weak interaction, a fundamental force of nature that underlies some forms of radioactivity, governs the decay of unstable subatomic particles such as mesons, and initiates the nuclear fusion reaction that fuels the Sun. The weak interaction acts upon left-handed fermions—i.e., elementary particles with half-integer values of intrinsic angular momentum, or spin—and right-handed antifermions. Particles interact through the weak interaction by exchanging force-carrier particles known as the W and Z particles. These particles are heavy, with masses about 100 times the mass of a proton, and it is their heaviness that defines the extremely short-range nature of the weak interaction and that makes the weak interaction appear weak at the low energies associated with radioactivity.

The effectiveness of the weak interaction is confined to a distance range of 10−17 metre, about 1 percent of the diameter of a typical atomic nucleus. In radioactive decays the strength of the weak interaction is about 100,000 times less than the strength of the electromagnetic force. However, it is now known that the weak interaction has intrinsically the same strength as the electromagnetic force, and these two apparently distinct forces are believed to be different manifestations of a unified electroweak force.

Most subatomic particles are unstable and decay by the weak interaction, even if they cannot decay by the electromagnetic force or the strong force. The lifetimes for particles that decay via the weak interaction vary from as little as 10−13 second to 896 seconds, the mean life of the free neutron. Neutrons bound in atomic nuclei can be stable, as they are when they occur in the familiar chemical elements, but they can also give rise through weak decays to the type of radioactivity known as beta decay. In this case the lifetimes of the nuclei can vary from a thousandth of a second to millions of years. Although low-energy weak interactions are feeble, they occur frequently at the heart of the Sun and other stars where both the temperature and the density of matter are high. In the nuclear fusion process that is the source of stellar energy production, two protons interact via the weak interaction to form a deuterium nucleus, which reacts further to generate helium with the concomitant release of large amounts of energy.

Italian-born physicist Dr. Enrico Fermi draws a diagram at a blackboard with mathematical equations. circa 1950.
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The characteristics of the weak interaction, including its relative strength and effective range and the nature of the force-carrier particles, are summarized in the Standard Model of particle physics.

Christine Sutton

electroweak theory, in physics, the theory that describes both the electromagnetic force and the weak force. Superficially, these forces appear quite different. The weak force acts only across distances smaller than the atomic nucleus, while the electromagnetic force can extend for great distances (as observed in the light of stars reaching across entire galaxies), weakening only with the square of the distance. Moreover, comparison of the strength of these two fundamental interactions between two protons, for instance, reveals that the weak force is some 10 million times weaker than the electromagnetic force. Yet one of the major discoveries of the 20th century has been that these two forces are different facets of a single, more-fundamental electroweak force.

The electroweak theory arose principally out of attempts to produce a self-consistent gauge theory for the weak force, in analogy with quantum electrodynamics (QED), the successful modern theory of the electromagnetic force developed during the 1940s. There are two basic requirements for the gauge theory of the weak force. First, it should exhibit an underlying mathematical symmetry, called gauge invariance, such that the effects of the force are the same at different points in space and time. Second, the theory should be renormalizable; i.e., it should not contain nonphysical infinite quantities.

During the 1960s Sheldon Lee Glashow, Abdus Salam, and Steven Weinberg independently discovered that they could construct a gauge-invariant theory of the weak force, provided that they also included the electromagnetic force. Their theory required the existence of four massless “messenger” or carrier particles, two electrically charged and two neutral, to mediate the unified electroweak interaction. The short range of the weak force indicates, however, that it is carried by massive particles. This implies that the underlying symmetry of the theory is hidden, or “broken,” by some mechanism that gives mass to the particles exchanged in weak interactions but not to the photons exchanged in electromagnetic interactions. The assumed mechanism involves an additional interaction with an otherwise unseen field, called the Higgs field, that pervades all space.

Italian physicist Guglielmo Marconi at work in the wireless room of his yacht Electra, c. 1920.
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In the early 1970s Gerardus ’t Hooft and Martinus Veltman provided the mathematical foundation to renormalize the unified electroweak theory proposed earlier by Glashow, Salam, and Weinberg. Renormalization removed the physical inconsistencies inherent in earlier calculations of the properties of the carrier particles, permitted precise calculations of their masses, and led to more-general acceptance of the electroweak theory. The existence of the force carriers, the neutral Z particles and the charged W particles, was verified experimentally in 1983 in high-energy proton-antiproton collisions at the European Organization for Nuclear Research (CERN). The masses of the particles were consistent with their predicted values.

The characteristics of the unified electroweak force, including the strength of the interactions and the properties of the carrier particles, are summarized in the Standard Model of particle physics.

Christine Sutton