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Download Chapter 43 Elementary Particles. High-Energy Particles and Accelerators Beginnings of Elementary Particle Physics – Particle Exchange Particles and Antiparticles

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  • Slide 1
  • Chapter 43 Elementary Particles
  • Slide 2
  • High-Energy Particles and Accelerators Beginnings of Elementary Particle Physics Particle Exchange Particles and Antiparticles Particle Interactions and Conservation Laws Neutrinos Recent Results Particle Classification Units of Chapter 43
  • Slide 3
  • Particle Stability and Resonances Strange Particles? Charm? Toward a New Model Quarks The Standard Model: Quantum Chromodynamics (QCD) and the Electroweak Theory Grand Unified Theories Strings and Supersymmetry Units of Chapter 43
  • Slide 4
  • If an incoming particle in a nuclear reaction has enough energy, new particles can be produced. This effect was first observed in cosmic rays; later particle accelerators were built to provide the necessary energy. 43.1 High-Energy Particles and Accelerators
  • Slide 5
  • As the momentum of a particle increases, its wavelength decreases, providing details of smaller and smaller structures: In addition, with additional kinetic energy more massive particles can be produced. 43.1 High Energy Particles and Accelerators
  • Slide 6
  • One early particle accelerator was the cyclotron. Charged particles are maintained in near-circular paths by magnets, while an electric field accelerates them repeatedly. The voltage is alternated so that the particles are accelerated each time they traverse the gap. 43.1 High-Energy Particles and Accelerators
  • Slide 7
  • Larger accelerators are a type called synchrotrons. Here, the magnetic field is increased as the particles accelerate, so that the radius of the path stays constant. This allows the construction of a narrow circular tunnel to house a ring of magnets. 43.1 High-Energy Particles and Accelerators
  • Slide 8
  • Synchrotrons can be very large, up to several miles in diameter. These pictures are of Fermilab, a synchrotron outside Chicago, Illinois. 43.1 High-Energy Particles and Accelerators
  • Slide 9
  • Accelerating particles radiate; this causes them to lose energy. This is called synchrotron radiation for particles in a circular path. For protons this is usually not a problem, but the much lighter electrons can lose substantial amounts. One solution is to construct a linear accelerator for electrons; the largest is about 3 km long. 43.1 High-Energy Particles and Accelerators
  • Slide 10
  • The maximum possible energy is obtained from an accelerator when two counter-rotating beams of particles collide head-on. Fermilab is able to obtain 1.8 TeV in protonantiproton collisions; a new accelerator called the Large Hadron Collider (LHC) will reach energies of 14 TeV.
  • Slide 11
  • The electromagnetic force acts over a distance direct contact is not necessary. How does that work? Because of waveparticle duality, we can regard the electromagnetic force between charged particles as due to: 1. an electromagnetic field, or 2. an exchange of photons. 43.2 Beginnings of Elementary Particle Physics Particle Exchange
  • Slide 12
  • This is a crude analogy for how particle exchange would work to transfer energy and momentum. The force can be either attractive or repulsive. 43.2 Beginnings of Elementary Particle Physics Particle Exchange
  • Slide 13
  • Physicists visualize interactions using Feynman diagrams, which are a kind of x - t graph. Here is a Feynman diagram for photon exchange by electrons: 43.2 Beginnings of Elementary Particle Physics Particle Exchange
  • Slide 14
  • The photon is emitted by one electron and absorbed by the other; it is never visible and is called a virtual photon. The photon carries the electromagnetic force. Originally, the strong force was thought to be carried by mesons. The mesons have nonzero mass, which is what limits the range of the force, as conservation of energy can only be violated for a short time. 43.2 Beginnings of Elementary Particle Physics Particle Exchange
  • Slide 15
  • The mass of the meson can be calculated, assuming the range, d, is limited by the uncertainty principle: For d = 1.5 x 10 -15 m, this gives 130 MeV. 43.2 Beginnings of Elementary Particle Physics Particle Exchange
  • Slide 16
  • This meson was soon discovered, and is called the pi meson, or pion, with the symbol . Pions are created in interactions in particle accelerators. Here are two examples: 43.2 Beginnings of Elementary Particle Physics Particle Exchange
  • Slide 17
  • The weak nuclear force is also carried by particles; they are called the W +, W -, and Z 0. They have been directly observed in interactions. A carrier for the gravitational force, called the graviton, has been proposed, but there is as yet no theory that will accommodate it. 43.2 Beginnings of Elementary Particle Physics Particle Exchange
  • Slide 18
  • This picture shows the reconstruction of the creation of a Z particle, and the detector that discovered it.
