained by fitting observables to experimental data; they cannot be calculated using perturbative QCD. Recently, it has been found that they can be calculated directly in lattice QCD using large-momentum effective field theory. [11] [12]
Experimentally determined parton distribution functions are available from various groups worldwide. The major unpolarized data sets are:
The LHAPDF [13] library provides a unified and easy-to-use Fortran/C++ interface to all major PDF sets.
Generalized parton distributions (GPDs) are a more recent approach to better understand hadron structure by representing the parton distributions as functions of more variables, such as the transverse momentum and spin of the parton. [14] They can be used to study the spin structure of the proton, in particular, the Ji sum rule relates the integral of GPDs to angular momentum carried by quarks and gluons. [15] Early names included "non-forward", "non-diagonal" or "skewed" parton distributions. They are accessed through a new class of exclusive processes for which all particles are detected in the final state, such as the deeply virtual Compton scattering. [16] Ordinary parton distribution functions are recovered by setting to zero (forward limit) the extra variables in the generalized parton distributions. Other rules show that the electric form factor, the magnetic form factor, or even the form factors associated to the energy-momentum tensor are also included in the GPDs. A full 3-dimensional image of partons inside hadrons can also be obtained from GPDs. [17]
Parton showers simulations are of use in computational particle physics either in automatic calculation of particle interaction or decay or event generators, in order to calibrate and interpret (and thus understand) processes in collider experiments. [18] They are particularly important in large hadron collider (LHC) phenomenology, where they are usually explored using Monte Carlo simulation.
The scale at which partons are given to hadronization is fixed by the Shower Monte Carlo program. Common choices of Shower Monte Carlo are PYTHIA and HERWIG. [19] [20]
A gluon is a type of 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 of 1. Through the strong interaction, gluons bind quarks into groups according to quantum chromodynamics (QCD), forming hadrons such as protons and neutrons.
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 field theory called a non-abelian gauge theory, with symmetry group SU(3). The QCD analog of electric charge is a property called color. Gluons are the force carriers of the theory, just as photons are for the electromagnetic force in quantum electrodynamics. The theory is an important part of the Standard Model of particle physics. A large body of experimental evidence for QCD has been gathered over the years.
The up quark or u quark is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons and protons of atomic nuclei. It is part of the first generation of matter, has an electric charge of +2/3 e and a bare mass of 2.2+0.5
−0.4 MeV/c2. Like all quarks, the up quark is an elementary fermion with spin 1/2, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. The antiparticle of the up quark is the up antiquark, which differs from it only in that some of its properties, such as charge have equal magnitude but opposite sign.
The down quark is a type of elementary particle, and a major constituent of matter. The down quark is the second-lightest of all quarks, and combines with other quarks to form composite particles called hadrons. Down quarks are most commonly found in atomic nuclei, where it combines with up quarks to form protons and neutrons. The proton is made of one down quark with two up quarks, and the neutron is made up of two down quarks with one up quark. Because they are found in every single known atom, down quarks are present in all everyday matter that we interact with.
In quantum field theory, asymptotic freedom is a property of some gauge theories that causes interactions between particles to become asymptotically weaker as the energy scale increases and the corresponding length scale decreases.
In physics, an effective field theory is a type of approximation, or effective theory, for an underlying physical theory, such as a quantum field theory or a statistical mechanics model. An effective field theory includes the appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale or energy scale, while ignoring substructure and degrees of freedom at shorter distances. Intuitively, one averages over the behavior of the underlying theory at shorter length scales to derive what is hoped to be a simplified model at longer length scales. Effective field theories typically work best when there is a large separation between length scale of interest and the length scale of the underlying dynamics. Effective field theories have found use in particle physics, statistical mechanics, condensed matter physics, general relativity, and hydrodynamics. They simplify calculations, and allow treatment of dissipation and radiation effects.
In particle physics, a glueball is a hypothetical composite particle. It consists solely of gluon particles, without valence quarks. Such a state is possible because gluons carry color charge and experience the strong interaction between themselves. Glueballs are extremely difficult to identify in particle accelerators, because they mix with ordinary meson states. In pure gauge theory, glueballs are the only states of the spectrum and some of them are stable.
Lattice QCD is a well-established non-perturbative approach to solving the quantum chromodynamics (QCD) theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time. When the size of the lattice is taken infinitely large and its sites infinitesimally close to each other, the continuum QCD is recovered.
