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Abdus Salam developed the gauge-theory unification of the weak interaction and electromagnetic interaction. The common electroweak framework relates charged and neutral currents, while spontaneous symmetry breaking accounts for massive weak carriers and a massless photon. His authored lecture traces how symmetry principles become quantitative relations among apparently different interactions. This is unification through one underlying structure, rather than a claim that weak and electromagnetic processes have identical observed strengths.^[ref]

A physicist.^[ref]

Beta decay is a weak interaction process changing the proton-to-neutron composition of an atomic nucleus while preserving its mass number. In beta-minus decay, a neutron converts to a proton with an emitted electron and electron antineutrino; in beta-plus decay, a proton converts to a neutron with an emitted Positron and electron neutrino, when the complete energy balance permits it.^[ref]

The emitted beta particle is created in the transition rather than being an ordinary orbital electron expelled from the atom. Its kinetic energy varies because the available Q value is shared with the neutrino and daughter recoil. A magnetic electron spectrometer can separate different emitted momenta; a detector then records a continuous distribution rather than one universal electron energy.

Conservation of electric charge, energy and angular momentum constrains the final state. Nuclear binding matters: a free proton cannot undergo ordinary beta-plus decay, although an energetically allowed proton-to-neutron transition can occur within an appropriate atomic nucleus.

Big Bang nucleosynthesis forms light atomic nuclei during the first minutes of the expanding, cooling universe. Weak interactions initially maintain neutron-proton chemical equilibrium. After cosmological weak freeze-out, free-neutron decay after freeze-out reduces the available neutron supply.

The deuterium bottleneck delays efficient assembly: energetic photons break up newly formed deuterium until sufficient cooling occurs. Most surviving neutrons then enter helium-4, with smaller residual abundances of other light nuclei. If $r=n_n/n_p$ at assembly and nearly all neutrons enter helium, counting two neutrons per helium nucleus gives the approximate helium mass fractionThis links nuclear reactions and particle decay to an astronomical abundance measurement. Comparing primordial deuterium and helium with the independently inferred cosmological baryon density tests the thermal history; the cosmological lithium problem remains an important qualification to the comparison.^[ref]

The bottom quark $b$ is the third-generation down-type quark, with electric charge $-e/3$, spin $1/2$ and baryon number $1/3$. It is a heavy quark; its weak interaction decays make bottom-containing hadrons unstable. The charge assignment is listed alongside the other flavors in ^[ref].

A bottom quark combined with a light antiquark forms the minimal valence content of a bottom meson. Measuring the decay products and displaced decay point of such a hadron helps identify bottom production in a particle detector. The observed particles are hadrons and their descendants, rather than an isolated bottom quark propagating freely through the detector.

Carlo Rubbia helped establish the experimental existence of the massive carriers of the weak interaction, the W boson and Z boson. His lecture explains how proton–antiproton collisions produced candidate events whose energetic leptons and reconstructed kinematics distinguished them from abundant hadronic backgrounds. For decays involving a neutrino, missing transverse momentum supplied information about an unseen particle. The discovery connected electroweak theory with identifiable short-lived particles in a collider detector.^[ref]

A physicist.^[ref]

A Standard Model charged-current box diagram converts $d\bar{s}$ into $s\bar{d}$. Internal quarks run over $u,c,t$, and the flavor sum contains ${\lambda }_i{\lambda }_j$ with ${\lambda }_i=V_{is}^{\ast }{V}_{id}$. The loop function depends on internal masses; CKM matrix unitarity gives ${\sum }_i{\lambda }_i=0$, producing the GIM mechanism cancellation of flavor-independent terms. The amplitude is of order $g^4$ and second order in the weak interaction.

Charles Louis Critchfield and Hans Bethe calculated the rate at which two protons can form a deuteron, initiating the proton–proton chain. In modern notation the reaction is $p+p\to d+e^{+}+{\nu }_e$: one proton changes into a neutron through the weak interaction. Their 1938 calculation combined penetration of the repulsive potential barrier with the probability of the nuclear transition. This joins two physically different obstacles to stellar nuclear fusion: getting the charged particles close together and converting their nuclear composition.^[ref]

A physicist.^[ref][ref]

Chien-Shiung Wu and collaborators tested parity conservation by measuring the beta decay of oriented cobalt-60 nuclei at very low temperature. Their apparatus placed a beta counter inside a demagnetization cryostat and used the accompanying gamma ray anisotropy to monitor nuclear orientation. The emitted electrons favored one direction relative to the nuclear spin. Because parity reverses momentum while leaving angular momentum unchanged, that directional asymmetry distinguishes a process from its mirror image and demonstrated parity violation in the weak interaction.^[ref]

A physicist.^[ref]

The combined operation of charge conjugation and parity symmetry in quantum field theory. A weak interaction can preserve this symmetry while violating each constituent separately.

