Particle & Nuclear Physics

The Standard Model

The Standard Model is the theoretical framework describing all known elementary particles and three of the four fundamental forces (electromagnetic, weak, strong). It classifies particles into fermions (matter particles, half-integer spin) and bosons (force carriers, integer spin).

Overview table

Class	Category	Particles
Fermions	Quarks	up, down, charm, strange, top, bottom
Fermions	Leptons	electron, muon, tau + their neutrinos
Bosons	Gauge	photon, gluon, W±, Z
Bosons	Scalar	Higgs

• Generations — fermions are organized into three generations of increasing mass. Generation I (electron, electron neutrino, up, down) makes up stable matter; Generations II and III are heavier and unstable.
• Antiparticles — every fermion has an antiparticle with the same mass but opposite charge (e.g., positron is the electron’s antiparticle). Bosons are their own antiparticles (except W+ and W−, which are each other’s).

Quarks

• Six flavors — up (u), down (d), charm (c), strange (s), top (t), bottom (b). Grouped in pairs by generation: (u, d), (c, s), (t, b).
• Charges — up-type (u, c, t) carry +2/3 charge; down-type (d, s, b) carry −1/3 charge.
• Color charge — quarks carry one of three “color” charges (red, green, blue) — not actual color, but the quantum number of the strong force. Gluons carry combinations of color and anticolor. The concept of color charge as a new quantum number was introduced by Oscar Greenberg (1964) to resolve the spin-statistics problem in baryon wave functions (e.g., Δ++ = uuu).
• Confinement — quarks are never observed in isolation; they are always bound in hadrons (baryons or mesons). Attempting to pull quarks apart creates new quark-antiquark pairs.
• Top quark — heaviest elementary particle known; too short-lived to form hadrons; decays almost exclusively to a W boson and a bottom quark.
• Bottom (beauty) quark — discovered at Fermilab (1977) via the Υ (Upsilon) meson. Charm quark — discovered in 1974 via the J/ψ particle (Nobel Prize to Richter and Ting).
• Quark model — proposed independently by Murray Gell-Mann and George Zweig in 1964.
• J/ψ meson — charmonium state (cc̄); simultaneously discovered at BNL by Ting (called J) and at SLAC by Richter (called ψ) on November 11, 1974; its anomalously long lifetime proved the existence of the charm quark and triggered the “November Revolution.”
• November Revolution (1974) — the simultaneous independent discovery of the J/ψ meson, confirming the charm quark and the GIM mechanism; transformed the field by establishing the four-quark model as correct and galvanizing the Standard Model consensus.
• Top quark discovery (1995) — discovered at Fermilab’s Tevatron by the CDF and DØ collaborations using proton-antiproton collisions at 1.8 TeV; mass ~173 GeV/c², making it the heaviest known elementary particle.
• GIM mechanism — proposed by Glashow, Iliopoulos, and Maiani (1970) to suppress flavor-changing neutral currents; predicted a fourth quark (charm) as required; confirmed by J/ψ discovery.

Leptons

• Six leptons — electron (e−), muon (μ−), tau (τ−), and their respective neutrinos: νe, νμ, ντ.
• Electron — lightest charged lepton; stable; mass ~0.511 MeV/c².
• Muon — ~207 times heavier than the electron; decays to an electron plus two neutrinos; mean lifetime ~2.2 μs.
• Tau — heaviest lepton (~1.78 GeV/c²); mean lifetime ~2.9 × 10⁻¹³ s; discovered at SLAC (1975) by Martin Perl.
• Neutrinos — electrically neutral; nearly massless (but non-zero mass, confirmed by oscillation experiments); interact only via the weak force and gravity. Solar neutrino problem resolved by SNO (2001) showing flavor oscillation.
• Lepton number — conserved separately for each generation in Standard Model interactions; neutrino oscillation violates generational lepton numbers but preserves total.

