= Michel spectrum
{c}
{title2=$f(x)=2x^2(3-2x)$}
For an unpolarized parent <charged lepton> undergoing a three-body leptonic <V-A interaction> decay, neglecting daughter <masses> and radiative corrections, the normalized charged-daughter <energy> distribution in the parent rest frame is $f(x)=2x^2(3-2x)$, where $x=2E_\ell^*/m_{\rm parent}$ lies in $[0,1]$. Direct integration gives $\int_0^1f(x)\,dx=1$ and $\int_0^1xf(x)\,dx=7/10$. A specified parent <spin> orientation also produces angular terms, so this one-variable law alone does not describe polarized samples.
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