Color charge

	Email		Print
	Bookmark		Link

Color charge

In particle physics

Particle physics

Particle physics is a branch of physics that studies the elementary particle constituents of matter and radiation, and the interactions between them....
, color charge is a property of quark

Quark

Quarks are a type of elementary particle and major constituents of matter. They are the only particles in the Standard Model to experience all four fundamental interaction, which are also known as fundamental interactions....
s and gluon

Gluon

Gluons are elementary particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei....
s which are related to their strong interaction

Strong interaction

In particle physics, the strong interaction, or strong force, or color force, holds quarks and gluons together to form protons, neutrons and other particles....
s in the context of quantum chromodynamics

Quantum chromodynamics

Quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons ....
(QCD). This has analogies with the notion of electric charge

Electric charge

Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields....
of particles, but because of the mathematical complications of QCD, there are many technical differences. The "butt" of quarks and gluons is completely unrelated to visual perception of color; rather, it is a whimsical name for a property which has almost no manifestation at distances above the size of an atomic nucleus

Atomic nucleus

Discussion

Ask a question about 'Color charge'

Start a new discussion about 'Color charge'

Answer questions from other users

Full Discussion Forum

Encyclopedia

In particle physics

Particle physics

Particle physics is a branch of physics that studies the elementary particle constituents of matter and radiation, and the interactions between them....
, color charge is a property of quark

Quark

Gluon

Strong interaction

Quantum chromodynamics

Electric charge

Atomic nucleus

The nucleus of an atom is the very dense region, consisting of nucleons , at the center of an atom. Although the size of the nucleus varies considerably according to the mass of the atom, the size of the entire atom is comparatively constant....
. The term "color" itself is simply derived from the fact that the property it describes has three aspects (analogous to the three primary colors

Primary Colors

Primary Colors: A Novel of Politics is a 1996 in literature novel by "Anonymity" ....
), as opposed to the single "aspect" of electromagnetic charge.

Shortly after the existence of quarks was first proposed in 1964, Oscar W. Greenberg

Oscar W. Greenberg

Oscar Wallace Greenberg is an American physicist and professor at University of Maryland, College Park. He is famous for positing the existence of a property of subatomic particles called color charge....
introduced the notion of color charge to explain how quarks could coexist inside some hadron

Hadron

In particle physics, a hadron is a bound state of quarks. Hadrons are held together by the strong interaction, similarly to how molecules are held together by the electromagnetic force....
s in otherwise identical states

Quark model

In physics, the quark model is a classification scheme for hadrons in terms of their valence quarks, i.e., the quarks which give rise to the quantum numbers of the hadrons....
and still satisfy the Pauli exclusion principle

Pauli exclusion principle

The Pauli exclusion principle is a quantum mechanics principle formulated by Wolfgang Pauli in 1925. It states that no two identical particles fermions may occupy the same quantum state simultaneously....
. The concept turned out to be useful. Quantum chromodynamics has been under development since the 1970s and constitutes an important ingredient in the standard model

Standard Model

The Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions....
of particle physics.

Red, Green, and Blue

One can say that a quark's color can take one of three values: "red", "green", or "blue"; and that an antiquark can take one of three "anticolors", sometimes called "antired", "antigreen" and "antiblue" (occasionally represented as cyan, magenta and yellow, respectively). In the same vein it can be said that gluons are mixtures of two colors: for example red-antigreen, and this constitutes their color charge. QCD considers eight gluons of the possible nine color/anti-color combinations to be unique; see Gluon
Gluon

Gluons are elementary particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei....
for the reason of this. Also see compelling strikes's for clarification of the following material.

Coupling constant and charge

In a quantum field theory
Quantum field theory

Quantum field theory or QFT provides a theoretical framework for constructing quantum mechanics models of systems classically described by field or of Many-body problem....
the notion of a coupling constant
Coupling constant

In physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. Usually the Lagrangian or the Hamiltonian mechanics of a system can be separated into a kinetic part and an interaction part....
and a charge
Coupling constant

In physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. Usually the Lagrangian or the Hamiltonian mechanics of a system can be separated into a kinetic part and an interaction part....
are different but related. The coupling constant sets the magnitude of the force of interaction; for example, in quantum electrodynamics
Quantum electrodynamics

Quantum electrodynamics is a relativity theory quantum field theory of electrodynamics. QED was developed by a number of physicists, beginning in the late 1920s....
, the fine structure constant is a coupling constant
Coupling constant

In physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. Usually the Lagrangian or the Hamiltonian mechanics of a system can be separated into a kinetic part and an interaction part....
. The charge in a gauge theory
Gauge theory

In physics, gauge theory is a quantum field theory where the Lagrangian is invariant under certain transformations.The transformations form a Lie group which is referred to as the symmetry group or the gauge group of the theory....
has to do with the way a particle transforms under the gauge symmetry; i.e., its representation
Group representation

In the mathematics field of representation theory, group representations describe abstract group in terms of linear transformations of vector spaces; in particular, they can be used to represent group elements as matrix so that the group operation can be represented by matrix multiplication....
under the gauge group. For example, the electron
Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
has charge -1 and the positron
Positron

The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1, a spin of 1/2, and the same mass as an electron....
has charge +1, implying that the gauge transformation has opposite effects on them in some sense. Specifically, if a local gauge transformation f(x) is applied in electrodynamics, then one finds
, and
where is the photon
Photon

In physics, the photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation....
field, and is the electron field with (a bar over denotes its antiparticle — the positron). Since QCD is a non-Abelian
Abelian

Abelian may refer to:* Abelians, a 4th century Christian sect* Hovhannes Abelian, an Armenian actor* A number of different mathematic terms named after Niels Henrik Abel::*Abelian group, a group in which the binary operation is commutative:**Category of abelian groups Ab has abelian groups as objects and group homomorphisms as morph...
theory, the representations, and hence the color charges, are more complicated. They are dealt with in the next section.

Quark and gluon fields and color charges

In QCD the gauge group is the non-Abelian group SU(3). The running coupling

Coupling constant

In physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. Usually the Lagrangian or the Hamiltonian mechanics of a system can be separated into a kinetic part and an interaction part....
is usually denoted by a_s. Each flavor of quark belongs to the fundamental representation
Fundamental representation

In representation theory of Lie groups and Lie algebras, a fundamental representation is an irreducible finite-dimensional representation of a semisimple Lie group...
(3) and contains a triplet of fields together denoted by ?. The antiquark field belongs to the complex conjugate representation
Group representation

In the mathematics field of representation theory, group representations describe abstract group in terms of linear transformations of vector spaces; in particular, they can be used to represent group elements as matrix so that the group operation can be represented by matrix multiplication....
(3^*) and also contains a triplet of fields. We can write
and
The gluon contains an octet of fields, belongs to the adjoint representation
Adjoint representation

In mathematics, the adjoint representation of a Lie group G is the natural group representation of G on its own Lie algebra. This representation is the linearized version of the group action of G on itself by conjugation ....
(8), and can be written using the Gell-Mann matrices
Gell-Mann matrices

The Gell-Mann matrices, named for Murray Gell-Mann, are one possible representation of the Lie group#The Lie algebra associated to a Lie groups of the special unitary group called SU....
as

All other particle
Subatomic particle

A subatomic particle is an elementary particle or composite particle particle smaller than an atom. Particle physics and nuclear physics are concerned with the study of these particles, their interactions, and non-atomic QCD matter....
s belong to the trivial representation
Trivial representation

In the mathematics field of representation theory, a trivial representation is a group representation of a Group G on which all elements of G act as the identity mapping of V....
(1) of color SU(3). The color charge of each of these fields is fully specified by the representations. Quarks and antiquarks have color charge 4/3, whereas gluons have color charge 8. All other particles have zero color charge. Mathematically speaking, the color charge of a particle is the value of a certain quadratic Casimir operator in the representation of the particle.

In the simple language introduced previously, the three indices "1", "2" and "3" in the quark triplet above are usually identified with the three colors. The colorful language misses the following point. A gauge transformation in color SU(3) can be written as ? ? U?, where U is a 3X3 matrix which belongs to the group SU(3). Thus, after gauge transformation, the new colors are linear combinations of the old colors. In short, the simplified language introduced before is not gauge invariant.

Color charge is conserved, but the book-keeping involved in this is more complicated than just adding up the charges, as is done in quantum electrodynamics. One simple way of doing this is to look at the interaction vertex in QCD and replace it by a color line representation. The meaning is the following. Let ?_i represent the i-th component of a quark field (loosely called the i-th color). The color of a gluon is similarly given by a which corresponds to the particular Gell-Mann matrix it is associated with. This matrix has indices i and j. These are the color labels on the gluon. At the interaction vertex one has q_i?g_ij+q_j. The color-line representation tracks these indices. Color charge conservation means that the ends of these color-lines must be either in the initial or final state, equivalently, that no lines break in the middle of a diagram.

Since gluons carry color charge, two gluons can also interact. A typical interaction vertex (called the three gluon vertex) for gluons involves g+g?g. This is shown here, along with its color line representation. The color-line diagrams can be restated in terms of conservation laws of color; however, as noted before, this is not a gauge invariant language. Note that in a typical non-Abelian gauge theory the gauge boson

Gauge boson

In particle physics, gauge bosons are bosonic particles that act as carriers of the fundamental interactions of nature. More specifically, elementary particles whose interactions are described by gauge theory exert forces on each other by the exchange of gauge bosons, usually as virtual particles....
carries the charge of the theory, and hence has interactions of this kind; for example, the W boson in the electroweak theory. In the electroweak theory, the W also carries electric charge, and hence interacts with a photon.

Color charge

Red, Green, and Blue

Coupling constant and charge

Quark and gluon fields and color charges

See also