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The electroweak interaction is the $SU(2{)}_L\times U(1{)}_Y$ gauge theory that unifies the weak and electromagnetic interactions.

An instructive tension motivates the unification: the photon is massless, while the W boson and Z boson are massive; electromagnetic interaction and weak interaction also have different observed low-energy strengths and chiral couplings. A gauge theory can relate their interactions while the Higgs field gives different masses after electroweak symmetry breaking. The resulting theory predicts relationships between processes, not merely a common name for observed phenomena.

Its useful organizing principle is the combination of weak isospin and hypercharge. The surviving electromagnetic generator is $Q=T_3+Y$ in the convention used here. The photon and Z boson are different mixtures of neutral gauge fields, while the charged current uses the charged combinations. The electroweak doublet gauge-boson mass matrix explains why one neutral mixture stays massless. At low energies, a massive W boson propagator instead produces the approximate Fermi interaction, withHere $g$ is the $SU(2{)}_L$ gauge coupling and $v$ the Higgs doublet vacuum expectation value. The inverse squared mediator mass suppresses low-energy weak rates. Unification does not require setting $g$ equal to the electromagnetic coupling $e$: their relation is $e=g\sin {\theta }_W$.

A striking experimental consequence is a neutral current in addition to the familiar charged current. The Gargamelle experiment tests this extra channel through visible charged secondaries, with a background problem fundamentally different from directly identifying a Z boson. The muon-neutrino electron recoil spectrum shows how the same Weinberg angle enters another process quantitatively.

Sheldon Lee Glashow's 1979 lecture, Towards a Unified Theory: Threads in a Tapestry, recalls his 1961 $SU(2)\times U(1)$ construction: a photon, a massive neutral intermediary and a mixing angle. It also stresses its unresolved mass problem: symmetry-breaking masses had been inserted, rather than generated by the Higgs mechanism. Establishing the gauge structure and explaining the gauge boson masses were complementary steps.^[ref]

Steven Weinberg's A Model of Leptons (1967) builds a spontaneously broken gauge theory for the electron-type leptons, using a scalar doublet to give masses while retaining a massless photon. It derives neutral and charged couplings and discusses neutrino-electron scattering. Its closing question about renormalizability is explicitly unresolved; the paper is neither a proof of that property nor a complete treatment of quarks.^[ref]

Abdus Salam's 1979 lecture, Gauge Unification of Fundamental Forces, describes his and Steven Weinberg's completion of the spontaneously broken $SU(2)\times U(1)$ formulation for leptons. He separates that construction from the subsequent establishment of renormalizability. Its role is to connect the mass mechanism with predictive neutral and charged couplings, not to identify a photographed event. The Gargamelle evidence subsequently tested the predicted neutral current.^[ref]

The 1999 recognition of Gerard 't Hooft and Martinus J. G. Veltman concerned the theoretical quantum structure of the electroweak interaction.^[ref] Renormalizability makes repeated radiative corrections predictive from a fixed set of inputs. It does not imply that the perturbation series converges, nor that renormalization alone chooses the correct particle content or excludes all physics beyond the Standard Model.

The Gargamelle collaboration prepared an experimental test by exposing its heavy-liquid bubble chamber to neutrino and antineutrino beams at CERN. The readout selected events producing hadrons without an identified muon or electron, compared with events containing a charged lepton. Its 1974 report assessed alternative origins from neutrons and long-lived neutral kaons, finding them insufficient to explain the sample. That supported neutral currents; it did not isolate a radiative correction or demonstrate renormalizability.^[ref]

The distinction matters when using the tree-level electroweak gauge-boson masses:Beyond tree level, self-energies, interaction vertices and counterterms alter how chosen input observables predict another observable. The Weinberg angle then needs a specified renormalization scheme; inserting its value from one convention into a formula from another can create an apparent discrepancy. A meaningful higher-order test must state both its independent experimental inputs and the calculated loop order.

