The standard model alludes to a mechanism underpinning the strong, weak and electromagnetic interactions, which is said to be ‘mediated’ through the exchange of spin-1 gauge bosons, i.e.
- Electromagnetic: photons
- strong interaction: gluons
- weak interactions: W± and Z bosons
Shortly, after the idea of quarks was first proposed, in 1964, the concept of ‘colour charge’ was used to explain how quarks might coexist in identical quantum states without violating the Pauli exclusion principle. As such, the semantics of the word ‘colour’ is now used in the context of Quantum Chromo-Dynamics (QCD) to be representative of a property of quarks and gluons, which provides a mechanism of strong interactions between hadrons.
Only by way of initial introduction, we may liken the concept of colour charge to the notion of electric charge of particles; although in the case of strong interaction, the colour of a quark, or gluon, has almost no measurable effect outside the atomic nucleus. Therefore, the term ‘colour’ was simply chosen because it represents an abstract property that has three aspects, which are then identified by the three primary colour, i.e. red, green, and blue. In practice, electric charge only has a single aspect, which is either positive or negative. So, retrospectively trying to apply this QCD terminology to QED would lead to the description of a photon, as a single gauge boson, which mediates the electric charge. This is in contrast to the gauge bosons associated with the strong interactions, which are identified by 3 colours that mediate the weak interactions. Weak gauge bosons interact with quarks and leptons and in the process of being emitted or absorbed, some of them can transform one kind of quark, or lepton, into another. When these gauge bosons are exchanged between leptons and quarks, they are said to account for the force between them. So while we initially made an analogy between the notion of electric charge and colour charge, QCD is only describing the strong interactions between the six quarks in terms of the 3 colours, which exists in addition to the fractional electric charge assigned to each type of quark.
As already outlined, there are six types or flavours of quarks, i.e. up, down, strange, charm, bottom and top. The up and down quarks have the lowest masses, but all heavier quarks quickly ‘decay’ into up and down quarks, which might be described as a transition from a higher to a lower ‘mass-frequency’ energy state. As such, up and down quarks are essentially representative of a stable state, while the remaining ‘flavours’ are usually only produced in high-energy collisions associated with cosmic rays or as transient by-products in particle accelerators. So, as a generalisation, a quark of one flavour can transform into a quark of another flavour by undergoing a weak interaction, this process occurs by absorbing or emitting a W-boson, such that an up-type quark, i.e. up, charm and top, transforms into a down-type quark, i.e. down, strange and bottom, and vice versa. This flavour change mechanism is the cause of the radioactive process of ‘beta decay’ and explains how neutron transforms into a proton, an electron and an electron anti-neutrino.
By definition, the hadron class of particles must have zero colour charge, which is the net results of the combination of the three underlying types of colour charge, i.e. red, green and blue. It is also highlighted that anti-quarks have a corresponding anti-colour, i.e. anti-red anti-green and anti-blue. The system of attraction and repulsion between quarks with different combination of colour charge defines the process of strong interaction, which is in-turn mediated by a force carrying a gauge boson particle known as a gluon. As indicated, the details associated with strong interaction are described by the theory called QCD. Within the particle hierarchy, a gluon is a type of gauge boson, which sits under the meson class that comprises of just 2 quarks, quark and anti-quark. In contrast, baryons are made up of 3 quarks, which in the case of neutrons and protons, comprise of different combinations of the stable up and down quarks. In essence, the gluon ‘glues’ together up and down quarks, via the strong nuclear force, to form the stable components of all atomic nuclei.
So is the nature of particle interaction really that simple?
Unfortunately, no, but it provides a visualisation that can be helpful in contrast to all the mathematical abstractions that any detailed discussion of QFT/QCD demands. For example, modern particle physics talks of gauge symmetries that belong in different symmetry groups, which then require various gauge theories in order to describe the interactions between all the various classes of particles. In this context, ‘colour SU(3)’ is the label given to the gauge symmetry that relates the colour charge in quarks, where the implied symmetry is analogous to a fundamental law of physics, which is independent of any given orientation in spacetime, i.e. a given coordinate system. As such, the physics of QCD is said to be independent of the orientation in ‘colour space’ although the actual mathematics will require a description of ‘complex space’, where a colour transformation corresponds to a ‘rotation’ in complex/colour space. So, while acknowledging the simplicity of the previous description in the face of mounting mathematical complexity, we might initially say that QCD is a gauge invariant theory, which defines three colours and eight massless gluons, i.e. the gauge bosons, where six can affect the attribute of colour, while the other two simply react to colour. A colour of a quark is said to change, when it absorbs, or emits, a colour-changing gluon.