An experimental nuclear physicist who coauthored the Wu experiment. Comparing beta decay from oriented cobalt-60 nuclei with the opposite orientation tested whether a mirror-reflected process would have the same probability. The observed directional asymmetry showed failure of parity conservation in the weak interaction.^[ref]

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.

A flavor eigenstate is a state with a definite flavor label in the specified interaction or flavor-charge basis. For neutral kaons, $K^0=d\bar{s}$ and ${\bar{K}}^0=s\bar{d}$ have definite opposite strangeness. Weak interactions mix them, so these flavor states are not the mass eigenstates. The precise flavor basis must be specified when discussing mixing.

A hadronic form factor parametrizes a current matrix element between hadron states, separating allowed Lorentz covariant tensor structures from scalar-valued functions of Lorentz invariant combinations of momentum transfer. These functions encode composite structure and strong interaction dynamics. Equal kinematic phase space does not imply equal form factors for different weak interaction transitions.

A matrix element of an interaction operator between hadron states. It contains strong interaction dynamics beyond the explicit CKM matrix factors of a weak interaction decay; its magnitude and strong phase need not be identical for two different channels.

A baryon containing one or more strange quarks and no heavier quark. The light hyperon multiplets include lambda, sigma, xi and omega baryons. Strong and electromagnetic interactions conserve strangeness, whereas a weak interaction can change it; a photon in the final state does not by itself make a hyperon decay electromagnetic.

In the reaction displayed here, an electron antineutrino interacts with a proton to produce a Positron and a neutron. It turns an otherwise hard-to-observe incident particle into two products with different detection signatures. A prompt Positron signal followed by neutron capture can identify a candidate much more selectively than a single energy deposit. Inverse beta decay is a weak interaction; the incident electron antineutrino is detected through its reaction products.

James Cronin coauthored the 1964 discovery of CP violation in neutral kaon decays. Magnetic analysis and spark chambers reconstructed two charged tracks from a long-lived neutral beam. The small two-pion component was incompatible with the simple CP-conserving assignment of that state. A rare decay channel thus tested a fundamental symmetry of the weak interaction.^[ref]

A physicist.^[ref]

A theoretical particle physicist who, with Abdus Salam, proposed treating leptons as a fourth color alongside the three quark colors. The Pati-Salam model places quarks and leptons in related symmetry multiplets, seeking a common organization of the strong interaction, weak interaction and electromagnetic interaction. The enlarged symmetry also motivates possible processes beyond the Standard Model.^[ref]

In 1956, Tsung-Dao Lee and Chen Ning Yang examined the evidence for parity conservation in the weak interaction. Their paper, Question of Parity Conservation in Weak Interactions, distinguished an untested assumption from an established experimental result and proposed comparing electron emission along opposite directions relative to oriented nuclear spins, explicitly considering cobalt-60. It proposed a test; it did not report the later Wu experiment.^[ref]

The conceptual improvement is the choice of observable. An angle between two polar vectors is preserved by spatial reflection; its distribution can conceal parity violation. An angle between an axial vector and a polar vector changes to its supplement. Thus a spin-resolved directional measurement asks a different question from an unpolarized total decay rate.

Neutral B meson mixing converts the flavor eigenstates $B^0=\bar{b}d$ and ${\bar{B}}^0=b\bar{d}$ into one another through the weak interaction. The propagating mass eigenstates are $|B_L⟩=p|B^0⟩+q|{\bar{B}}^0⟩$ and $|B_H⟩=p|B^0⟩-q|{\bar{B}}^0⟩$, with $|p{|}^2+|q{|}^2=1$.

In units $\hslash =c=1$, neglect their width difference and take $|q/p|=1$. Evolution of an initial $B^0$ at proper time $t$ gives the undecayed flavour probabilitieswhere $\Delta m=m_H-m_L$ and $\Gamma$ is the common decay width. Their sum is the survival probability, not one.

For a common final state, mixing and direct decay interfere through ${\lambda }_f=(q/p)({\bar{A}}_f/A_f)$. In the Standard Model, $f=J/\psi K_S$ combines a J/psi meson with a short-lived neutral kaon. Neglecting subleading weak amplitudes and small kaon CP violation,Here the $V_{ij}$ are CKM matrix elements; the minus sign reflects the approximate negative CP parity of this specified final state. A different charmonium-kaon final state need not have that same sign.^[ref]