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).The quark colors (red, green, blue) combine to be colorlessThe quark anticolors (antired, antigreen, antiblue) also combine to be colorlessA hadron with 3 quarks (red, green, blue) before a color changeBlue quark emits a blue-antigreen gluonGreen quark has absorbed the blue-antigreen gluon and is now blue; color remains conservedAn animation of the interaction inside a neutron. The gluons are represented as circles with the color charge in the center and the anti-color charge on the outside. Color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics (QCD). The 'color charge' of quarks and gluons is completely unrelated to the everyday meaning of color. The term color and the labels red, green, and blue became popular simply because of the loose analogy to the primary colors. Richard Feynman referred to his colleagues as 'idiot physicists' for choosing the confusing name. Particles have corresponding antiparticles. A particle with red, green, or blue charge has a corresponding antiparticle in which the color charge must be the anticolor of red, green, and blue, respectively, for the color charge to be conserved in particle-antiparticle creation and annihilation. Particle physicists call these antired, antigreen, and antiblue. All three colors mixed together, or any one of these colors and its complement (or negative), is 'colorless' or 'white' and has a net color charge of zero. Due to a property of the strong interaction called color confinement, free particles must have a color charge of zero: a baryon is composed of three quarks, which must be one each of red, green, and blue colors; likewise an antibaryon is composed of three antiquarks, one each of antired, antigreen and antiblue. A meson is made from one quark and one antiquark; the quark can be any color, and the antiquark has the matching anticolor. This color charge differs from electric charge in that electric charge has only one kind of value. However color charge is also similar to electric charge in that color charge also has a negative charge corresponding to each kind of value. Shortly after the existence of quarks was first proposed in 1964, Oscar W. Greenberg introduced the notion of color charge to explain how quarks could coexist inside some hadrons in otherwise identical quantum states without violating the Pauli exclusion principle. The theory of quantum chromodynamics has been under development since the 1970s and constitutes an important component of the Standard Model of particle physics. In quantum chromodynamics, a quark's color can take one of three values or charges: red, green, and blue. An antiquark can take one of three anticolors: called antired, antigreen, and antiblue (represented as cyan, magenta, and yellow, respectively). Gluons are mixtures of two colors, such as red and antigreen, which constitutes their color charge. QCD considers eight gluons of the possible nine color–anticolor combinations to be unique; see eight gluon colors for an explanation. The following illustrates the coupling constants for color-charged particles: Analogous to an electric field and electric charges, the strong force acting between color charges can be depicted using field lines. However, the color field lines do not arc outwards from one charge to another as much, because they are pulled together tightly by gluons (within 1 fm). This effect confines quarks within hadrons. In a quantum field theory, a coupling constant and a charge are different but related notions. The coupling constant sets the magnitude of the force of interaction; for example, in quantum electrodynamics, the fine-structure constant is a coupling constant. The charge in a gauge theory has to do with the way a particle transforms under the gauge symmetry; i.e., its representation under the gauge group. For example, the electron has charge −1 and the positron has charge +1, implying that the gauge transformation has opposite effects on them in some sense. Specifically, if a local gauge transformation ϕ(x) is applied in electrodynamics, then one finds (using tensor index notation): where A μ {displaystyle A_{mu }} is the photon field, and ψ is the electron field with Q = −1 (a bar over ψ denotes its antiparticle — the positron). Since QCD is a non-abelian theory, the representations, and hence the color charges, are more complicated. They are dealt with in the next section.