Weak nuclear force

From Example Problems
Jump to navigation Jump to search

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 1013 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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 3 \times 10^{-25}} seconds by the uncertainty principle:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta E\cdot\Delta t\geq\hbar/2}

Even at the speed of light this effectively limits the range of the weak nuclear force to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 10^{-18}} meters, 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. In 1957, Robert Marshak and George Sudarshan and, somewhat later, Richard Feynman and Murray Gell-Mann proposed a V-A Lagrangian for weak interactions.

The Standard Model of particle physics describes the electromagnetic force and the weak nuclear force as two different aspects of a single electroweak force, the theory of which was developed around 1968 by Sheldon Glashow, Abdus Salam and Steven Weinberg (more at W and Z bosons). They were awarded the 1979 Nobel Prize in Physics for their work.

Decay via the weak force

A weak decay is the process of decomposing a heavier particle into lighter particles (plus energy) by the weak force.

A typical example is that of beta decay.

References

  • Griffiths, David J. (1987). Introduction to Elementary Particles, Wiley, John & Sons, Inc. ISBN 0471603864.

See also

External links

ca:Força nuclear feble cs:Slabá interakce da:Svag kernekraft de:Schwache Wechselwirkung es:Interacción nuclear débil eo:Malforta nuklea forto fr:Interaction faible ko:약한 상호작용 it:Forza nucleare debole he:הכוח הגרעיני החלש hu:Gyenge kölcsönhatás nl:Zwakke kernkracht ja:弱い相互作用 pl:Oddziaływanie słabe pt:Força nuclear fraca ru:Слабое взаимодействие sl:Šibka jedrska sila sv:Svag växelverkan zh:弱相互作用