Bosons and Force Carriers

• Photon (γ) — carrier of the electromagnetic force; massless; spin-1; travels at speed c; range is infinite.
• Gluon (g) — carrier of the strong nuclear force; massless; spin-1; carries color charge itself, so gluons interact with each other. Range is very short (~10⁻¹⁵ m).
• W± and Z bosons — carriers of the weak nuclear force; massive (W: ~80.4 GeV/c², Z: ~91.2 GeV/c²); this mass gives the weak force its very short range. Discovered at CERN (1983) by Carlo Rubbia and Simon van der Meer (Nobel Prize).
• Higgs boson — scalar (spin-0) boson; gives mass to other particles via the Higgs mechanism; mass ~125 GeV/c²; discovered at CERN’s LHC in 2012 (ATLAS and CMS collaborations); Nobel Prize to Peter Higgs and François Englert (2013).
• Graviton — hypothesized carrier of gravity; not part of the Standard Model; not yet detected.

The Four Fundamental Forces

Force	Carrier	Relative strength	Range	Acts on
Strong	Gluon	1	~10⁻¹⁵ m	Quarks, gluons
Electromagnetic	Photon	~10⁻²	Infinite	Charged particles
Weak	W±, Z	~10⁻⁶	~10⁻¹⁸ m	All fermions
Gravity	Graviton (hypothetical)	~10⁻³⁸	Infinite	All mass-energy

• Electroweak unification — electromagnetic and weak forces are unified at energies above ~100 GeV; described by Glashow, Salam, and Weinberg (Nobel Prize 1979).
• QCD (Quantum Chromodynamics) — the quantum field theory of the strong force; analogous to QED for electromagnetism. Asymptotic freedom — quarks interact more weakly at shorter distances (higher energies); Gross, Politzer, Wilczek (Nobel 2004).
• QED (Quantum Electrodynamics) — the quantum theory of the electromagnetic force; most precisely tested theory in physics. Developed principally by Feynman, Schwinger, and Tomonaga (Nobel 1965).
• Renormalization — mathematical procedure in QFT that absorbs the infinite self-energy corrections that arise in loop diagrams into redefined (physical) masses and coupling constants; originally resisted by Dirac but essential to making QED and the Standard Model predictive; ‘t Hooft and Veltman proved it works for the non-Abelian electroweak theory.
• Running coupling constant — in QFT, coupling “constants” vary with the energy scale at which they are measured; in QED the fine-structure constant α increases slightly at high energies; in QCD the strong coupling αs decreases at high energies (asymptotic freedom) and increases at low energies (confinement).
• Feynman diagrams — pictorial representation of the perturbative terms in a QFT calculation; external lines are particles, internal lines are propagators, vertices are interaction points; the amplitude for a process is the sum over all topologically distinct diagrams.
• V-A theory — the “vector minus axial-vector” structure of the weak charged-current interaction, proposed by Feynman and Gell-Mann (1958) and independently by Sudarshan and Marshak; explains maximal parity violation and correctly predicts the helicity of neutrinos (left-handed only in Standard Model).
• Higgs mechanism — process by which gauge bosons acquire mass through spontaneous symmetry breaking of a scalar field; in the electroweak theory the Higgs field has a non-zero vacuum expectation value (~246 GeV), breaking SU(2)×U(1) down to U(1)_EM and giving the W and Z masses while leaving the photon massless; proposed independently in 1964 by Higgs, and by Brout and Englert, and by Guralnik, Hagen, and Kibble.
• Spontaneous symmetry breaking — a phenomenon in which the ground state (vacuum) of a system does not share the symmetry of the governing equations; Nambu applied this concept from condensed matter (superconductivity) to particle physics; leads to Goldstone bosons (massless) in global symmetry breaking, or to massive gauge bosons when a local (gauge) symmetry is broken (Higgs mechanism).
• CKM matrix — Cabibbo-Kobayashi-Maskawa matrix; a 3×3 unitary matrix parameterizing the mixing of quark flavors in charged weak interactions; originally the Cabibbo angle (mixing d and s) was extended to three generations by Kobayashi and Maskawa (1973) to accommodate CP violation; Nobel Prize 2008.
• CP violation — violation of the combined charge-conjugation and parity symmetry; first observed in neutral kaon decays (1964) by Cronin and Fitch (Nobel 1980); also seen in B mesons; the matter-antimatter asymmetry of the universe requires CP violation beyond what the Standard Model provides.
• Color confinement — the property of QCD that prevents free quarks or gluons from being observed; at large distances the QCD potential grows linearly with separation, so separating a quark-antiquark pair creates new pairs rather than free quarks; no analytic proof from first principles, but confirmed by lattice QCD simulations.
• Hypercharge — the weak hypercharge Y is the quantum number that, combined with weak isospin, gives electric charge via the Gell-Mann-Nishijima relation Q = I₃ + Y/2; the U(1) factor in the Standard Model gauge group SU(3)×SU(2)×U(1) corresponds to hypercharge.
• Cabibbo angle — the mixing angle θ_C ≈ 13° between the d and s quark mass eigenstates in charged weak currents, introduced by Nicola Cabibbo in 1963; explained why strangeness-changing weak decays are suppressed relative to strangeness-preserving ones; later generalized to the CKM matrix.

