| Electroweak interaction | |
|---|---|
| Name | Electroweak interaction |
| Caption | Schematic of electroweak unification with symmetry breaking |
| Type | Fundamental interaction |
| Discovered | 1960s |
| Theoretical origin | Gauge theory; Glashow–Weinberg–Salam model |
| Key people | Sheldon Glashow, Steven Weinberg, Abdus Salam |
Electroweak interaction
The electroweak interaction is the unified description of two of the four fundamental forces: the electromagnetic force and the weak nuclear force. Formulated within the framework of quantum field theory, it explains phenomena ranging from beta decay to the properties of the W and Z bosons and provides essential input to the Standard Model of particle physics. Electroweak theory underpins precision tests of quantum interactions and guides searches for physics beyond the Standard Model.
The electroweak interaction emerged from efforts in the mid-20th century to reconcile disparate descriptions of weak and electromagnetic phenomena. Early work on weak processes such as beta decay and the discovery of parity violation by Chien-Shiung Wu highlighted the need for a quantum theory of weak interactions. The theoretical synthesis known as the Glashow–Weinberg–Salam model was developed by Sheldon Glashow, Steven Weinberg, and Abdus Salam in the 1960s, employing non-Abelian gauge theory techniques introduced by Yang–Mills theory. The subsequent prediction and discovery of heavy gauge bosons (W and Z) at the Super Proton Synchrotron and measurements at the Large Electron–Positron Collider (LEP) and Large Hadron Collider (LHC) established electroweak theory as a cornerstone of modern particle physics.
Electroweak theory is based on the gauge group SU(2)_L × U(1)_Y, combining an isospin-like SU(2) symmetry acting on left-handed fermions with a hypercharge U(1) symmetry. The gauge fields associated with SU(2)_L are the triplet W^a, while the U(1)_Y field is denoted B. Gauge invariance constrains interactions and ensures renormalizability, following methods developed by Gerard 't Hooft and Martinus Veltman. Spontaneous symmetry breaking via the Higgs mechanism reduces SU(2)_L × U(1)_Y to the electromagnetic U(1)_EM, giving mass to the W and Z bosons while leaving the photon massless. This mechanism involves the Higgs field and its scalar excitation, the Higgs boson, discovered at the CERN ATLAS and CMS collaborations.
The electroweak Lagrangian combines kinetic terms for gauge fields, chiral fermion kinetic and Yukawa terms, and the Higgs scalar potential. Left-handed fermions form SU(2)_L doublets (e.g., e_L with ν_e_L), while right-handed components are SU(2)_L singlets with distinct hypercharges. Yukawa couplings generate fermion masses after symmetry breaking, linking to the pattern observed in quark and lepton masses through the Cabibbo–Kobayashi–Maskawa matrix (CKM) and potential leptonic mixing in the Pontecorvo–Maki–Nakagawa–Sakata matrix (PMNS). The Lagrangian predicts interactions mediated by the photon, W^±, and Z^0, and accommodates radiative corrections treated within renormalization.
Key parameters include the SU(2) and U(1) gauge couplings (g and g'), the Higgs vacuum expectation value (v ≈ 246 GeV), the weak mixing angle (θ_W, often sin^2θ_W), and fermion Yukawa couplings. Precision electroweak observables—such as the Z boson mass (m_Z), the W mass (m_W), Z-pole asymmetries measured at LEP and the SLC, and the muon lifetime—provide stringent tests of the theory. Calculations of radiative corrections employ techniques from perturbative quantum field theory and higher-order computations by groups using frameworks like Dimensional regularization. Discrepancies in precision fits can signal contributions from oblique parameters (S, T, U) or from virtual effects of new particles predicted in extensions like supersymmetry.
Electroweak interactions govern a wide set of processes: charged-current interactions (mediated by W bosons) such as nuclear beta decay and neutrino scattering; neutral-current interactions mediated by the Z boson observed in deep inelastic scattering and e^+e^- annihilation; and electromagnetic processes unified at high energies. Phenomenology includes computations of cross sections, decay widths, and branching ratios for processes studied at facilities like Fermilab and CERN. Electroweak radiative corrections are essential for interpreting results in collider physics, neutrino physics, and precision low-energy experiments (e.g., measurements of parity violation in atomic systems and the anomalous magnetic moment of the muon (g−2)).
Electroweak theory is a stepping stone toward grander unification schemes. Grand Unified Theories (GUTs) such as SU(5) and SO(10) embed SU(2)_L × U(1)_Y into larger gauge groups, predicting relations among couplings and proton decay. Electroweak symmetry breaking and the naturalness of the Higgs mass motivate proposals like supersymmetry (SUSY), composite Higgs models, and extra-dimensional scenarios (e.g., Randall–Sundrum model). Electroweak baryogenesis mechanisms seek to explain the cosmic matter–antimatter asymmetry via CP-violating processes during the electroweak phase transition, connecting to cosmology and searches at colliders and in low-energy experiments.
The discovery of neutral currents in 1973 at the Gargamelle bubble chamber first confirmed electroweak neutral interactions. The direct observation of W and Z bosons at the Super Proton Synchrotron in 1983 provided decisive evidence for the gauge bosons predicted by the Glashow–Weinberg–Salam model. Precision measurements at LEP, SLC, and the Tevatron refined determinations of sin^2θ_W, m_Z, and electroweak couplings, while the 2012 discovery of the Higgs boson at CERN completed the particle content. Ongoing experiments at LHC, neutrino facilities like NOvA and DUNE, and precision low-energy programs continue to probe electroweak parameters and search for deviations indicating new physics.
Category:Quantum physics Category:Standard Model Category:Gauge theories