Forbidden beta decay (${\pi }_f/{\pi }_i=(-1{)}^l$)

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 $l=n\ge 1$ 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.