Beyond the Standard Model

• Grand Unified Theories (GUTs) — frameworks that unify the strong, electromagnetic, and weak forces into a single gauge symmetry at very high energies (~10¹⁵–10¹⁶ GeV). The simplest GUT, SU(5) by Georgi and Glashow (1974), predicts proton decay; proton lifetime bounds from Super-Kamiokande have ruled out minimal SU(5).
• Seesaw mechanism — theoretical explanation for the extreme lightness of neutrino masses; posits that each light neutrino is paired with a very heavy right-handed Majorana neutrino; the product of the two masses is fixed by the electroweak scale, so a very heavy partner implies a very light active neutrino. The mechanism naturally generates the tiny neutrino masses seen in oscillation experiments.
• Majorana neutrino — a fermion that is its own antiparticle, as proposed by Ettore Majorana (1937); if neutrinos are Majorana particles, neutrinoless double-beta decay (0νββ) should occur; current experiments (KamLAND-Zen, GERDA) have not confirmed it.
• Hierarchy problem — the unsolved puzzle of why the Higgs mass (~125 GeV) is so much lighter than the Planck mass (~10¹⁹ GeV) despite quantum corrections that naturally push it toward the higher scale; a primary motivation for supersymmetry, extra dimensions, and other beyond-Standard-Model theories.
• Theory of Everything (TOE) — a hypothetical framework that would unify all four fundamental forces including gravity; no experimentally confirmed TOE exists. String theory is the leading candidate but lacks testable low-energy predictions to date.
• Supersymmetry (SUSY) — a theoretical symmetry that pairs every Standard Model boson with a fermionic “superpartner” and vice versa (e.g., selectron for electron, neutralino for photon/Higgs/Z). Would resolve the hierarchy problem and provide a dark matter candidate (lightest supersymmetric particle, LSP). No superpartners have been found at the LHC as of 2026.
• String/superstring theory — a theoretical framework in which the fundamental objects are one-dimensional strings rather than point particles; requires extra spatial dimensions and naturally incorporates gravity. Superstring theory adds supersymmetry; five consistent versions were unified under M-theory by Witten (1995). Not yet experimentally testable at accessible energies.
• Dark matter — non-luminous, non-baryonic matter making up ~27% of the universe’s energy content; inferred from galactic rotation curves, gravitational lensing, and large-scale structure. Its particle identity is unknown; no confirmed direct detection as of 2026.
• WIMPs (Weakly Interacting Massive Particles) — hypothetical dark matter candidates with masses ~10–1000 GeV and weak-scale cross sections; naturally arise in supersymmetric models (LSP). Extensively searched via underground direct-detection experiments (LUX-ZEPLIN, XENONnT) with no confirmed signal.

Hadrons

Hadrons are composite particles made of quarks; divided into baryons and mesons.