Hadronization 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 of quark-gluon plasma into hadrons is studied in lattice QCD numerical simulations, which are explored in relativistic heavy-ion experiments. Quark-gluon plasma hadronization occurred shortly after the Big Bang when the quark–gluon plasma cooled down to the Hagedorn temperature when free quarks and gluons cannot exist. In string breaking new hadrons are forming out of quarks, antiquarks and sometimes gluons, spontaneously created from the vacuum.
Exotic hadrons are subatomic particles composed of quarks and gluons, but which – unlike "well-known" hadrons such as protons, neutrons and mesons – consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.
The Drell–Yan process occurs in high energy hadron–hadron scattering. It takes place when a quark of one hadron and an antiquark of another hadron annihilate, creating a virtual photon or Z boson which then decays into a pair of oppositely-charged leptons. Importantly, the energy of the colliding quark-antiquark pair can be almost entirely transformed into the mass of new particles. This process was first suggested by Sidney Drell and Tung-Mow Yan in 1970 to describe the production of lepton–antilepton pairs in high-energy hadron collisions. Experimentally, this process was first observed by J. H. Christenson et al. in proton–uranium collisions at the Alternating Gradient Synchrotron.
In particle physics phenomenology, chiral color is a speculative model which extends quantum chromodynamics (QCD), the generally accepted theory for the strong interactions of quarks. QCD is a gauge field theory based on a gauge group known as color SU(3)C with an octet of colored gluons acting as the force carriers between a triplet of colored quarks.
The light-front quantization of quantum field theories provides a useful alternative to ordinary equal-time quantization. In particular, it can lead to a relativistic description of bound systems in terms of quantum-mechanical wave functions. The quantization is based on the choice of light-front coordinates, where plays the role of time and the corresponding spatial coordinate is . Here, is the ordinary time, is one Cartesian coordinate, and is the speed of light. The other two Cartesian coordinates, and , are untouched and often called transverse or perpendicular, denoted by symbols of the type . The choice of the frame of reference where the time and -axis are defined can be left unspecified in an exactly soluble relativistic theory, but in practical calculations some choices may be more suitable than others.
Quark–gluon plasma is an interacting localized assembly of quarks and gluons at thermal and chemical (abundance) equilibrium. The word plasma signals that free color charges 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—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 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 colour charge open for a new state of matter to be referred to as QGP.
The NA49 experiment was a particle physics experiment that investigated the properties of quark–gluon plasma. The experiment's synonym was Ions/TPC-Hadrons. It took place in the North Area of the Super Proton Synchrotron (SPS) at CERN from 1991-2002.
In strong interaction physics, light front holography or light front holographic QCD is an approximate version of the theory of quantum chromodynamics (QCD) which results from mapping the gauge theory of QCD to a higher-dimensional anti-de Sitter space (AdS) inspired by the AdS/CFT correspondence proposed for string theory. This procedure makes it possible to find analytic solutions in situations where strong coupling occurs, improving predictions of the masses of hadrons and their internal structure revealed by high-energy accelerator experiments. The most widely used approach to finding approximate solutions to the QCD equations, lattice QCD, has had many successful applications; however, it is a numerical approach formulated in Euclidean space rather than physical Minkowski space-time.
The proton spin crisis is a theoretical crisis precipitated by a 1987 experiment by the European Muon Collaboration (EMC), which tried to determine the distribution of spin within the proton.
The light-front quantization of quantum field theories provides a useful alternative to ordinary equal-time quantization. In particular, it can lead to a relativistic description of bound systems in terms of quantum-mechanical wave functions. The quantization is based on the choice of light-front coordinates, where plays the role of time and the corresponding spatial coordinate is . Here, is the ordinary time, Failed to parse : {\displaystyle z} is a Cartesian coordinate, and is the speed of light. The other two Cartesian coordinates, and , are untouched and often called transverse or perpendicular, denoted by symbols of the type . The choice of the frame of reference where the time and -axis are defined can be left unspecified in an exactly soluble relativistic theory, but in practical calculations some choices may be more suitable than others. The basic formalism is discussed elsewhere.
Xiangdong Ji is a Chinese theoretical nuclear and elementary particle physicist. He is a Distinguished University Professor at the University of Maryland, College Park.
Color transparency is a phenomenon observed in high-energy particle physics, where hadrons created in a nucleus propagate through that nucleus with less interaction than expected. It suggests that hadrons are first created with a small size in the nucleus, and then grow to their nominal size. Here, color refers to the color charge, the property of quarks and gluons that determines how strongly they interact through the nuclear strong force.
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