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Fermi's interaction
In physics, Fermi's interaction is an old explanation of the weak force, proposed by Enrico Fermi. Four fermions directly interact with one another. For example, this interaction is directly able to split a neutron (or a down-quark) to an electron, antineutrino and a proton (or an up-quark).
Tree Feynman diagrams describe the interaction remarkably well. Unfortunately, loop diagrams cannot be calculated reliably because Fermi's interaction is not renormalizable. The solution is to replace the four-fermion contact interaction by a more complete theory - an exchange of a W boson or a Z boson as explained in the electroweak theory. The electroweak theory is renormalizable.
Before the electroweak theory and the Standard Model were constructed, Richard Feynman and Murray Gell-Mann were able to determine the correct tensor structure (vector minus axial vector, V-A) of the four-fermion interaction.
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