• Baryons — made of three quarks; examples: proton (uud), neutron (udd), lambda (Λ), sigma (Σ), xi (Ξ), omega (Ω−). Baryons have half-integer spin.
• Mesons — made of one quark and one antiquark; examples: pion (π), kaon (K), J/ψ, Υ (Upsilon). Mesons have integer spin.
• Proton — stable (no confirmed decay); charge +1; mass ~938.3 MeV/c².
• Neutron — stable inside nuclei; free neutron decays to proton + electron + antineutrino with half-life ~10 min; mass ~939.6 MeV/c² (slightly heavier than proton).
• Pion (π) — lightest meson; mediates the residual strong force between nucleons. Three charge states: π+, π−, π0.
• Baryon number — conserved in all known interactions; protons and neutrons each have B = 1.
• Eightfold Way — classification scheme for hadrons by strangeness and charge proposed by Gell-Mann (and independently Yuval Ne’eman, 1961); predicted the Ω− particle, confirmed 1964.
• Isospin — approximate symmetry quantum number (I) reflecting the near-equal masses of the up and down quarks; proton and neutron form an isospin doublet (I = 1/2). Introduced by Heisenberg (1932); conserved by the strong force but not by the electromagnetic or weak forces.
• Strangeness — quantum number (S) assigned to hadrons containing strange quarks; S = −1 per strange quark. Proposed by Gell-Mann and Nishijima (1953) to explain why certain particles were produced rapidly (strong force) but decayed slowly (weak force, which changes strangeness).
• Resonances — short-lived hadron excitations (e.g., Δ(1232), ρ meson) with lifetimes ~10⁻²³ s; too brief to leave a detector track and detected only as peaks in cross-section vs. energy plots. They represent excited states of ordinary hadrons.

Nuclear Structure

• Atomic nucleus — composed of protons (Z) and neutrons (N); together called nucleons. Mass number A = Z + N.
• Isotopes — atoms of the same element (same Z) with different N; e.g., carbon-12 (6p, 6n) and carbon-14 (6p, 8n).
• Nuclear binding energy — energy required to completely separate a nucleus into individual nucleons. Expressed via the mass defect: actual nuclear mass is less than the sum of constituent masses (E = mc²).
• Binding energy per nucleon — peaks at iron-56 (⁵⁶Fe), the most tightly bound nucleus. Fusing lighter nuclei or splitting heavier ones both release energy, because both move toward this peak.
• Nuclear radius — scales as r ≈ r₀ A^(1/3), where r₀ ≈ 1.2 fm (femtometers). Nuclear density is roughly constant regardless of size.
• Shell model — nuclei have “magic numbers” of protons or neutrons (2, 8, 20, 28, 50, 82, 126) at which shells are filled and nuclei are especially stable; analogous to electron shells in atoms.
• Strong nuclear force (residual) — holds nucleons together despite proton-proton electrostatic repulsion; mediated by pion exchange; effective only over ~1–3 fm.

Radioactive Decay

• Alpha (α) decay — emission of a helium-4 nucleus (2p + 2n); reduces A by 4 and Z by 2. Cannot penetrate a sheet of paper; stopped by skin.
• Beta-minus (β−) decay — a neutron converts to a proton, emitting an electron and an electron antineutrino; Z increases by 1. Penetrates paper; stopped by a few mm of aluminum.
• Beta-plus (β+) decay — a proton converts to a neutron, emitting a positron and an electron neutrino; Z decreases by 1. Positron annihilates with an electron, producing two 511 keV photons (used in PET scanning).
• Electron capture — proton captures an orbital electron, emitting a neutrino; Z decreases by 1; alternative to β+ in proton-rich nuclei.
• Gamma (γ) decay — emission of a high-energy photon from an excited nuclear state; no change in Z or A. Penetrates deep into matter; requires lead or thick concrete shielding.
• Half-life (t½) — time for half of a radioactive sample to decay; governed by N(t) = N₀ e^(−λt), where λ = ln2 / t½.
• Carbon-14 dating — exploits the 5,730-year half-life of ¹⁴C to date organic material up to ~50,000 years old. ¹⁴C is continuously produced in the atmosphere by cosmic-ray neutrons.
• Decay series — heavy nuclides decay through a chain of alpha and beta decays to a stable lead isotope; e.g., uranium-238 → lead-206 through 14 steps.

