= Beta decay transition
{wiki}
A nuclear <beta decay> connects initial and final <quantum states> through vector and axial-vector parts of the <weak charged current>. At leading order in the lepton wavelength expansion, <Fermi transitions> carry lepton-pair <spin> $S=0$, while <Gamow-Teller transitions> carry $S=1$. If the lepton pair carries <orbital angular momentum> $l$, its total <angular momentum> is $J=l$ for $S=0$, or $J=|l-1|,\ldots,l+1$ for $S=1$. The nuclear states must obey $|J_i-J_f|\leq J\leq J_i+J_f$, and their <parities> satisfy $\pi_f/\pi_i=(-1)^l$ in this leading orbital classification. The least permitted $l$ labels the degree of forbiddenness. Real forbidden transitions involve several weak multipole operators; the elementary spin classification does not assert that only one operator or nuclear matrix element exists.
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