Allowed beta decay
= Allowed beta decay
A <beta decay transition> whose leading lepton orbital term has $l=0$. <Fermi transitions> allow $\Delta J=0$ and <Gamow-Teller transitions> allow $\Delta J=0,1$, excluding a Gamow-Teller $0\to0$ transition. Both preserve nuclear <parity>. Allowed describes the selection rules; cancellations in the nuclear matrix element can still make an allowed transition slow.