Fission

• Nuclear fission — a heavy nucleus splits into two or more lighter fragments, releasing energy (2–3 neutrons and ~200 MeV per fission of ²³⁵U).
• Critical mass — the minimum mass of fissile material needed to sustain a chain reaction; depends on geometry and surrounding materials (reflectors).
• Chain reaction — each fission releases neutrons that can trigger further fissions. Controlled in a reactor; uncontrolled in a bomb.
• Fissile vs fissionable — fissile nuclides (²³⁵U, ²³⁹Pu) can be split by thermal (slow) neutrons; fissionable nuclides (²³⁸U) require fast neutrons.
• Chicago Pile-1 — first self-sustained nuclear chain reaction; achieved under Enrico Fermi at the University of Chicago, December 2, 1942.
• Nuclear reactor — uses controlled fission; moderator (water, graphite) slows neutrons; control rods (boron, cadmium) absorb neutrons. PWR (pressurized water reactor) and BWR (boiling water reactor) are the most common designs.
• Manhattan Project — U.S. program (1942–1945) to develop nuclear weapons; produced two bomb designs: gun-type (Little Boy, ²³⁵U) and implosion (Fat Man, ²³⁹Pu).

Fusion

• Nuclear fusion — light nuclei combine to form a heavier nucleus, releasing energy; most efficient for hydrogen isotopes deuterium (²H) and tritium (³H) → helium-4 + neutron + 17.6 MeV.
• Stellar nucleosynthesis — stars fuse hydrogen to helium (main sequence); heavier elements are built up through successive fusion stages; elements heavier than iron are produced in supernova explosions or neutron-star mergers (r-process).
• pp chain — proton-proton chain: the dominant fusion pathway in stars like the Sun; net result is 4¹H → ⁴He + 2e+ + 2νe + energy.
• CNO cycle — carbon-nitrogen-oxygen catalytic cycle; dominates fusion in stars more massive than ~1.3 solar masses.
• Magnetic confinement (tokamak) — uses magnetic fields to confine hot plasma; major projects: JET (UK) and ITER (under construction in France). ITER aims at 500 MW output from 50 MW input (Q = 10).
• Inertial confinement — uses intense lasers to compress and ignite a fuel pellet; demonstrated at NIF (National Ignition Facility); achieved ignition (Q > 1) in December 2022.
• H-bomb (thermonuclear weapon) — uses a fission primary to ignite fusion fuel (Li deuteride); yields in the megaton range.

