Published in 2011 by Britannica Educational Publishing (a trademark of Encyclopædia Britannica, Inc.) in association with Rosen Educational Services, LLC 29 East 21st Street, New York, NY 10010. Copyright © 2011 Encyclopædia Britannica, Inc. Britannica, Encyclopædia Britannica, and the Thistle logo are registered trademarks of Encyclopædia Britannica, Inc. All rights reserved. Rosen Educational Services materials copyright © 2011 Rosen Educational Services, LLC. All rights reserved. Distributed exclusively by Rosen Educational Services. For a listing of additional Britannica Educational Publishing titles, call toll free (800) 237-9932. First Edition Britannica Educational Publishing Michael I. Levy: Executive Editor J.E. Luebering: Senior Manager Marilyn L. Barton: Senior Coordinator, Production Control Steven Bosco: Director, Editorial Technologies Lisa S. Braucher: Senior Producer and Data Editor Yvette Charboneau: Senior Copy Editor Kathy Nakamura: Manager, Media Acquisition Erik Gregersen: Associate Editor, Science and Technology Rosen Educational Services Heather M. Moore Niver: Editor Nelson Sá: Art Director Cindy Reiman: Photography Manager Matthew Cauli: Designer, Cover Design Introduction by Erik Gregersen Library of Congress Cataloging-in-Publication Data The Britannica guide to particle physics / edited by Erik Gregersen. p. cm.—(Physics explained) “In association with Britannica Educational Publishing, Rosen Educational Services.” Includes bibliographical references and index. ISBN /-.#'#,'+)&#).(#)[8eea 1. Particles (Nuclear physics)—Popular works. I. Gregersen, Erik. II. Title: Guide to par- ticle physics. III. Title: Particle physics. QC793.26.B75 2011 539.7'2—dc22 2010030482 Cover, p. iii © www.istockphoto.com/Kasia Biel On page x: One integral facet of particle physics is string theory, which considers particles more like “strings” than points, such as strings on a piano. Bloomberg via Getty Images On page xviii: The Large Hadron Collider (model shown here) may help scientists under- stand the basic structure of matter. Johannes Simon/Getty Images On pages 1, 49, 107, 165, 198, 204, 207, 211: The Milky Way Galaxy seems to be completely made up of matter, as scientists find no evidence of areas where matter and antimatter meet and extinguish. NASA/CXC/UMass/D. Wang et al., NASA/ESA/STScl/D. Wang et al., NASA/ JPL-Caltech/SSC/S.Stolovy CONTENTS Introduction x Chapter 1: Basic Concepts of Particle Physics 1 The Divisible Atom 2 Size 4 Elementary Particles 5 Spin 6 9 Antiparticles 8 Four Basic Forces 13 Field Theory 14 22 The Basic Forces and Their Messenger Particles 15 Gravity 15 Electromagnetism 17 The Weak Force 19 The Strong Force 23 Feynman Diagrams 25 Classes of Subatomic Particles 29 Leptons and Antileptons 29 Hadrons 36 Quarks and Antiquarks 44 50 Chapter 2: The Development of Modern Particle Theory 49 Quantum Electrodynamics: Describing the Electromagnetic Force 49 Quantum Chromodynamics: Describing the Strong Force 52 The Nuclear Binding Force 53 “Strangeness” 56 SU(3) Symmetry 57 The Development of Quark Theory 59 Colour 62 Asymptotic Freedom 64 Electroweak Theory: Describing the Weak Force 66 Beta Decay 67 A Universal Weak Force 67 Early Theories 69 Hidden Symmetry 73 Finding the Messenger Particles 77 Current Research in Particle Physics 80 76 Experiments 81 Theory 94 103 Chapter 3: Particle Accelerators 107 Principles of Particle Acceleration 107 Generating Particles 108 Accelerating Particles 109 Guiding Particles 111 Colliding Particles 113 Detecting Particles 114 History 115 Constant-Voltage Accelerators 119 Voltage Multipliers (Cascade Generators) 119 Van de Graaff Generators 120 122 Betatrons 123 Cyclotrons 124 Classical Cyclotrons 125 Synchrocyclotrons 127 Sector-Focused Cyclotrons 128 Linear Resonance Accelerators 129 Linear Electron Accelerators 130 Linear Proton Accelerators 132 Synchrotrons 135 Electron Synchrotrons 138 Proton Synchrotrons 140 Colliding-Beam Storage Rings 142 Electron Storage Rings 144 Proton Storage Rings 146 Electron-Proton Storage Rings 148 Impulse Accelerators 149 Famous Particle Accelerators 149 Argonne National Laboratory 150 CERN and the Large Hadron Collider 151 DESY 156 Fermi National Accelerator Laboratory 158 151 SLAC 160 Conclusion 162 Chapter 4: Biographies 165 Patrick M.S. Blackett 165 Sir James Chadwick 167 Owen Chamberlain 168 Sir John Douglas Cockcroft 168 Raymond Davis, Jr. 169 Sheldon Glashow 171 David J. Gross 172 William Webster Hansen 173 166 Gerardus ’t Hooft 175 Koshiba Masatoshi 176 Ernest Orlando Lawrence 177 170 Tsung-Dao Lee 179 Edwin Mattison McMillan 180 Simon van der Meer 181 Yoichiro Nambu 182 H. David Politzer 183 Carlo Rubbia 184 Abdus Salam 185 Emilio Segrè 186 Robert Jemison Van de Graaff 188 Ernest Thomas Sinton Walton 189 Steven Weinberg 190 Frank Wilczek 191 Edward Witten 193 Chen Ning Yang 194 Yukawa Hideki 196 Appendix 198 Degenerate Gas 198 Fermi-Dirac Statistics 199 Hyperon 199 Isospin 200 192 J/psi Particle 201 Neutron Optics 201 197 Renormalization 