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Chapter 32. Physics: Principles with. Applications, 6th edition. Giancoli ...
Lecture PowerPoint Chapter 32
Physics: Principles with Applications, 6th edition Giancoli © 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials.
Chapter 32 Elementary Particles
Units of Chapter 32 • 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 32 • Particle Stability and Resonances • Strange Particles? Charm? Maybe a New Model Is Needed! • Quarks
• The “Standard Model”: Quantum Chromodynamics (QCD) and the Electroweak Theory • Grand Unified Theories • Strings and Supersymmetry
32.1 High Energy Particles and Accelerators
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.
32.1 High Energy Particles and Accelerators As the momentum of a particle increases, its wavelength decreases, providing details of smaller and smaller structures: (32-1)
In addition, with additional kinetic energy more massive particles can be produced.
32.1 High Energy Particles and Accelerators 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.
32.1 High Energy Particles and Accelerators The frequency of the applied voltage must equal that of the circulating particles, and is given by:
This is called the cyclotron frequency.
32.1 High Energy Particles and Accelerators Larger accelerators are of 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. Synchrotrons can be very large, up to several miles in diameter.
32.1 High Energy Particles and Accelerators 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.
32.2 Beginnings of Elementary Particle Physics – Particle Exchange The electromagnetic force acts over a distance – direct contact is not necessary. How does that work? Because of wave-particle duality, we can regard the electromagnetic force between charged particles as due to: 1. an electromagnetic field, or 2. an exchange of photons
32.2 Beginnings of Elementary Particle Physics – Particle Exchange This is a crude analogy for how particle exchange would work to transfer energy and momentum. The force can either be attractive or repulsive.
32.2 Beginnings of Elementary Particle Physics – Particle Exchange Physicists visualize interactions using Feynman diagrams, which are a kind of x-t graph. Here is a Feynman diagram for photon exchange by electrons:
32.2 Beginnings of Elementary Particle Physics – Particle Exchange 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.
32.2 Beginnings of Elementary Particle Physics – Particle Exchange 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.
32.2 Beginnings of Elementary Particle Physics – Particle Exchange The mass of the meson can be calculated, assuming the range, d, is limited by the uncertainty principle: (32-3)
For d = 1.5 x 10-15 m, this gives 130 MeV.
32.2 Beginnings of Elementary Particle Physics – Particle Exchange This meson was soon discovered, and is called the pi meson, or pion, symbol π. Pions are created in interactions in particle accelerators; here are two examples:
32.2 Beginnings of Elementary Particle Physics – Particle Exchange The weak nuclear force is also carried by particles; they are called the W+, W-, and Z0. They have been directly observed in interactions.
32.2 Beginnings of Elementary Particle Physics – Particle Exchange A carrier for the gravitational force, called the graviton, has been proposed, but there is as yet no theory that will accommodate it.
32.2 Beginnings of Elementary Particle Physics – Particle Exchange This table details the four known forces, their relative strength for two protons in a nucleus, and their field particle.
32.3 Particles and Antiparticles The positron is the same as the electron, except for having opposite charge (and lepton number). We call the positron the antiparticle of the electron.
32.3 Particles and Antiparticles Every type of particle has its own antiparticle, with the same mass but most quantum numbers being opposite. A few particles, such as the photon and the π0, are their own antiparticles, as all the relevant quantum numbers are zero for them.
32.3 Particles and Antiparticles This is a drawing of an interaction between an incoming antiproton and a proton (not seen) that results in the creation of several different particles and antiparticles.
32.4 Particle Interactions and Conservation Laws 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.
32.4 Particle Interactions and Conservation Laws 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.
32.4 Particle Interactions and Conservation Laws 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.
32.4 Particle Interactions and Conservation Laws This accounts for the following decays:
Decays that have an unequal mix of e-type and μ-type leptons are not allowed.
32.5 Neutrinos – Recent Results 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.
32.5 Neutrinos – Recent Results This suggests that the individual lepton numbers are sometimes not strictly conserved, although there is no evidence that the total lepton number is no.
In addition, these oscillations cannot take place unless at least one neutrino type has a nonzero mass.
32.6 Particle Classification 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).
32.7 Particle Stability and Resonances 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.
32.7 Particle Stability and Resonances Strongly decaying particles do not travel far enough to be observed; their existence is inferred from their decay products.
32.7 Particle Stability and Resonances The lifetime of strongly decaying particles is calculated from the variation in their effective mass using the uncertainty principle. These particles are often called resonances.
32.8 Strange Particles? Charm? Maybe a New Model Is Needed! When the K, Λ, and Σ particles were first discovered in the early 1950s, there were mysteries associated with them: • They are always produced in pairs
• They are created in a strong interaction, decay to strongly interacting particles, but have lifetimes characteristic of the weak interaction To explain this, a new quantum number, called strangeness, S, was introduced.
