W and Z Bosons

The W and Z bosons (together known as the weak bosons or, less specifically, the intermediate vector bosons) are the elementary particles that mediate the weak interaction; their symbols are W+, W− and Z. The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticles. The Z boson is electrically neutral and is its own antiparticle. All three of these particles are very short-lived with a half-life of about 3×10−25 s. Their discovery was a major success for what is now called the Standard Model of particle physics.

The W bosons are named after the weak force. The physicist Steven Weinberg named the additional particle the "Z particle", later giving the explanation that it was the last additional particle needed by the model – the W bosons had already been named – and that it has zero electric charge.

The two W bosons are best known as mediators of neutrino absorption and emission, where their charge is associated with electron or positron emission or absorption, always causing nuclear transmutation. The Z boson is not involved in the absorption or emission of electrons and positrons.

The Z boson is most easily detected as a necessary theoretical force-mediator whenever neutrinos scatter elastically from matter, something that must happen without the production or absorption of new, charged particles. Such behavior (which is almost as common as inelastic neutrino interactions) is seen in bubble chambers irradiated with neutrino beams. Whenever an electron simply "appears" in such a chamber as a new free particle suddenly moving with kinetic energy, and moves in the direction of the neutrinos as the apparent result of a new impulse, and this behavior happens more often when the neutrino beam is present, it is inferred to be a result of a neutrino interacting directly with the electron. Here, the neutrino simply strikes the electron and scatters away from it, transferring some of the neutrino's momentum to the electron. Because neither neutrinos nor electrons are affected by the strong force – because neutrinos are electrically neutral, and because the incredibly small masses of these particles makes negligible any gravitational force between them – such an interaction can only happen via the weak force. Since such an electron is not created from a nucleon, and is unchanged except for the new force impulse imparted by the neutrino, this weak force interaction between the neutrino and the electron must be mediated by a weak-force boson particle with no charge. Thus, this interaction requires a Z boson.