Gargamelle was a heavy-liquid bubble chamber at CERN. Its 1973 collaboration report, Observation of neutrino-like interactions without muon or electron in the Gargamelle neutrino experiment, by F. J. Hasert and colleagues, described hadronic events with no identified muon or electron and evaluated rates relative to charged current events. This is evidence for a neutral current, rather than direct production or photography of a Z boson.^[ref]

CERN's Gargamelle account identifies the target liquid as $\mathrm{C}{\mathrm{F}}_3\mathrm{Br}$ and distinguishes hadronic events from the separate isolated-electron signal. The latter is an antineutrino transferring momentum to an electron, whose electromagnetic particle shower supplies visible tracks. These are different final states, not two names for one event.^[ref]

A neutrino beam uses protons striking a target to produce charged pions and kaons; focusing selects parents that undergo particle decay in flight. Their neutrinos pass through shielding into the target. Scanning photographs locates vertices and classifies outgoing tracks.^[ref]

Expansion prepares a bubble chamber's superheated liquid; ionization by charged secondaries seeds bubbles for photography.^[ref]

The recorded light reveals bubbles, not a directly visible incoming or outgoing neutrino. Reconstructing a vertex and classifying bubble-chamber tracks connects that optical record to a hypothesis about a particle interaction.

The collaboration compared samples with matched hadronic cuts. A flat longitudinal vertex distribution resembled neutrino interactions, but sideways neutrons and multi-step cascades could also flatten it. The background estimate combined visible parent-interaction/secondary-neutron pairs with a cascade calculation including surrounding material. Later proton exposures tested the predicted cascade lengths. Escaping charged hadrons could imitate muons; there was no independent perfect muon tag.^[ref]

The inference combines positive evidence for an interaction with controlled alternatives for its visible final state. Shielding suppresses unwanted beam components, yet interactions in surrounding material can generate secondary neutrons; shielding alone cannot certify a neutral current. Nor does a successful neutral current search by itself establish every coupling or the Higgs mechanism. Its importance for the electroweak interaction is that the theory predicts an additional weak channel, with coupling relations that further cross section measurements can test.

For an especially simple neutral current example, scatter a muon neutrino elastically from an initially stationary unpolarized electron:Here $T_e$ is the final electron kinetic energy, $E_{\nu }$ is the incident neutrino energy, and ${\theta }_W$ is the Weinberg angle. These are electron chiral couplings in the $g/\cos {\theta }_W$ vertex convention of neutral-current vector and axial couplings. The chosen muon neutrino elastic channel has no additional W boson exchange amplitude; the electron-neutrino elastic channel would have one. Other charged-current final states can become kinematically accessible at sufficiently high incident energies and are not part of this elastic model.

Use natural units $\hslash =c=1$, retain only tree-level Feynman diagrams, and take the contact limit $|q^2|\ll m_Z^2$ with $E_{\nu }\gg m_e$. Neglect electron-mass terms relative to $E_{\nu }$, radiative corrections and detector response. Then$G_F$ is the Fermi constant. The approximate endpoint $y=1$ replaces the exact elastic endpoint $y_{\max }=2E_{\nu }/(m_e+2E_{\nu })$. The omitted leading mass-dependent interference term in $d{\sigma }_{\nu }/dy$ is $-K{g}_L{g}_R{m}_{e}y/E_{\nu }$. Thus the approximation is not appropriate for arbitrarily soft neutrinos.

The shape follows from the two chiral contributions to the massless scattering amplitude. The spin sum gives terms proportional to $s^2$ and $u^2$ in Mandelstam variables; in this limit $u/s=-(1-y)$, giving the square $(1-y{)}^2$. The two electron chiralities do not interfere when the relevant mass corrections are dropped. Replacing the neutrino by its antineutrino exchanges which coupling multiplies each kinematic term. This is the same chiral angular weighting that appears in neutrino scattering on an antiquark, now with a target whose couplings are particularly simple.

