= Fermi transition
{c}
{title2=$M_F=\langle f|\sum_k\tau_k^\pm|i\rangle$}
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>.
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