<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> In particle physics, the weak interaction (the weak force or weak nuclear force) is the mechanism of interaction between sub-atomic particles that causes radioactive decay and thus plays an essential role in nuclear fission . The theory of the weak interaction is sometimes called quantum flavourdynamics (QFD), in analogy with the terms quantum chromodynamics (QCD) dealing with the strong interaction and quantum electrodynamics (QED) dealing with the electromagnetic force . However, the term QFD is rarely used because the weak force is best understood in terms of electro - weak theory (EWT). </P> <P> The weak interaction takes place only at very small, sub-atomic distances, less than the diameter of a proton . It is one of the four known fundamental interactions of nature, alongside the strong interaction, electromagnetism, and gravitation . </P> <P> The Standard Model of particle physics provides a uniform framework for understanding the electromagnetic, weak, and strong interactions . An interaction occurs when two particles, typically but not necessarily half - integer spin fermions, exchange integer - spin, force - carrying bosons . The fermions involved in such exchanges can be either elementary (e.g. electrons or quarks) or composite (e.g. protons or neutrons), although at the deepest levels, all weak interactions ultimately are between elementary particles . </P>

Where do the nuclear and weak interaction forces occur