The weak interaction is one of the four fundamental interactions of nature. In particle physics, it is described within the Standard Model alongside electromagnetism and the strong interaction; gravity is not included in that framework. The weak interaction causes beta decay, participates in the production and detection of neutrinos, and allows certain elementary particles to change their identity. Its mediators are the electrically charged W⁺ and W⁻ bosons and the electrically neutral Z boson. Unlike familiar macroscopic forces, it is often observed through particle transformations rather than attraction or repulsion. (home.web.cern.ch)
Mediators, range, and strength
The weak interaction acts on quarks and leptons, including electrically neutral neutrinos. Its W and Z bosons have spin 1 and masses of approximately 80 and 91 GeV/c², respectively. These large masses suppress exchange processes at low energies and give the interaction a characteristic range of roughly 10⁻¹⁸ metres, substantially smaller than the diameter of a proton. The range represents a characteristic suppression scale, not a sharp boundary beyond which every weak effect disappears. (pdg.lbl.gov)
The name “weak” therefore depends on the physical circumstances. At energies typical of nuclear decay, weak reactions generally occur much less readily than electromagnetic or strong reactions. The underlying coupling is not extraordinarily small: the heavy mediators are central to the low-energy suppression. By contrast, the electromagnetic mediator, the photon, is massless. At sufficiently high energies, the description of the weak interaction as simply much weaker than electromagnetism becomes inadequate. (home.web.cern.ch)
Charged and neutral currents
Weak processes are divided into two principal classes. Charged-current interactions, mediated by W⁺ or W⁻, transfer electric charge between particles. They can turn an up-type quark into a down-type quark, or connect a charged lepton with its associated neutrino. Such transformations obey overall electric-charge conservation. Neutral-current interactions, mediated by the Z boson, transfer no electric charge; an example is a neutrino scattering from an electron while both remain the same particle types. (energy.gov)
In the Standard Model, W-mediated charged currents couple to left-chiral fermion fields, with the corresponding right-chiral antiparticle participation. Chirality is a property of relativistic quantum fields and should not be identified indiscriminately with the direction of a massive particle’s spin relative to its motion. Z-mediated neutral currents can couple to both left- and right-chiral charged fermions, with different strengths. Thus, saying that the weak interaction acts “only on left-handed particles” is an incomplete description. (pdg.lbl.gov)
Beta decay and particle transformations
A characteristic example is the decay of a free neutron into a proton, an electron, and an electron antineutrino:
At the quark level, one down quark in the neutron becomes an up quark. The neutron’s quark composition consequently changes from udd to uud, the composition of a proton. The electron and antineutrino are produced in the reaction; they are not constituents stored inside the neutron. (energy.gov)
Within an atomic nucleus, beta-minus decay increases the proton number by one, while beta-plus decay decreases it by one and emits a positron and an electron neutrino. These changes can transform one chemical element into another. Beta decay is therefore an important form of radioactivity, although other radioactive processes have different mechanisms. Whether a particular nuclear transformation is possible depends on the available energy. (energy.gov)
At low energies, the W mediator is represented as a virtual particle, not an independently detectable W boson produced with its full rest mass. This explains why nuclear beta decay can proceed even though its released energy is far below that required to produce a real W boson. (nobelprize.org)
Electroweak theory and symmetry
The weak and electromagnetic interactions are unified mathematically in the electroweak interaction, a gauge theory based on the symmetry group SU(2)ₗ × U(1)ᵧ. The Higgs field, through the Higgs mechanism, gives mass to the W and Z bosons while leaving the photon massless. Their contrasting masses explain much of the difference between low-energy weak and electromagnetic phenomena. (home.web.cern.ch)
A distinctive property is violation of parity, the symmetry corresponding to spatial reflection. In 1956, Tsung-Dao Lee and Chen Ning Yang questioned whether parity was conserved in weak processes. Experiments by Chien-Shiung Wu and collaborators demonstrated its violation in 1957. Certain weak decays also violate CP symmetry, which combines spatial reflection with exchanging particles and antiparticles; this was first observed in neutral-kaon decay in 1964. (nobelprize.org)
Historical and astrophysical significance
Enrico Fermi developed an early theory of beta decay in 1933–1934, treating the interaction as a local coupling of four matter-particle fields. It remains useful as a low-energy approximation, but a complete high-energy description requires electroweak theory. Evidence for weak neutral currents was announced by the Gargamelle collaboration at CERN on July 19, 1973. Direct discoveries of the W and Z bosons followed in 1983. (home.web.cern.ch)
The weak interaction is essential to nuclear fusion in the Sun because hydrogen-to-helium conversion requires some protons to become neutrons. These reactions also produce neutrinos, which escape readily from the solar interior. Detecting them through weak interactions provides information about nuclear processes in the solar core that ordinary observations of sunlight cannot directly reveal. (nobelprize.org)