FermiTheorie

Fermi-Theorie is the phenomenological theory of weak interactions associated with beta decay, introduced by Enrico Fermi in 1934. It describes beta decay as a pointlike four-fermion interaction, in which a neutron converts into a proton while emitting an electron and an antineutrino. The strength of this interaction is set by the Fermi coupling constant G_F, and the theory provides a simple framework for calculating decay rates and spectra.

In its original form, Fermi’s theory treated the weak interaction as a current-current contact interaction. Later refinements incorporated vector and axial-vector components, leading to the modern V-A (vector minus axial-vector) structure that underpins the universal description of weak processes. The decay rate predicted by the theory scales with the square of G_F, and the spectral shape of emitted electrons is a direct test of the model.

Fermi-Theorie played a crucial role in establishing the weak force as a fundamental interaction and in guiding early experimental tests of beta decay. It also introduced a useful classification of beta decays into Fermi transitions (no spin change) and Gamow-Teller transitions (spin change), a distinction that remains in effective descriptions of weak processes.

Limitations and legacy: Fermi theory is nonrenormalizable in the modern sense and is valid only at energies well below the W boson mass. At higher energies, the interaction is understood as mediated by W boson exchange in the electroweak theory. Today, Fermi theory is viewed as an effective low-energy approximation within the Standard Model, with G_F extracted from muon decay and other low-energy weak processes.