In particle physics, the parton model is a model of

hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the e ...

s, such as protons and neutrons, proposed by

Richard Feynman Richard Phillips Feynman (; May 11, 1918 – February 15, 1988) was an American theoretical physicist, known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of the superfl ...

. It is useful for interpreting the cascades of radiation (a parton shower) produced from

quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the theory 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 o ...

(QCD) processes and interactions in high-energy particle collisions.

Model

Parton showers are simulated extensively in Monte Carlo event generators, in order to calibrate and interpret (and thus understand) processes in collider experiments. As such, the name is also used to refer to algorithms that approximate or simulate the process.

Motivation

The parton model was proposed by

in 1969 as a way to analyze high-energy hadron collisions. Any hadron (for example, a proton) can be considered as a composition of a number of point-like constituents, termed "partons". The parton model was immediately applied to electron- proton

deep inelastic scattering Deep inelastic scattering is the name given to a process used to probe the insides of hadrons (particularly the baryons, such as protons and neutrons), using electrons, muons and neutrinos. It provided the first convincing evidence of the realit ...

by Bjorken and Paschos.

Component particles

A hadron is composed of a number of point-like constituents, termed "partons". Later, with the experimental observation of

Bjorken scaling James Daniel "BJ" Bjorken (born 1934) is an American theoretical physicist. He was a Putnam Fellow in 1954, received a BS in physics from MIT in 1956, and obtained his PhD from Stanford University in 1959. He was a visiting scholar at the Inst ...

, the validation of the

quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Ei ...

, and the confirmation of

asymptotic freedom 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. Asymptotic fre ...

, partons were matched to

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 nuclei. All commonly o ...

s and

gluon A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. Gluons bind q ...

s. The parton model remains a justifiable approximation at high energies, and others have extended the theory over the years. 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.

Reference frame

The

is defined in a

reference frame In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system whose origin, orientation, and scale are specified by a set of reference points― geometric points whose position is identified both math ...

where it has infinite momentum—a valid approximation at high energies. Thus, parton motion is slowed by

time dilation In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them ( special relativistic "kinetic" time dilation) or to a difference in gravitational ...

, and the hadron charge distribution is Lorentz-contracted, so incoming particles will be scattered "instantaneously and incoherently". Partons are defined with respect to a physical scale (as probed by the inverse of the momentum transfer). 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. 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. ::And ''erratum'' in

History

The parton model was proposed by

in 1969, used originally for analysis of high-energy collisions. It was applied to electron/ proton

by Bjorken and Paschos. Later, with the experimental observation of

, the validation of the

, and the confirmation of

in quantum chromodynamics, partons were matched to quarks and gluons. The parton model remains a justifiable approximation at high energies, and others have extended the theory over the years. It was recognized that partons describe the same objects now more commonly referred to as quarks and gluons. A more detailed presentation of the properties and physical theories pertaining indirectly to partons can be found under

s. In 1994, partons were used by

Leonard Susskind Leonard Susskind (; born June 16, 1940)his 60th birthday was celebrated with a special symposium at Stanford University.in Geoffrey West's introduction, he gives Suskind's current age as 74 and says his birthday was recent. is an American physicis ...

to model

holography Holography is a technique that enables a wavefront to be recorded and later re-constructed. Holography is best known as a method of generating real three-dimensional images, but it also has a wide range of other Holography#Applications, applic ...

Parton distribution functions

A parton distribution function (PDF) within so called ''collinear factorization'' is defined as the

probability density In probability theory, a probability density function (PDF), or density of a continuous random variable, is a function whose value at any given sample (or point) in the sample space (the set of possible values taken by the random variable) can ...

for finding a particle with a certain longitudinal momentum fraction ''x'' at resolution scale ''Q''². Because of the inherent

non-perturbative In mathematics and physics, a non-perturbative function or process is one that cannot be described by perturbation theory. An example is the function : f(x) = e^, which does not have a Taylor series at ''x'' = 0. Every coefficient of the Tay ...

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 A virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle. The concept of virtual particles arises in the perturb ...

with virtuality ''Q''² 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; 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. Experimentally determined parton distribution functions are available from various groups worldwide. The major unpolarized data sets are:
''ABM''
by S. Alekhin, J. Bluemlein, S. Moch *
CTEQ
', from the CTEQ Collaboration

from M. Glück, P. Jimenez-Delgado, E. Reya, and A. Vogt
''HERA''
PDFs, by H1 and ZEUS collaborations from the Deutsches Elektronen-Synchrotron center (DESY) in Germany
MSHT/MRST/MSTW/MMHT
from A. D. Martin, R. G. Roberts, W. J. Stirling, R. S. Thorne, and collaborators
''NNPDF''
from the NNPDF Collaboration Th
''LHAPDF''
library provides a unified and easy-to-use Fortran/

C++ C, or c, is the third letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''cee'' (pronounced ), plural ''cees''. History "C" ...

interface to all major PDF sets. ''Generalized parton distributions'' (GPDs) are a more recent approach to better understand

structure by representing the parton distributions as functions of more variables, such as the transverse momentum and

spin Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

of the parton. 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. 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. 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.

Simulation

Parton showers simulations are of use in computational particle physics either in automatic calculation of particle interaction or decay or event generators, and are particularly important in

large hadron collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundre ...

(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 Pythia (; grc, Πυθία ) was the name of the high priestess of the Temple of Apollo at Delphi. She specifically served as its oracle and was known as the Oracle of Delphi. Her title was also historically glossed in English as the Pythoness ...

and HERWIG.M Morett
Understunding events at the LHC: Parton Showers and Matrix Element tools for physics simulation at the hadronic colliders
p. 19. 28/11/2006.

References

This article contains material from Scholarpedia.

External links

* * Event Generator Physics (http://www.hep.phy.cam.ac.uk/theory/webber/MCnet/MClecture2.pdf) * * http://www.kceta.kit.edu/grk1694/img/2013_10_01_Hangst.pdf * http://d-nb.info/1008230227/34 * {{DEFAULTSORT:Parton (Particle Physics) Quantum chromodynamics Richard Feynman