positive-energy solutions, is $u^\dagger(l) u(l)=2 E$, $E$ being the particle’s total energy (where $l$ now stands for the neutrini as well). Assuming this normalisation condition, one can prove that $\bar{u}(l) u(l)=2 m$, $m$ being the particle’s rest mass and $\bar{u}(l) \coloneqq u^\dagger(l) \gamma_0$.

On the other hand, the leptonic NC follows the generic form $$\bra{u(l)} J_\mu \ket{u(l)} = \frac{g_N}{2} \bar{u}(l) \gamma_\mu (g_V - g_A \gamma_5) u(l) \, \, \, ,$$ where the constant $g_N$ determines the overall strength of the NC processes, and $g_V$ and $g_A$ are known as vector and axial-vector couplings, respectively. These couplings are different for the neutrini and for the other leptons: the neutrino NC remains purely $V-A$ in character, i.e., $$g_V^\nu = 1/2 \, \, \, , g_A^\nu = 1/2 \, \, \, ,$$ whereas for all other leptons $$g_V^l = -1/2 + 2\xi \, \, \, , g_A^l = -1/2 \, \, \, .$$

According to the Standard Model of the Electroweak Interactions, developed in the 1960s by Sheldon Lee Glashow, Abdus Salam, and Steven Weinberg, the couplings $g$ and $g_N$ are related via the expression: $g=g_N \xi$. A similar relation holds for the masses of the IVBs: $M_W = M_Z \xi$. As a 