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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 predicate "weak" derives from the fact that the field strength is some 1013 times less than that of the strong force.
Properties
Interaction types 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. Violation of symmetry 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. Electroweak Theory The Standard Model of particle physics describes the electromagnetic interaction and the weak interaction as two different aspects of a single electroweak interaction, the theory of which was developed around 1968 by Sheldon Glashow, Abdus Salam and Steven Weinberg (see W and Z bosons). They were awarded the 1979 Nobel Prize in Physics for their work. According to the electroweak theory, at very high energies, the universe has four identical massless gauge bosons similar to the photon and a scalar Higgs field. However, at low energies the symmetry of the Higgs field is spontaneously broken by the Higgs mechanism. This symmetry breaking produces three massless Goldstone bosons which are "eaten" by three of the photon-like fields, giving them mass. These three fields become the W and Z bosons of the weak interaction, while the fourth field remains massless and is the photon of electromagnetism. Although this theory has made a number of impressive predictions, including a prediction of the mass of the Z boson before its discovery, the Higgs boson itself has never been observed. Producing Higgs bosons will be a major goal of the Large Hadron Collider being built at CERN. See also | ||||||||||
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