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Weak Interactions

Fundamental weak interactions occur for all fundamental particles except gluons and photons. Weak interactions involve the exchange or production of W or Z bosons.

Weak forces are very short-ranged. In ordinary matter, their effects are negligible except in cases where they allow an effect that is otherwise forbidden. There are a number of conservation laws that are valid for strong and electromagnetic interactions, but broken by weak processes. So, despite their slow rate and short range, weak interactions play a crucial role in the make-up of the world we observe.

 

Any process where the number of particles minus the number of antiparticles of a given quark or lepton type changes is a weak decay process and involves a W-boson. Weak decays are thus responsible for the fact that ordinary stable matter contains only up and down type quarks and electrons. Matter containing any more massive quark or lepton types is unstable.  If there were no weak interactions, then many more types of matter would be stable.

 

Processes involving Z-bosons (called "neutral current processes") are even more elusive than W-boson effects, and were not recognized until after the electroweak theory had predicted they must exist in the early 1970s. Careful searches then found events that could not be explained without such processes.

Later, higher energy experiments produced W and Z bosons and studied their decays. Carlo Rubbia and Simon van der Meer were awarded the 1984 Nobel Prize for their development of the facility at CERN which first produced and detected W and Z bosons, and for their leadership of the discovery experiment.

 

The weak interaction was first recognized in cataloging the types of nuclear radioactive decay chains, as alpha, beta, and gamma. decays. Alpha and gamma decays can be understood in terms of other known interactions (residual strong and electromagnetic, respectively). But, to explain beta decay required the introduction of an additional rare type of interaction -- called the weak interaction.

Beta decay is a process in which a neutron (two down quarks and one up) disappears and is replaced by a proton (two up quarks and one down), an electron, and an anti-electron neutrino. According to the Standard Model, a down quark disappears in this process and an up quark and a virtual W boson is produced.. The W boson then decays to produce an electron and an anti-electron type neutrino. This can be represented by the Feynman diagram:

 

Quark flavor never changes except by weak interactions, like beta decay, that involve W bosons.

Any quark type can convert to any other quark type with a different electric charge by emitting or absorbing a W boson. The relative probabilities of the different possible transitions are shown schematically in the diagram below.

Quarks and their weak decays

Decay processes always proceed from a more massive quark to a less massive quark, as indicated in the diagram, because the reverse process would violate conservation of energy. Scattering processes can involve the reverse transitions, provided sufficient energy is available.

 

The set of observed lepton processes is much simpler. Each charged lepton is converted only to its own neutrino type by emitting or absorbing a W boson. This leads to the three lepton number conservation laws.