Fermi coupling constant

In particle physics, Fermi's interaction (also the Fermi theory of beta decay) is an explanation of the beta decay, proposed by Enrico Fermi in 1933. The theory posits four fermions directly interacting with one another, at one vertex.
For example, this interaction explains beta decay of a neutron by direct coupling of a neutron with:

1. an electron,
2. an antineutrino and
3. a proton.