QUANTUM CHROMODYNAMICS

(QCD) the theory that describes the action of the strong nuclear force. QCD was constructed on analogy to quantum electrodynamics (QED), the quantum theory of the electromagnetic force. In QED, the electromagnetic interactions of charged particles are described through the emission and subsequent absorption of massless photons, best known as the particles of light; such interactions are not possible between uncharged, electrically neutral particles. The strong force is observed to behave in a similar way, acting only upon certain particles, principally quarks that are bound together in the protons and neutrons of the atomic nucleus, as well as in less stable, more exotic forms of matter. So by analogy with QED, quantum chromodynamics has been built upon the concept that quarks interact via the strong force because they carry a form of strong charge, which has been given the name of colour; other particles, such as the electron, which do not carry the colour charge, do not interact in this way. In QED there are only two values for electric charge, positive and negative, or charge and anticharge. To explain the behaviour of quarks in QCD, by contrast, there need to be three different types of colour charge, each of which can occur as colour or anticolour. The three types of charge are called red, green, and blue in analogy to the primary colours of light, although there is no connection whatsoever with colour in the usual sense. Colour-neutral particles occur in one of two ways. In baryons (i.e., particles built from three quarks, as, for example, protons and neutrons), the three quarks are each of a different colour, and a mixture of the three colours produces a particle that is neutral. Mesons, on the other hand, are built from pairs of quarks and antiquarks, and in these the anticolour of the antiquark neutralizes the colour of the quark, much as positive and negative electric charges cancel each other to produce an electrically neutral object. Quarks interact via the strong force by exchanging particles called gluons. In contrast to QED, where the photons exchanged are electrically neutral, the gluons of QCD also carry colour charges. To allow all the possible interactions between the three colours of quarks, there must be eight gluons, each of which generally carries a mixture of a colour and an anticolour of a different kind. Because gluons carry colour, they can interact among themselves, and this makes the behaviour of the strong force subtly different from the electromagnetic force. QED describes a force that becomes weaker as the distance between two charges increases (obeying an inverse square law), but in QCD the interactions between gluons emitted by colour charges prevent those charges from being pulled apart. Instead, if sufficient energy is invested in the attempt to knock a quark out of a proton, for example, the result is the creation of a quark-antiquark pairin other words a meson. Like QED, quantum chromodynamics is a gauge-invariant theory, which means that its basic equations give the same results at different points in space and time. The fundamental symmetry in the equations of QCD corresponds to the mathematical symmetry group SU(3), or special unitary group in three dimensions, where the three dimensions correspond to the three colours. In comparison, the underlying symmetry in QED is represented by the one-dimensional unitary group U(1). See also quantum field theory; quark. Christine Sutton