Gluon

and A gluon (/ˈɡluːɒn/) 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. In layman's terms, they 'glue' quarks together, forming hadrons such as protons and neutrons. A gluon (/ˈɡluːɒn/) 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. In layman's terms, they 'glue' quarks together, forming hadrons such as protons and neutrons. In technical terms, gluons are vector gauge bosons that mediate strong interactions of quarks in quantum chromodynamics (QCD). Gluons themselves carry the color charge of the strong interaction. This is unlike the photon, which mediates the electromagnetic interaction but lacks an electric charge. Gluons therefore participate in the strong interaction in addition to mediating it, making QCD significantly harder to analyze than QED (quantum electrodynamics). The gluon is a vector boson; like the photon, it has a spin of 1. While massive spin-1 particles have three polarization states, massless gauge bosons like the gluon have only two polarization states because gauge invariance requires the polarization to be transverse. In quantum field theory, unbroken gauge invariance requires that gauge bosons have zero mass (experiments limit the gluon's rest mass to less than a few meV/c2). The gluon has negative intrinsic parity. Unlike the single photon of QED or the three W and Z bosons of the weak interaction, there are eight independent types of gluon in QCD. This may be difficult to understand intuitively. Quarks carry three types of color charge; antiquarks carry three types of anticolor. Gluons may be thought of as carrying both color and anticolor. This gives nine possible combinations of color and anticolor in gluons. The following is a list of those combinations (and their schematic names): These are not the actual color states of observed gluons, but rather effective states. To correctly understand how they are combined, it is necessary to consider the mathematics of color charge in more detail. It is often said that the stable strongly interacting particles (such as the proton and the neutron, i.e. hadrons) observed in nature are 'colorless', but more precisely they are in a 'color singlet' state, which is mathematically analogous to a spin singlet state. Such states allow interaction with other color singlets, but not with other color states; because long-range gluon interactions do not exist, this illustrates that gluons in the singlet state do not exist either. The color singlet state is: