Sargent's rule ($\Gamma \propto G_F^2{Q}^5$)

In a low-energy allowed weak three-body decay, negligible final-state masses and a nearly constant effective matrix element give a fifth-power energy-release dependence. For muon decay in natural units, $\Gamma \simeq G_F^2{m}_{\mu }^5/(192{\pi }^3)$. This four-fermion approximation requires momentum transfers small compared with the W boson mass. Extrapolation to a heavy particle able to emit a real W changes the decay kinematics and the mass scaling, so it is not a reliable heavy-particle lifetime prediction.