Fermi constant

The Fermi constant is the number that measures how strong the weak force effectively is at the low energies of everyday radioactivity. It is named after Enrico Fermi, who in the 1930s wrote down the first working theory of beta decay long before anyone knew about W bosons. His description treated beta decay as if four particles met at a single point and interacted with a strength set by this one constant.

Fermi's picture is what physicists call the four-fermion description: a neutron, a proton, an electron, and a neutrino all touching at one spot, with the Fermi constant fixing the rate. We now know this is an approximation — really a W boson is exchanged between two pairs of particles — but because the W is so heavy, at low energies that exchange happens over such a tiny distance that it looks point-like. The Fermi constant turns out to combine the true weak coupling strength with the W boson's mass, which is why it is small: a strong intrinsic coupling divided by a very large mass.

The Fermi constant is one of the most precisely measured numbers in all of physics, pinned down chiefly by timing how long muons take to decay. Because it links to the W mass and the weak coupling, it is a cornerstone input to the Standard Model: many predictions are anchored to its value. Fermi's four-fermion theory is the textbook example of an effective theory — a simpler stand-in that works beautifully at low energy but quietly fails when energies climb high enough to resolve the W boson directly.