W Particle is one of two electrically charged subatomic particles that are thought to transmit the weak nuclear force—that is, the force that governs radioactive decay in certain kinds of atomic nuclei. These particles are also referred to as intermediate vector bosons, or weakons.

The existence of intermediate vector bosons and their properties were predicted in the late 1960s by the physicists Sheldon Glashow, Steven Weinberg, and Abdus Salam. Their theoretical efforts, now called the electroweak theory, explain that the electromagnetic force and the weak force, long considered separate entities, are actually manifestations of the same basic interaction. Just as the electromagnetic force is transmitted by means of carrier particles known as photons, the weak force is exchanged via three types of intermediate vector bosons. In addition to carrying the weak force, two of these bosons bear either a positive or negative electrical charge and are designated W+ and W−, respectively. The third type, called Z0, is electrically neutral. Unlike photons, each such intermediate vector boson has a large mass, and this characteristic is responsible for the extremely short range of the nuclear weak force (i.e., its influence is confined to a distance on the order of only about 10−16 cm). As established by quantum mechanics, the range of any given force tends to be inversely proportional to the mass of the particle transmitting it.

In low-energy processes such as beta decay, the heavy W particles can be exchanged only because quantum mechanics allows fluctuations in mass-energy over sufficiently short time scales (see uncertainty principle). Such W particles can never be observed directly. However, detectable W particles can be produced in collisions between subatomic particles provided that the energy is high enough. A W particle of this kind then typically decays into a charged lepton (e.g., electron, muon, or tau) and an associated neutrino.

In 1983 a group of investigators led by Carlo Rubbia and Simon van der Meer at the European Laboratory for Particle Physics (CERN) in Geneva, Switz., detected characteristics closely approximating those predicted for the formation and decay of W and Z particles. Their findings constituted the first direct evidence of weak bosons and as such provided strong support for the electroweak theory. The CERN scientists reported having observed numerous clear-cut instances of weak bosons in proton-antiproton collision experiments that were carried out on a 540-billion-electron-volt (GeV) colliding-beam storage ring. All of the observed W particles had a mass of about 81 GeV, or approximately 80 times the mass of the proton, as had been predicted by the electroweak model. The electrically neutral Z particles detected, with a rest mass of 93 GeV, were also consistent with prediction.