 Solution

The electroweak gauge group is $SU(2{)}_L\times U(1{)}_Y$. Left-handed fermions form weak-isospin doublets and right-handed charged fermions are singlets under $SU(2{)}_L$. Before symmetry breaking the gauge fields are $W^1,W^2,W^3$ and $B$, with couplings $g,g^{\prime }$. A Higgs-doublet vacuum expectation value breaks the symmetry to electromagnetic $U(1)$.

In units $\hslash =c=1$, the physical combinations and tree-level relations areThe photon remains massless. charged currents change members of weak doublets, whereas the $Z$ mediates weak neutral currents with parity-dependent couplings. At low energies $W$ exchange reduces to the Fermi contact interaction, $G_F/\sqrt{2}=g^2/(8M_W^2)$; the apparent weak suppression reflects the massive mediator. This unifies the gauge description, not the numerical equality of all low-energy couplings.^[ref]

Neutral-current neutrino interactions and subsequent observation of massive weak vector bosons tested this structure. In a neutral-current neutrino experiment, an isolated hadronic interaction without an accompanying charged lepton is a candidate signal; entering neutral hadrons and missed charged-current leptons are important backgrounds. Gargamelle's neutral-current evidence supplied a direct experimental check beyond ordinary beta decay.^[ref]