Integrating the two elementary terms givesFor an original teaching example choose $s_W^2=1/4$, so $g_L=-1/4$ and $g_R=1/4$. This is a stipulated parameter, not a measurement of the Weinberg angle. ThenThe normalized probability density function is $p(y)=3[1+(1-y{)}^2]/4$, not the unnormalized differential cross section. Its endpoints have ratio two, and the fraction with $y>1/2$ is $13/32$. The antineutrino spectrum happens to coincide at this chosen parameter because $g_L^2=g_R^2$; this equality is not a general prediction for arbitrary Weinberg angles.

Unlike a visible electron recoil, the scattered neutrino is unrecorded. A measured recoil spectrum must therefore be compared with incident neutrino flux, energy distribution, acceptance, detection efficiency and backgrounds. A shape or rate tests a specified theoretical model only after these experimental ingredients have been supplied.

The weak neutral current couples the Z boson to a fermion without changing its electric charge. Its vector and axial-vector coefficients depend on weak isospin, electric charge, and the weak mixing angle.

The neutral current transfers no electric charge at a fermion vertex; “neutral” does not mean that every participating particle is uncharged. For example, ${\nu }_{\mu }{e}^{-}\to {\nu }_{\mu }{e}^{-}$ transfers energy and momentum to an electron through Z boson exchange. By contrast, a charged current can convert a muon neutrino into a muon, as in ${\nu }_{\mu }n\to {\mu }^{-}p$. Both processes conserve total electric charge. A photon also carries no electric charge, but an active neutrino has no electromagnetic charge coupling: its neutral current is a distinct weak interaction.

The experimentally useful contrast on a nucleon target iswhere $N$ is a nucleon and $X$ is an allowed final state of hadrons. The charged current makes a charged lepton available for identification; the outgoing neutrino in a neutral current leaves no bubble-chamber track. A sample of hadronic events without an identified muon therefore poses an experimental question: how many could instead be neutron interactions, or charged current events whose muon was missed? The answer requires a detector-response and background analysis, not merely an inspection for an absent line.

Awarded in equal shares to Sheldon Lee Glashow, Abdus Salam and Steven Weinberg for contributions to unified weak interactions and electromagnetic interactions, including the prediction of the neutral current.^[ref]

The weak interaction changes particle species through the charged current and also acts through the neutral current. Familiar examples include beta-minus decay and neutrino scattering. It is short-range and chiral: its W boson and Z boson couplings distinguish chirality. Above the electroweak scale it belongs to the unbroken $SU(2{)}_L\times U(1{)}_Y$ electroweak interaction. At much lower energies, W boson exchange is approximated by the Fermi interaction.

A particularly direct window into the weak interaction is the Wu experiment: a sample with oriented nuclear spins emits electrons preferentially toward one end of its spin axis. The comparison tests parity conservation, because spin is an axial vector and momentum is a polar vector. The polarized beta-decay angular distribution translates that geometric distinction into a measurable counting asymmetry.

The Weinberg angle rotates the neutral gauge fields into the photon and Z boson:$g$ belongs to $SU(2{)}_L$, $g^{\prime }$ to $U(1{)}_Y$, and $e$ is the positive magnitude of the electron electric charge. These definitions use $Q=T_3+Y$; doubling the definition of hypercharge requires halving its coefficient in the covariant derivative. The electromagnetic coupling of a weak multiplet follows by substituting this rotation.

For a fermion component, the corresponding neutral coefficient isFor a left-handed charged lepton, $T_3=-1/2$ and $Q=-1$, giving $g_L=-1/2+{\sin }^2{\theta }_W$. Its right-handed component has $T_3=0$, giving $g_R={\sin }^2{\theta }_W$. This algebra links the neutral current to the electromagnetic charge assignment. Together with the Higgs doublet, it also gives the tree-level electroweak gauge-boson masses with $m_W/m_Z=\cos {\theta }_W$; precise comparisons require a defined renormalization convention and radiative corrections.