The weak interaction (often called the weak force or sometimes the weak nuclear force) is one of the four fundamental interactions of nature. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons. Its most familiar effect is beta decay (of neutrons in atomic nuclei) and the associated radioactivity. The word "weak" derives from the fact that the field strength is some 10 times less than that of the strong force.
There are three basic types of weak interaction vertices (up to charge conjugation and crossing symmetry). Two of them involve charged bosons, they are called "charged current interactions." The third type is called "neutral current interaction."
Two charged-current interactions together are responsible for the beta decay phenomenon. The neutral current interaction was first observed in neutrino scattering experiments in 1974 and in collider experiments in 1983.
A charged lepton (such as an electron or a muon) can emit or absorb a W boson and convert into a corresponding neutrino.
A down-type quark (with charge -1/3) can emit or absorb a W boson and convert into a superposition of up-type quarks. Conversely, an up-type quark can convert into a superposition of down-type quarks. The exact content of this superposition is given by CKM matrix.
Either a lepton or a quark can emit or absorb a Z boson. Interaction types
The laws of nature were long thought to remain the same under mirror reflection, the reversal of all spatial axes. The results of an experiment viewed via a mirror were expected to be identical to the results of a mirror-reflected copy of the experimental apparatus. This so-called law of parity conservation was known to be respected by classical gravitation and electromagnetism; it was assumed to be a universal law. However, in the mid-1950's Chen Ning Yang and Tsung-Dao Lee suggested that the weak interaction might violate this law. Chien Shiung Wu and collaborators in 1957 discovered that the weak interaction in fact maximally violates parity, earning Yang and Lee the 1957 Nobel Prize in Physics.
Although the weak interaction used to be described by Fermi's theory of a contact four-fermion interaction, the discovery of parity violation and renormalization theory suggested a new approach was needed. In 1957, Robert Marshak and George Sudarshan and, somewhat later, Richard Feynman and Murray Gell-Mann proposed a V−A (vector minus axial vector or left-handed) Lagrangian for weak interactions. In this theory, the weak interaction acts only on left-handed particles (and right-handed antiparticles). Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity.
However, this theory allowed a compound symmetry CP to be conserved. CP combines parity P (switching left to right) with charge conjugation C (switching particles with antiparticles). Physicists were again surprised when in 1964, James Cronin and Val Fitch provided clear evidence in kaon decays that CP symmetry could be broken too, winning them the 1980 Nobel Prize in Physics. Unlike parity violation, CP violation is a very small effect.