Electroweak Interaction

The electroweak interaction is the unified description of electromagnetic and weak interactions within the Standard Model of particle physics. It explains how two interactions with markedly different everyday manifestations arise from a common theoretical framework. Electromagnetic interactions are mediated by the massless photon, whereas weak interactions involve the massive W and Z bosons. Their contrasting properties result from the way electroweak symmetry is realized in the vacuum, rather than from two wholly independent theories. Electroweak unification does not include the strong interaction or gravity. (nobelprize.org)

Gauge structure

Electroweak theory is a quantum field theory based on the gauge symmetry

The first factor is associated with weak isospin, and the second with weak hypercharge. The subscript $L$ indicates that the weak-isospin interaction acts on left-handed fermion fields. The two factors have separate coupling constants, conventionally denoted $g$ and $g'$; unification therefore does not mean that electromagnetic and weak interactions share one identical coupling. (indico.cern.ch)

Before symmetry breaking, the gauge fields are three weak-isospin fields, $W^1_\mu,W^2_\mu,W^3_\mu$, and a hypercharge field, $B_\mu$. The charged W fields are combinations of $W^1_\mu$ and $W^2_\mu$. The photon and Z fields are different mixtures of $W^3_\mu$ and $B_\mu$. Consequently, the photon cannot simply be identified with the original hypercharge field. (indico.cern.ch)

The mixing is characterized by the weak mixing angle, $\theta_W$. At tree level,

where $e$ is the electromagnetic coupling. These relations connect electromagnetic measurements with weak-interaction parameters and make the unified theory experimentally testable. (pdg.lbl.gov)

Symmetry breaking and particle masses

The Higgs field provides the mechanism that distinguishes the physical electromagnetic and weak sectors. In the minimal Standard Model, it is a complex weak-isospin doublet with nonzero hypercharge. Its nonzero vacuum expectation value leads to

The surviving electromagnetic symmetry keeps the photon massless, while the Higgs mechanism gives masses to the W and Z bosons. This is conventionally described as spontaneous symmetry breaking: the underlying equations retain their gauge structure, but the vacuum selects a particular realization of it. (nobelprize.org)

The electroweak vacuum scale is approximately $v=246$ GeV. At tree level, in natural units,

so that $m_W=m_Z\cos\theta_W$. A physical scalar excitation remains: the Higgs boson. Charged leptons and quarks obtain masses through Yukawa interactions with the Higgs field. These couplings are independent parameters, so the mechanism does not itself predict the observed fermion mass pattern. (pdg.lbl.gov)

“Electroweak unification at high energy” should not be understood as an abrupt merger at one collision energy. At energies much larger than the W and Z masses, those masses become less important to many scattering processes, making the shared gauge structure more apparent. The distinct couplings and quantum numbers nevertheless remain. (nobelprize.org)

Charged and neutral currents

Weak processes fall into charged-current and neutral-current classes. Charged-current interactions exchange a $W^+$ or $W^-$, transferring electric charge between particles. They connect members of weak-isospin doublets, such as a neutrino and its associated charged lepton, or up-type and down-type quark fields. These interactions underlie beta decay and many particle decays. Their chiral structure produces violation of parity, the symmetry associated with spatial reflection. (pdg.lbl.gov)

Weak neutral-current interactions exchange a Z boson without transferring electric charge. They allow neutrinos to scatter from matter without becoming charged leptons. The Z generally couples differently to left- and right-handed fermion components, unlike the photon’s electromagnetic coupling. The large W and Z masses suppress weak processes at low momentum transfer; thus “weak” describes their low-energy effectiveness, not an exceptionally tiny fundamental gauge coupling. (pdg.lbl.gov)

Historical development and experimental evidence

Sheldon Glashow, Steven Weinberg, and Abdus Salam developed the principal electroweak framework through separate contributions during the 1960s. Combining gauge symmetry with symmetry breaking made it possible to describe massive weak-force carriers alongside a massless photon. They shared the 1979 Nobel Prize in Physics for contributions to the unified theory, including its prediction of weak neutral currents. (nobelprize.org)

A decisive experimental milestone was the announcement of weak neutral currents by the Gargamelle collaboration at CERN on July 19, 1973. The UA1 and UA2 experiments subsequently discovered the W and Z bosons in 1983. These observations tested both the existence of the predicted carriers and the structure of their interactions. Precision measurements at the Large Electron–Positron collider later confirmed important quantum corrections. In 2012, the ATLAS and CMS experiments at the Large Hadron Collider discovered the Higgs boson, supplying direct evidence for the physical scalar associated with electroweak symmetry breaking. (home.cern)

Quantum consistency and precision tests

Renormalization makes it possible to express quantum corrections in terms of measurable parameters and obtain finite predictions. Gerardus ’t Hooft and Martinus Veltman established essential aspects of the quantum consistency of electroweak gauge theory; their work was recognized by the 1999 Nobel Prize in Physics. Calculations beyond the simplest approximation became indispensable for comparing theory with precise experiments. (nobelprize.org)

Electroweak tests compare W and Z masses, decay rates, scattering asymmetries, and low-energy neutral-current measurements. Because quantum corrections depend on particles circulating in internal loops, these observables can constrain particles and interactions that are not directly produced. Higgs measurements test the symmetry-breaking sector, while neutrino masses require an extension of the minimal model’s original massless-neutrino formulation. (pdg.lbl.gov)