Key Experiments and Accelerators

• Rutherford gold-foil experiment (1909–1911) — fired alpha particles at thin gold foil; large-angle scattering proved the atom has a small, dense, positive nucleus, disproving Thomson’s “plum pudding” model. Performed by Geiger and Marsden under Rutherford.
• J.J. Thomson (1897) — discovered the electron via cathode ray deflection; measured charge-to-mass ratio; Nobel Prize 1906.
• Millikan oil-drop experiment (1909) — measured the fundamental electric charge e ≈ 1.6 × 10⁻¹⁹ C.
• Chadwick’s neutron (1932) — James Chadwick identified the neutron by bombarding beryllium with alpha particles; Nobel Prize 1935.
• Positron discovery (1932) — Carl Anderson observed the positron in cosmic ray tracks, confirming Dirac’s prediction; Nobel Prize 1936.
• Cowan-Reines neutrino detection (1956) — first direct detection of the neutrino (electron antineutrino) using reactor flux at Savannah River; Nobel Prize to Reines (1995).
• Wu experiment (1956) — Chien-Shiung Wu demonstrated parity violation in weak interactions (beta decay of ⁶⁰Co), confirming Lee and Yang’s theoretical prediction (Nobel 1957, to Lee and Yang; Wu was excluded).
• SLAC deep inelastic scattering (1968) — electron-proton scattering showed protons have internal point-like structure (quarks); Nobel Prize to Friedman, Kendall, Taylor (1990).
• LHC (Large Hadron Collider) — CERN, Geneva; world’s highest-energy particle collider; 27 km circumference; proton-proton collisions at up to 13.6 TeV. Hosts ATLAS, CMS, ALICE, and LHCb detectors.
• Higgs discovery (2012) — announced July 4, 2012, by ATLAS and CMS at CERN; Higgs mass ~125 GeV/c².
• Tevatron — Fermilab proton-antiproton collider; discovered top quark (1995); operated 1983–2011.
• Bubble chamber / cloud chamber — early particle detectors using superheated liquid / supersaturated vapor to visualize charged-particle tracks.
• Cyclotron — early circular accelerator invented by Ernest Lawrence (Nobel 1939); accelerates particles in a spiral path using alternating electric fields and a static magnetic field.
• UA1 and UA2 experiments — proton-antiproton collider experiments at CERN’s SPS (1983) led by Carlo Rubbia (UA1); discovered the W± and Z bosons, confirming electroweak unification; van der Meer’s stochastic cooling technique was essential for producing the intense antiproton beam.
• Homestake experiment / Davis experiment — Raymond Davis Jr. used a 100,000-gallon chlorine tank in the Homestake Mine, South Dakota, to detect solar neutrinos (1968–1994); consistently measured only ~1/3 of the flux predicted by the Standard Solar Model, establishing the solar neutrino problem; Nobel Prize 2002 (shared with Koshiba).
• Solar neutrino problem — the decades-long discrepancy between theoretical solar neutrino flux (Bahcall) and observed flux (Davis); resolved in 2001–2002 when SNO (Sudbury Neutrino Observatory) proved neutrinos oscillate between flavors, implying non-zero mass; Davis and Koshiba shared the 2002 Nobel Prize.
• Super-Kamiokande — large underground water Cherenkov detector in Japan; in 1998 reported strong evidence for atmospheric neutrino oscillation (νμ → ντ), the first convincing demonstration that neutrinos have mass; also monitors solar and supernova neutrinos.
• SNO (Sudbury Neutrino Observatory) — heavy-water Cherenkov detector in Ontario; in 2001–2002 measured both charged-current (νe only) and neutral-current (all flavors) solar neutrino fluxes, proving the total flux matched solar models and that electron neutrinos had oscillated into other flavors; Nobel Prize 2015 to Art McDonald.
• Takaaki Kajita and Arthur McDonald — shared the Nobel Prize in Physics 2015 for discovery of neutrino oscillations (Kajita: Super-K atmospheric; McDonald: SNO solar), establishing that neutrinos have mass.
• IceCube Neutrino Observatory — cubic-kilometer neutrino detector buried 1.5–2.5 km deep in the Antarctic ice sheet at the South Pole; detects high-energy astrophysical neutrinos; in 2013 reported the first detection of high-energy cosmic neutrinos of extragalactic origin.
• ATLAS and CMS — the two general-purpose detectors at the LHC; each has ~3,000–4,000 collaborating scientists; both independently confirmed the Higgs boson signal on July 4, 2012; designed to look for new physics at the TeV scale.
• LHCb — dedicated LHC experiment designed to study b-hadron decays and CP violation; has made precision measurements of the CKM matrix and discovered several exotic hadrons (tetraquarks, pentaquarks).
• ALICE — LHC detector optimized for heavy-ion (Pb-Pb) collisions; studies the quark-gluon plasma (QGP), the state of matter that existed microseconds after the Big Bang.
• Fermilab (Fermi National Accelerator Laboratory) — premier U.S. high-energy physics laboratory in Batavia, Illinois; home to the Tevatron (proton-antiproton collider, 1987–2011); discovered the top quark (1995) and tau neutrino (DONUT, 2000); current focus on neutrino physics (NOvA, DUNE) and muon g-2.
• SLAC (Stanford Linear Accelerator Center) — 3.2 km linear electron accelerator in California; site of the 1968 deep inelastic scattering experiments that revealed quark structure; also discovered the J/ψ, the tau lepton, and contributed to B-meson CP violation (BaBar experiment).
• CERN — European Organization for Nuclear Research near Geneva, Switzerland; world’s largest particle physics laboratory; built the SPS, LEP, and LHC; birthplace of the World Wide Web (Tim Berners-Lee, 1989).
• Quark-gluon plasma (QGP) — a state of matter in which quarks and gluons are deconfined and move freely; existed in the early universe (~10⁻⁶ s after the Big Bang); recreated in heavy-ion collisions at RHIC (Brookhaven) and ALICE at the LHC; behaves as a near-perfect liquid with very low viscosity.
• verify: Cronin-Fitch kaon CP violation (1964) — confirm that Cronin and Fitch observed CP violation in K_L → π+π− decays at BNL, and that they received the Nobel Prize in 1980 (not an earlier year).
• verify: DONUT experiment (2000) — confirm that the tau neutrino was directly detected by the DONUT collaboration at Fermilab in 2000, completing the third-generation lepton sector.
• verify: Georgi-Glashow SU(5) GUT (1974) — confirm that Howard Georgi and Sheldon Glashow published the minimal SU(5) grand unified theory in 1974, not 1973 or 1975.

