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11.S: Particle Physics and Cosmology (Summary)

  • Page ID
    10324
  • Key Terms

    antiparticlesubatomic particle with the same mass and lifetime as its associated particle, but opposite electric charge
    baryon numberbaryon number has the value \(B=+1\) for baryons, \(–1\) for antibaryons, and 0 for all other particles and is conserved in particle interactions
    baryonsgroup of three quarks
    Big Bangrapid expansion of space that marked the beginning of the universe
    bosonparticle with integral spin that are symmetric on exchange
    colorproperty 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
    cosmologystudy of the origin, evolution, and ultimate fate of the universe
    dark energyform of energy believed to be responsible for the observed acceleration of the universe
    dark mattermatter in the universe that does not interact with other particles but that can be inferred by deflection of distance star light
    electroweak forceunification of electromagnetic force and weak-nuclear force interactions
    exchange symmetryproperty of a system of indistinguishable particles that requires the exchange of any two particles to be unobservable
    fermionparticle with half-integral spin that is antisymmetric on exchange
    Feynman diagramspace-time diagram that describes how particles move and interact
    fundamental forceone of four forces that act between bodies of matter: the strong nuclear, electromagnetic, weak nuclear, and gravitational forces
    gluonparticle that that carry the strong nuclear force between quarks within an atomic nucleus
    grand unified theorytheory of particle interactions that unifies the strong nuclear, electromagnetic, and weak nuclear forces
    hadrona meson or baryon
    Hubble’s constantconstant that relates speed and distance in Hubble’s law
    Hubble’s lawrelationship between the speed and distance of stars and galaxies
    leptona fermion that participates in the electroweak force
    lepton numberelectron-lepton number \(L_e\), the muon-lepton number \(L_μ\), and the tau-lepton number \(L_τ\) are conserved separately in every particle interaction
    mesonsa group of two quarks
    nucleosynthesiscreation of heavy elements, occurring during the Big Bang
    particle acceleratormachine designed to accelerate charged particles; this acceleration is usually achieved with strong electric fields, magnetic fields, or both
    particle detectordetector designed to accurately measure the outcome of collisions created by a particle accelerator; particle detectors are hermetic and multipurpose
    positronantielectron
    quantum chromodynamics (QCD)theory that describes strong interactions between quarks
    quantum electrodynamics (QED)theory that describes the interaction of electrons with photons
    quarka fermion that participates in the electroweak and strong nuclear force
    redshiftlengthening of the wavelength of light (or reddening) due to cosmological expansion
    Standard Modelmodel of particle interactions that contains the electroweak theory and quantum chromodynamics (QCD)
    strangenessparticle property associated with the presence of a strange quark
    strong nuclear forcerelatively strong attractive force that acts over short distances (about \(10^{−15}) m) responsible for binding protons and neutrons together in atomic nuclei
    synchrotroncircular accelerator that uses alternating voltage and increasing magnetic field strengths to accelerate particles to higher and higher energies
    synchrotron radiationhigh-energy radiation produced in a synchrotron accelerator by the circular motion of a charged beam
    theory of everythinga theory of particle interactions that unifies all four fundamental forces
    virtual particleparticle that exists for too short of time to be observable
    W and Z bosonparticle with a relatively large mass that carries the weak nuclear force between leptons and quarks
    weak nuclear forcerelative 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\(ds^2=c^2dt^2−a(t)^2d\sum^2\)

    Summary

    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.

    Contributors

    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).