The weak nuclear force or weak interaction is one of the four fundamental forces of nature. It is most commonly seen in beta decay and the associated radioactivity. The predicate weak derives from the fact that the field strength is some 109 times less than that of the strong nuclear force. The term nuclear indicates that it is a short-range force, limited to distances smaller than an atomic nucleus.
Properties
The weak nuclear force affects all leptons and quarks. It is the only force affecting neutrinos (except for gravitation, which is negligible on laboratory scales). The weak interaction enables all lepton and quark particles and antiparticles to interchange energy, mass, electric charge and flavor—effectively to change into each other.
Although the weak nuclear force used to be described by Fermi's theory of a contact four-fermion interaction, today we know that it is mediated by the W and Z bosons. Because of their large mass of about 90 GeV/c2, their mean life is limited to about seconds by the uncertainty principle:
Even at the speed of light this effectively limits the range of the weak nuclear force to 10 - 18meters, or about 1000 times smaller than the diameter of an atomic nucleus.
The laws of nature were long thought to remain the same under mirror reflection. 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 is called the law of parity conservation in physicists' jargon. 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 confirmed this prediction in 1957, earning Yang and Lee the 1957 Nobel Prize in Physics.
The electromagnetic force is a force of infinite range which obeys the inverse square law, and is of the same form as the gravity force.
The discovery of the W and Z particles in 1983 was hailed as a confirmation of the theories which connect the weakforce to the electromagnetic force in electroweak unification.
This suggests the weak interaction mechanism for the decay of the pion, which is observed to happen by the muon pathway.
In contrast to electromagnetic and strong forces, the strength of the weakforce is different for particles and anti-particles (Charge Violation), for a scattering process and its mirror image (Parity Violation), and for a scattering process and the time reversal of that scattering process (Time Violation).
The weak interaction finds practical application in the radioactive elements used in medicine and technology, which are in general beta-radioactive, and in the beta-decay of a carbon isotope into nitrogen, which is the basis for the carbon-14 method for dating of organic archaeological remains.
The weak charges of quarks and leptons are comparable to their electromagnetic charges, a manifestation of how electromagnetism and the weakforce are components of a unified electroweak force.
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