Antimatter

• Dirac equation (1928) — relativistic quantum equation for the electron predicted the existence of a positive-energy antielectron (positron).
• Annihilation — when a particle meets its antiparticle, both are destroyed and their mass converts entirely to energy (usually photons). Electron-positron annihilation yields two 511 keV gamma rays.
• Matter-antimatter asymmetry (CP violation) — the observable universe contains far more matter than antimatter, unexplained by the Standard Model. CP violation (violation of charge-parity symmetry) observed in kaon (1964) and B-meson systems.
• Antihydrogen — simplest antiatom; first created at CERN (1995); trapped and studied at ALPHA experiment.
• PET scan — positron emission tomography; a medical imaging technique exploiting the 511 keV photon pairs from positron annihilation.

Key Figures

• Ernest Rutherford — nuclear model of the atom; first artificial nuclear transmutation; discovered alpha/beta radiation; Nobel Prize in Chemistry 1908.
• Niels Bohr — early quantum model of atomic electron orbits; principle of complementarity; Nobel Prize 1922.
• Enrico Fermi — theory of beta decay; first controlled nuclear chain reaction (Chicago Pile-1); Nobel Prize 1938.
• Richard Feynman — formulated QED using path integrals and Feynman diagrams; Nobel Prize 1965 (shared with Schwinger and Tomonaga).
• Murray Gell-Mann — quark model; Eightfold Way classification of hadrons; Nobel Prize 1969.
• Paul Dirac — relativistic quantum mechanics; predicted the positron; Nobel Prize 1933.
• Wolfgang Pauli — exclusion principle; predicted the neutrino to conserve energy in beta decay; Nobel Prize 1945.
• Werner Heisenberg — uncertainty principle (ΔxΔp ≥ ℏ/2); matrix mechanics formulation of quantum mechanics; Nobel Prize 1932.
• Abdus Salam, Sheldon Glashow, Steven Weinberg — electroweak unification; Nobel Prize 1979.
• Peter Higgs and François Englert — Higgs mechanism (independently proposed 1964); Nobel Prize 2013.
• Carlo Rubbia and Simon van der Meer — discovery of W and Z bosons at CERN; Nobel Prize 1984.
• Chien-Shiung Wu — experimental proof of parity violation in weak interactions (1956).
• Ernest Lawrence — invented the cyclotron; Nobel Prize 1939.
• Oscar Greenberg — proposed color charge as a new quark quantum number (1964) to save the spin-statistics theorem for baryons containing identical quarks; foundational to QCD.
• Gerard ‘t Hooft — proved that spontaneously broken non-Abelian gauge theories (including the electroweak theory) are renormalizable (1971), placing the electroweak model on rigorous mathematical footing; Nobel Prize 1999 (shared with Martinus Veltman).
• Henri Becquerel — discovered radioactivity in 1896 by observing that uranium salts spontaneously fog photographic plates; Nobel Prize in Physics 1903 (shared with Pierre and Marie Curie).
• James Clerk Maxwell — formulated classical electromagnetism in Maxwell’s equations (1860s), unifying electricity, magnetism, and optics and predicting electromagnetic waves traveling at speed c; foundational to QED and gauge theory.
• Max Planck — introduced the quantum hypothesis (1900) to explain blackbody radiation, proposing energy is emitted in discrete quanta E = hf; initiated quantum mechanics; Nobel Prize 1918.
