A Fermi transition is a beta decay in which the spins of the emitted electron (positron) and anti-neutrino (neutrino) couple to total spin S = 0 , leading to an angular momentum change Δ Δ J = 0 between the initial and final states of the nucleus (assuming an allowed transition). In the non-relativistic limit, the nuclear part of the operator for a Fermi transition is given by O F = G V ∑ ∑ a τ τ ^ ^ a ± ± with G V the weak vector coupling constant, τ τ ± ± the isospin raising and lowering operators, and a running over all protons and neutrons in the nucleus.