203 Glossary 204 Bibliography 207 Index 211 7 Introduction 7 ince English physicist J.J. Thomson discovered the Selectron over 100 years ago, understanding of what makes up the atom has revealed ever more interesting lay- ers. This book in the Physics Explained series examines particle physics, the study of the astonishingly small sub- atomic particles that make up all matter. The word “atom” comes from the Greek word atomos meaning indivisible. That is, according to ancient Greek philosophers such as Democritus and Leucippus, the atoms were the irreducible pieces of matter. The atoms had no internal ingredients or components. Because atoms were regarded as philosophical concepts, there was no idea of actually testing the irreducible nature of the atom. That had to wait until much later. The first subatomic particle discovered was the elec- tron in 1897. Physicist J.J. Thomson was investigating cathode rays. These were rays that were emitted from an electrode in a glass tube. Thomson did a series of three experiments. In the first two, he found that the cathode rays were particles that carried a negative electric charge. The smallest particles then known were atoms. So what kind of atom was the cathode ray? He then did another experiment that allowed him to measure the ratio of the cathode ray’s mass to its charge. Much to everyone’s sur- prise, the ratio was extremely small. So either the cathode ray has a very small mass, much smaller than that of any atom, or the cathode ray had an enormous charge, much more than had been seen on any atom. It turned out to be the former. Thomson called these little particles corpus- cles, and they were later called electrons. The electron is the smallest of the famous three particles—protons, neutrons, and electrons—that make up atoms. It has a mass of 9.109 x 10-31 kg. If written out, there would be 30 zeros between the decimal point and the 9. xi 7 The Britannica Guide to Particle Physics 7 How the atom was actually constructed became clearer in 1911 when English physicist Ernest Rutherford posited that the atom was mostly empty space. There was a massive centre, and around it orbited the electrons. The atom was like the solar system in miniature, with the electrons as the planets. The centre was the nucleus, home to the larger pro- tons and neutrons. The proton has a mass about 1,837 times that of the electron, which is still an extremely small mass. However, even the proton and neutron could be divided further. Each of these two particles is made of quarks. Quarks are unusual particles that come in six flavours: up, down, top, bottom, charm, and strange. The proton is made up of two up quarks and one down. The neutron is one up and two down. The electron contained no quarks. It was found that subatomic particles could be divided into two types: hadrons and leptons. The proton and neutron were hadrons (from the Greek for “heavy”); the electrons were leptons (from the Greek for “light”). The dif- ference between the particles stems from how they relate to the fundamental forces. There are only four fundamen- tal forces: gravity, electromagnetism, the weak force, and the strong force. The most well known is gravity. Gravity explains the apple falling out of the tree, the Moon orbiting Earth, and the galaxies interacting with each other. Because it works on the smallest scales of thousands of kilometres in the case of the apple and the largest scales of billions of light-years in the case of the galaxies, one might think that gravity was the strongest or the most “fundamental” force. It is actually the weakest. The distinction of being the strongest force belongs to the aptly named strong force, which binds the quarks together in hadrons. The leptons are not affected by the strong force. The strong force is similar to electromagnetism in that the quarks carry a “charge.” However, the charge is much xii 7 Introduction 7 more complicated than the simple positive and negative charges that drive electricity. The strong charge is called colour and comes in three types: red, green, and blue. Quarks of the same colour repel each other, while those with different colours attract. A hadron must be colour- neutral. The heavier, three-quark hadrons, or the baryons, have a red, a blue, and a green quark. The lighter, two- quark hadrons—or the mesons—consist of, for example, a red quark and antiquark, which has the colour of antired. Therefore the mesons are colour-neutral. The four forces have a carrier particle that transmits the force. In the case of the strong force, that particle is called the gluon. The theory that describes how colour works is called quantum chromodynamics. Quantum chromodynamics, or QCD, is one of two theories that make up the Standard Model of particle physics. The other is electroweak theory, which describes electromagnetism and the weak force. Electromagnetism, like gravity, is a long-range force. The photons are the car- rier particles of electromagnetism and can be seen across the universe.