= Sargent's rule
{c}
{title2=$\Gamma\propto G_F^2Q^5$}
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>, $\Gamma\simeq G_F^2m_\mu^5/(192\pi^3)$. 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.
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