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11.S: Particle Physics and Cosmology (Summary)

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  • Key Terms

    antiparticle subatomic particle with the same mass and lifetime as its associated particle, but opposite electric charge
    baryon number baryon number has the value \(B=+1\) for baryons, \(–1\) for antibaryons, and 0 for all other particles and is conserved in particle interactions
    baryons group of three quarks
    Big Bang rapid expansion of space that marked the beginning of the universe
    boson particle with integral spin that are symmetric on exchange
    color property of particles and that plays the same role in strong nuclear interactions as electric charge does in electromagnetic interactions
    cosmic microwave background radiation (CMBR) thermal radiation produced by the Big Bang event
    cosmology study of the origin, evolution, and ultimate fate of the universe
    dark energy form of energy believed to be responsible for the observed acceleration of the universe
    dark matter matter in the universe that does not interact with other particles but that can be inferred by deflection of distance star light
    electroweak force unification of electromagnetic force and weak-nuclear force interactions
    exchange symmetry property of a system of indistinguishable particles that requires the exchange of any two particles to be unobservable
    fermion particle with half-integral spin that is antisymmetric on exchange
    Feynman diagram space-time diagram that describes how particles move and interact
    fundamental force one of four forces that act between bodies of matter: the strong nuclear, electromagnetic, weak nuclear, and gravitational forces
    gluon particle that that carry the strong nuclear force between quarks within an atomic nucleus
    grand unified theory theory of particle interactions that unifies the strong nuclear, electromagnetic, and weak nuclear forces
    hadron a meson or baryon
    Hubble’s constant constant that relates speed and distance in Hubble’s law
    Hubble’s law relationship between the speed and distance of stars and galaxies
    lepton a fermion that participates in the electroweak force
    lepton number electron-lepton number \(L_e\), the muon-lepton number \(L_μ\), and the tau-lepton number \(L_τ\) are conserved separately in every particle interaction
    mesons a group of two quarks
    nucleosynthesis creation of heavy elements, occurring during the Big Bang
    particle accelerator machine designed to accelerate charged particles; this acceleration is usually achieved with strong electric fields, magnetic fields, or both
    particle detector detector designed to accurately measure the outcome of collisions created by a particle accelerator; particle detectors are hermetic and multipurpose
    positron antielectron
    quantum chromodynamics (QCD) theory that describes strong interactions between quarks
    quantum electrodynamics (QED) theory that describes the interaction of electrons with photons
    quark a fermion that participates in the electroweak and strong nuclear force
    redshift lengthening of the wavelength of light (or reddening) due to cosmological expansion
    Standard Model model of particle interactions that contains the electroweak theory and quantum chromodynamics (QCD)
    strangeness particle property associated with the presence of a strange quark
    strong nuclear force relatively strong attractive force that acts over short distances (about \(10^{−15}) m) responsible for binding protons and neutrons together in atomic nuclei
    synchrotron circular accelerator that uses alternating voltage and increasing magnetic field strengths to accelerate particles to higher and higher energies
    synchrotron radiation high-energy radiation produced in a synchrotron accelerator by the circular motion of a charged beam
    theory of everything a theory of particle interactions that unifies all four fundamental forces
    virtual particle particle that exists for too short of time to be observable
    W and Z boson particle with a relatively large mass that carries the weak nuclear force between leptons and quarks
    weak nuclear force relative weak force (about \(10^{−6}\) the strength of the strong nuclear force) responsible for decays of elementary particles and neutrino interactions

    Key Equations

    Momentum of a charged particle in a cyclotron \(p=0.3Br\)
    Center-of-mass energy of a colliding beam machine \(W^2=2[E_1E_2+(p_1c)(p_2c)]+(m_1c^2)^2+(m_2c^2)^2\)
    Approximate time for exchange of a virtual particle between two other particles \(Δt=\frac{h}{E}\)
    Hubble’s law \(v=H_0d\)
    Cosmological space-time metric \(ds^2=c^2dt^2−a(t)^2d\sum^2\)


    11.1 Introduction to Particle Physics

    • The four fundamental forces of nature are, in order of strength: strong nuclear, electromagnetic, weak nuclear, and gravitational. Quarks interact via the strong force, but leptons do not. Both quark and leptons interact via the electromagnetic, weak, and gravitational forces.

    • Elementary particles are classified into fermions and boson. Fermions have half-integral spin and obey the exclusion principle. Bosons have integral spin and do not obey this principle. Bosons are the force carriers of particle interactions.

    • Quarks and leptons belong to particle families composed of three members each. Members of a family share many properties (charge, spin, participation in forces) but not mass.

    • All particles have antiparticles. Particles share the same properties as their antimatter particles, but carry opposite charge.

    11.2 Particle Conservation Laws

    • Elementary particle interactions are governed by particle conservation laws, which can be used to determine what particle reactions and decays are possible (or forbidden).

    • The baryon number conservation law and the three lepton number conversation law are valid for all physical processes. However, conservation of strangeness is valid only for strong nuclear interactions and electromagnetic interactions.

    11.3 Quarks

    • Six known quarks exist: up (u), down (d), charm (c), strange (s), top (t), and bottom (b). These particles are fermions with half-integral spin and fractional charge.

    • Baryons consist of three quarks, and mesons consist of a quark-antiquark pair. Due to the strong force, quarks cannot exist in isolation.

    • Evidence for quarks is found in scattering experiments.

    11.4 Particle Accelerators and Detectors

    • Many types of particle accelerators have been developed to study particles and their interactions. These include linear accelerators, cyclotrons, synchrotrons, and colliding beams.

    • Colliding beam machines are used to create massive particles that decay quickly to lighter particles.

    • Multipurpose detectors are used to design all aspects of high-energy collisions. These include detectors to measure the momentum and energies of charge particles and photons.

    • Charged particles are measured by bending these particles in a circle by a magnetic field.

    • Particles are measured using calorimeters that absorb the particles.

    11.5 The Standard Model

    • The Standard Model describes interactions between particles through the strong nuclear, electromagnetic, and weak nuclear forces.

    • Particle interactions are represented by Feynman diagrams. A Feynman diagram represents interactions between particles on a space-time graph.

    • Electromagnetic forces act over a long range, but strong and weak forces act over a short range. These forces are transmitted between particles by sending and receiving bosons.

    • Grand unified theories seek an understanding of the universe in terms of just one force.

    11.6 The Big Bang

    • The universe is expanding like a balloon—every point is receding from every other point.

    • Distant galaxies move away from us at a velocity proportional to its distance. This rate is measured to be approximately 70 km/s/Mpc. Thus, the farther galaxies are from us, the greater their speeds. These “recessional velocities” can be measure using the Doppler shift of light.

    • According to current cosmological models, the universe began with the Big Bang approximately 13.7 billion years ago.

    11.7 Evolution of the Early Universe

    • The early universe was hot and dense.

    • The universe is isotropic and expanding.

    • Cosmic background radiation is evidence for the Big Bang.

    • The vast portion of the mass and energy of the universe is not well understood.


    Samuel J. Ling (Truman State University), Jeff Sanny (Loyola Marymount University), and Bill Moebs with many contributing authors. This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).