Fermi transition ($M_F=⟨f|{\sum }_k{\tau }_k^{\pm }|i⟩$)

The spin-independent vector contribution to an allowed beta decay. The isospin operator ${\tau }_k^{\pm }$ changes a neutron into a proton or the reverse; the sign convention depends on the assigned nucleon isospin projection. Its rotational rank is zero, so $J_f=J_i$ and the parity is unchanged. The phrase Fermi-type forbidden contribution extends the $S=0$ classification to higher orbital angular momentum.