A gauge theory (sometimes called quantum flavourdynamics, or QFD) that gives a unified description of the electromagnetic and weak interactions (see fundamental interactions). A successful electroweak theory was proposed in 1967 by Steven Weinberg (1933– ) and Abdus Salam (1926–96), known as the Weinberg–Salam model or WS model. Because early developments of these ideas were put forward by Sheldon Glashow (1932– ), it is sometimes known as the Glashow–Weinberg–Salam model or GWS model. In this electroweak theory the gauge group is non-Abelian and the gauge symmetry is a broken symmetry. The electroweak interaction is mediated by photons and by intermediate vector bosons, called the W boson and the Z boson. The observation of these particles in 1983–84, with their predicted energies, was a major success of the theory. The theory successfully accounts for existing data for electroweak processes and also predicts the existence of a heavy particle with spin 0, the Higgs boson.