• Charles-Augustin de Coulomb — established the inverse-square law of electrostatic force (Coulomb’s law, 1785), F = kq₁q₂/r²; the SI unit of electric charge is named for him.
• Hideki Yukawa — predicted the existence of the pion (pi meson) in 1935 as the carrier of the residual strong nuclear force between nucleons; Nobel Prize 1949; first Japanese Nobel laureate in physics.
• Julian Schwinger — independently developed QED using an operator formalism distinct from Feynman’s path-integral approach; Nobel Prize 1965 (shared with Feynman and Tomonaga); known for extremely formal, dense lecture style.
• Sin-Itiro Tomonaga — independently developed QED in Japan during World War II; showed renormalization could tame infinities; Nobel Prize 1965 (shared with Feynman and Schwinger).
• George Zweig — independently proposed the quark model in 1964 (calling the constituents “aces”); his CERN preprint was not published in a journal at the time, giving Gell-Mann the greater share of credit.
• Chen-Ning Yang and Robert Mills — proposed the Yang-Mills gauge theory in 1954, generalizing Maxwell’s gauge invariance to non-Abelian (SU(2)) symmetry groups; the mathematical backbone of both the electroweak theory and QCD.
• Martinus Veltman — collaborator with ‘t Hooft on proving electroweak renormalizability; developed the Schoonschip computer algebra system for loop calculations; Nobel Prize 1999 (shared with ‘t Hooft).
• Yoichiro Nambu — pioneered the concept of spontaneous symmetry breaking in particle physics (1960–1961), inspiring the Higgs mechanism; also proposed the color degree of freedom and early string theory; Nobel Prize 2008.
• Robert Brout — with François Englert, published the Brout-Englert-Higgs mechanism in August 1964, showing how gauge bosons acquire mass; Brout died in 2011 before the Nobel was awarded.
• Martin Perl — discovered the tau lepton at SLAC (1975) using the SPEAR electron-positron storage ring; Nobel Prize 1995 (shared with Frederick Reines); established that there are (at least) three lepton generations.
• Frederick Reines — co-detected the neutrino with Clyde Cowan (1956) at the Savannah River reactor; Nobel Prize 1995 (shared with Perl); Cowan died in 1974 before the prize was awarded.
• Samuel Ting — co-discovered the J/ψ meson at Brookhaven (1974) simultaneously with Burton Richter at SLAC; Nobel Prize 1976 (shared with Richter); led to the “November Revolution.”
• Burton Richter — co-discovered the J/ψ at SLAC (1974) in the SPEAR ring; Nobel Prize 1976 (shared with Ting); the particle’s long lifetime implied a new conservation law (charm).
• David Gross, David Politzer, and Frank Wilczek — proved asymptotic freedom in QCD in 1973: the strong coupling constant decreases at short distances (high energies), explaining why quarks behave as free particles in deep inelastic scattering; Nobel Prize 2004.
• Carl Anderson — discovered the positron in 1932 by analyzing cosmic-ray cloud-chamber photographs; Nobel Prize 1936; also discovered the muon in 1936 (initially called the “mesotron”).
• James Chadwick — identified the neutron in 1932 by interpreting Joliot-Curie radiation as neutral particles ejected from beryllium; Nobel Prize 1935; the discovery enabled nuclear fission research.
• Lee Tsung-Dao and Yang Chen-Ning — proposed in 1956 that parity is not conserved in weak interactions; Nobel Prize 1957; their prediction was confirmed experimentally by Wu the same year.