= Forbidden beta decay
{title2=$\pi_f/\pi_i=(-1)^l$}
A <beta decay transition> for which the leading $l=0$ lepton orbital term vanishes by the <angular momentum> or <parity> selection rules. The least contributing orbital order $l=n\geq1$ gives the name $n$th forbidden. First-forbidden transitions change <parity> and can carry total lepton <angular momentum> $0,1,2$; second-forbidden transitions preserve <parity> and can carry $1,2,3$, with Fermi-type $S=0$ contributions restricted to $J=l$. Forbidden is suppression by higher powers of lepton momentum times nuclear size, rather than an absolute prohibition.
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