The standard model of matter accepted by most physicists today postulates two fundamental types of particles. One type includes those particles of which matter is made, namely quarks and leptons. The second type consists of mediating particles that transmit the four fundamental forces, namely the graviton (for the gravitational force), the gluon (for the strong force), the photon (for the electromagnetic force) and the W and Z bosons (for the weak force). Of these mediating particles, only the photon had actually been observed prior to the 1980s. Even today, the graviton and gluon are only hypothetical particles for whose existence there is very little or no empirical evidence.

The existence of W and Z bosons was predicted as a consequence of the electroweak theory developed by Steven Weinberg, Abdus Salam, and Sheldon Glashow in the early 1970s. Since the weak force acts...