Fermi Couplings and the Failure of Parity

Remarkably, the strength of the coupling for each of the three examples alone seems to be equal. It is measured by a constant G satisfying GM²/ħc = 1.01 ± 0.01 × 10⁻⁵, where M is the mass of the proton, included to make a dimensionless ratio. It is seen to be very small; the coupling very weak. The only couplings known for the neutrino (rest mass 0, spin 1/2) are Fermi couplings, so the interaction with matter of this particle is very small and its direct detection extremely difficult.