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chapter14 - CHAPTER 14 Elementary Particles 14.1 14.2 14.3...

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    14.1 Early Discoveries 14.2 The Fundamental Interactions 14.3 Classification of Elementary Particles 14.4 Conservation Laws and Symmetries 14.5 Quarks 14.6 The Families of Matter 14.7 Beyond the Standard Model 14.8 Accelerators CHAPTER 14 Elementary Particles Elementary Particles If I could remember the names of all these particles, I’d be a botanist. - Enrico Fermi
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    Elementary Particles We began our study of subatomic physics in Chapter 12. We investigated the nucleus in Chapters 12 and 13. We now delve deeper, because finding answers to some of the basic questions about nature is a foremost goal of science: What are the basic building blocks of matter? What is inside the nucleus? What are the forces that hold matter together? How did the universe begin? Will the universe end, and if so, how and when?
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    The Building Blocks of Matter We have thought of electrons, neutrons, and protons as elementary particles, because we believe they are basic building blocks of matter. However, in this chapter the term elementary particle is used loosely to refer to hundreds of particles, most of which are unstable.
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    14.1: Early Discoveries In 1930 the known elementary particles were the proton, the electron, and the photon. Thomson identified the electron in 1897, and Einstein’s work on the photoelectric effect can be said to have defined the photon (originally called a quantum ) in 1905. The proton is the nucleus of the hydrogen atom. Despite the rapid progress of physics in the first couple of decades of the twentieth century, no more elementary particles were discovered until 1932, when Chadwick proved the existence of the neutron, and Carl Anderson identified the positron in cosmic rays.
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    The Positron Dirac in 1928 introduced the relativistic theory of the electron when he combined quantum mechanics with relativity. He found that his wave equation had negative, as well as positive, energy solutions. His theory can be interpreted as a vacuum being filled with an infinite sea of electrons with negative energies. If enough energy is transferred to the “sea”, an electron can be ejected with positive energy leaving behind a hole that is the positron , denoted by e + .
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    Antiparticles Dirac’s theory, along with refinements made by others opened the possibility of antiparticles which: Have the same mass and lifetime as their associated particles Have the same magnitude but are opposite in sign for such physical quantities as electric charge and various quantum numbers All particles, even neutral ones (with some notable exceptions like the neutral pion), have antiparticles.
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    Cosmic Rays Cosmic rays are highly energetic particles, mostly protons, that cross interstellar space and enter the Earth’s atmosphere, where their interaction with particles creates cosmic “showers” of many distinct particles.
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    Positron-Electron Interaction The ultimate fate of positrons (antielectrons) is annihilation with electrons.
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