= Muon decay
{title2=$\mu^-\to e^-\bar\nu_e\nu_\mu$}
Ordinary negative-muon decay is $\mu^-\to e^-+\bar\nu_e+\nu_\mu$; the positive-muon channel has the conjugate particles. The process proceeds through a <weak charged current>. At energies much smaller than the W mass, a <four-fermion interaction> with the <Fermi constant> describes it.
At leading order, neglecting the electron mass and radiative corrections, the width is $\Gamma=G_F^2m_\mu^5/(192\pi^3)$ in natural units. The fifth power reflects the interaction dimension together with three-body phase space. The electron has a continuous energy distribution because two neutrinos share the missing energy and momentum. Precision extraction of the Fermi constant uses the measured lifetime and includes electron-mass and radiative corrections. A stopping experiment measures disappearance or decay timing, so nuclear capture must be distinguished from free decay when relevant.
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