32.8 Strange Particles? Charm? Maybe a New Model Is Needed! Particles such as the K, Λ, and Σ have S = 1 (and their antiparticles S = -1); other particles have S = 0.
32.8 Strange Particles? Charm? Maybe a New Model Is Needed! The strangeness number is conserved in strong interactions but not in weak ones; therefore these particles are produced in particle-antiparticle pairs, and decay weakly.
More recently, another new quantum number called charm was discovered to behave in the same way.
32.9 Quarks Due to the regularities seen in the particle tables, as well as electron scattering results that showed internal structure in the proton and neutron, a theory of quarks was developed.
32.9 Quarks There are six different “flavors” of quarks; each has baryon number B = ⅓.
Hadrons are made of three quarks; mesons are a quark-antiquark pair.
32.9 Quarks Here are the quark compositions for some baryons and mesons:
32.9 Quarks This table gives the properties of the six known quarks.
32.9 Quarks This is a list of some of the hadrons that have been discovered that contain c, t, or b quarks.
32.9 Quarks The particles that we now consider to be truly elementary – having no internal structure – are the quarks, the gauge bosons, and the leptons. The quarks and leptons are arranged in three “generations”; each has the same pattern of electric charge, but the masses increase from generation to generation.
32.10 The “Standard Model”: Quantum Chromodynamics (QCD) and the Electroweak Theory Soon after the quark theory was proposed, it was suggested that quarks have another property, called color, or color charge.
32.10 The “Standard Model”: Quantum Chromodynamics (QCD) and the Electroweak Theory Unlike other quantum numbers, color takes on three values. Real particles must be colorless; this explains why only 3-quark and quarkantiquark configurations are seen. Color also ensures that the exclusion principle is still valid.
32.10 The “Standard Model”: Quantum Chromodynamics (QCD) and the Electroweak Theory Each quark carries a color charge, and the force between them is called the color force – hence the name Quantum Chromodynamics. The particles that transmit the color force are called gluons; there are eight different ones, with all possible color-anticolor combinations.
32.10 The “Standard Model”: Quantum Chromodynamics (QCD) and the Electroweak Theory The color force becomes much larger as quarks separate; quarks are therefore never seen as individual particles, as the energy to separate them is less than the energy to create a new quark-antiquark pair. Conversely, when the quarks are very close together, the force is very small.
32.10 The “Standard Model”: Quantum Chromodynamics (QCD) and the Electroweak Theory These Feynman diagrams show a quark-quark interaction mediated by a gluon; a baryonbaryon interaction mediated by a meson; and the baryon-baryon interaction as mediated on the quark level by gluons.
32.10 The “Standard Model”: Quantum Chromodynamics (QCD) and the Electroweak Theory Beta decay is the result of a weak interaction, and is mediated by a W± particle. Here is a Feynman diagram of beta decay:
32.11 Grand Unified Theories A Grand Unified Theory (GUT) would unite the strong, electromagnetic, and weak forces into one. There would be (rare) transitions that would transform quarks into leptons and vice versa. This unification would occur at extremely high energies; at lower energies the forces would “freeze out” into the ones we are familiar with. This is called “symmetry breaking.”
32.11 Grand Unified Theories GUTs predict that the proton will eventually decay; in fact, the simplest GUT predicts a lifetime for the proton that is shorter than the measured limit, so a more complex GUT must be the correct theory.
32.12 Strings and Supersymmetry Finally, there are theories that attempt to include the gravitational force as well.
String theory models the fundamental particles as different resonances on tiny loops of “string.” Supersymmetry postulates a fermion partner for each boson, and vice versa. Neither of these theories has any experimental evidence either favoring or disfavoring it at the moment.
Summary of Chapter 32 • Particle accelerators accelerate particles to very high energy, to probe the detailed structure of matter and to produce new massive particles • Every particle has an antiparticle, with the same mass and opposite charge (and some other quantum numbers) • Other quantum numbers: baryon number; lepton number; strangeness; charm; topness; bottomness • Strong force is mediated by gluons
Summary of Chapter 32 • Fundamental force carriers are called gauge bosons
• Leptons interact weakly and electromagnetically • Hadrons are made of quarks, and interact strongly • Most particles decay quickly, either weakly, electromagnetically, or strongly • There are six quarks and six leptons
Summary of Chapter 32 • The quarks also carry color charge • Quantum chromodynamics is the theory of the strong interaction • Electroweak theory unites the electromagnetic and weak forces
• Grand unified theories attempt to unite all three forces