Electroweak Interaction

The electroweak interaction is the unified description of two of the four fundamental forces — the electromagnetic force and the weak nuclear force — within the framework of the Standard Model of particle physics. Proposed by Sheldon Glashow, Abdus Salam, and Steven Weinberg in the 1960s, the electroweak theory showed that at sufficiently high energies, these two forces are manifestations of a single electroweak force, mediated by four massless gauge bosons. At low energies, this symmetry is hidden by the Higgs mechanism, which gives mass to three of the bosons (the W+, W−, and Z0) while leaving the fourth (the photon) massless.

The electroweak symmetry group is SU(2) × U(1). The SU(2) factor governs the weak isospin, a quantum number that distinguishes left-handed fermion doublets (such as the electron and its neutrino, or the up and down quarks). The U(1) factor governs the weak hypercharge, a quantum number related to electric charge through the Gell-Mann–Nishijima formula. The photon and the Z0 boson are mixtures — quantum superpositions — of the gauge bosons associated with these two factors. The mixing angle, called the weak mixing angle or Weinberg angle, determines the relative contributions and is a fundamental parameter of the Standard Model.

The weak force, as manifested at low energies, is responsible for radioactive beta decay, neutrino interactions, and the fusion processes that power stars. It is the only force that changes the flavor of quarks (transforming up-type quarks into down-type and vice versa) and leptons (transforming electrons into neutrinos and vice versa). These flavor-changing interactions are described by the Cabibbo-Kobayashi-Maskawa (CKM) matrix for quarks and the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix for neutrinos.

The electroweak theory was experimentally confirmed through the discovery of the W and Z bosons at CERN in 1983, and through precision measurements of their properties and of weak interaction effects in particle scattering. The theory predicts relationships between the masses of the W and Z bosons, the weak mixing angle, and the Higgs vacuum expectation value that have been verified to high precision.

The unification of the electroweak force with the strong force (quantum chromodynamics) into a single "grand unified theory" (GUT) remains an open problem. In GUT proposals, the SU(3) × SU(2) × U(1) symmetry of the Standard Model is embedded in a larger simple group (such as SU(5) or SO(10)), which would unify all three forces at energies around 10^15 GeV — far beyond current experimental reach.