  • Slide 19
  • This table details the four known forces, their relative strengths for two protons in a nucleus, and their field particles. 43.2 Beginnings of Elementary Particle Physics Particle Exchange
  • Slide 20
  • The positron is the same as the electron, except for having the opposite charge (and lepton number). We call the positron the antiparticle of the electron. Every type of particle has its own antiparticle, with the same mass and most with the opposite quantum number. A few particles, such as the photon and the 0, are their own antiparticles, as all the relevant quantum numbers are zero for them. 43.3 Particles and Antiparticles
  • Slide 21
  • This drawing, from a bubble chamber photograph, is of an interaction between an incoming antiproton and a proton (not seen) that results in the creation of several different particles and antiparticles. 43.3 Particles and Antiparticles
  • Slide 22
  • In the study of particle interactions, it was found that certain interactions did not occur, even though they conserve energy and charge, such as: A new conservation law was proposed: the conservation of baryon number. Baryon number is a generalization of nucleon number to include more exotic particles. 43.4 Particle Interactions and Conservation Laws
  • Slide 23
  • Particles such as the proton and neutron have baryon number B = +1 ; antiprotons, antineutrons, and the like have B = -1 ; all other particles (electrons, photons, etc.) have B = 0. There are three types of leptons the electron, the muon (about 200 times more massive), and the tau (about 3000 electron masses). Each type of lepton is conserved separately. 43.4 Particle Interactions and Conservation Laws
  • Slide 24
  • This accounts for the following decays: Decays that have an unequal mix of e -type and -type leptons are not allowed. 43.4 Particle Interactions and Conservation Laws
  • Slide 25
  • Conceptual Example 43-5: Lepton number in muon decay. Which of the following decay schemes is possible for muon decay? (a) (b) (c) All of these particles have L = 0. 43.4 Particle Interactions and Conservation Laws
  • Slide 26
  • Example 43-6: Energy and momentum are conserved. In addition to the number conservation laws which help explain the decay schemes of particles, we can also apply the laws of conservation of energy and momentum. The decay of a + particle at rest with a mass of 1189 MeV/c 2 commonly yields a proton (mass = 938 MeV/c 2 ) and a neutral pion, (mass = 135 MeV/c 2 ): What are the kinetic energies of the decay products, assuming the + parent particle was at rest? 43.4 Particle Interactions and Conservation Laws
  • Slide 27
  • Neutrinos are currently a subject of active research. Evidence has shown that a neutrino of one type may change into a neutrino of another type; this is called flavor oscillation. This suggests that the individual lepton numbers are sometimes not strictly conserved, although there is no evidence that the total lepton number is not. In addition, these oscillations cannot take place unless at least one neutrino type has a nonzero mass. 43.5 Neutrinos Recent Results
  • Slide 28
  • As work continued, more and more particles of all kinds were discovered. They have now been classified into different categories. Gauge bosons are the particles that mediate the forces. Leptons interact weakly and (if charged) electromagnetically, but not strongly. Hadrons interact strongly; there are two types of hadrons, baryons ( B = 1) and mesons ( B = 0). The table of particle properties on the next slide gives some indication of the complexity of the known particles. 43.6 Particle Classification
  • Slide 29
  • Slide 30
  • Example 43-7: Baryon decay. Show that the decay modes of the + baryon given in Table 432 do not violate the conservation laws we have studied up to now: energy, charge, baryon number, lepton numbers. 43.6 Particle Classification
  • Slide 31
  • Almost all of the particles that have been discovered are unstable. If they decay weakly, their lifetimes are around 10 -13 s; if electromagnetically, around 10 -16 s; and if strongly, around 10 -23 s. Strongly decaying particles do not travel far enough to be observed; their existence is inferred from their decay products. 43.7 Particle Stability and Resonances
  • Sli

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