BBoM-halfttl.4/16/03 5/8/03 5:47 PM Page i B UILDING B LOCKS OF M ATTER EDITORIAL BOARD Editor in Chief John S. Rigden American Institute of Physics Editors Jonathan Bagger Johns Hopkins University Roger H. Stuewer University of Minnesota BBoM-ttl pg.4/16/03 5/8/03 5:58 PM Page 1 B UILDING B LOCKS OF M ATTER A Supplement to the MACMILLAN ENCYCLOPEDIA OF PHYSICS John S. Rigden Editor in Chief Building Blocks of Matter: A Supplement to the Macmillan Encyclopedia of Physics John S. Rigden, Editor in Chief ©2003 by Macmillan Reference USA. 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Macmillan encyclopedia of physics. QC793.2 .B85 2003 539.7’2—dc21 2002013396 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1 CONTENTS Preface......................................................................................................vii Introduction ................................................................................................ix Reader’s Guide............................................................................................xiii List of Articles ............................................................................................xvii List of Contributors .....................................................................................xxiii Common Abbreviations and Acronyms................................................................xxix Building Blocks of Matter ................................................................................1 Time Line.................................................................................................503 Glossary....................................................................................................509 Index......................................................................................................515 v EDITORIAL AND PRODUCTION STAFF Deirdre Graves, Brigham Narins (Project Editors) Shawn Beall (Editorial Support) Patti Brecht, Joseph Pomerance (Copy Editors) Carol Roberts (Indexer) Robyn Young (Project Manager, Imaging and Multimedia Content) Pam Galbreath (Art Director) GGS Information Services (Typesetter) Mary Beth Trimper (Composition Manager) Evi Seoud (Assistant Production Manager) Rhonda Williams (Buyer) Macmillan Reference USA Frank Menchaca (Vice President) Hélène G. Potter (Director of New Product Development) Jill Lectka (Director of Publishing) vi PREFACE The concepts and ideas of elementary particle physics that may be unknown to the reader, both in the field are abstract, and they are typically expressed in the of physics and in related sciences. A list of common language of mathematics. However, the goal of ele- abbreviations and acronyms at the beginning of the mentary particle physics is very simple, and all the ef- book is included to aid readers unfamiliar with those forts of elementary particle physicists are directed used in the book. Numerous tables, figures, illustra- toward that simple goal: to identify the basic build- tions, and photographs supplement the information ing blocks of matter and to understand how they in- contained within the articles and provide visual tools teract to produce the material world we observe. to better understand the material presented. This encyclopedia contains articles intended for Entries are arranged alphabetically and include a broad audience of general readers and is designed extensive cross-references to refer the reader to ad- to edify and give readers an appreciation for one of ditional discussions of related topics. In each arti- the most active and productive areas of physics cle, a bibliography directs the reader to books, throughout the twentieth century and to the present articles, and Web sites that provide additional time. On the one hand, most of the articles have sources of information. The articles themselves fo- been written in ordinary language and provide a cus on particular topics that, taken together, make solid base in particle physics concepts and history for up the intellectual framework called elementary par- those who are new to the field. On the other hand, ticle physics. Articles such as those on accelerators, some topics in particle physics are difficult to express quarks, leptons, antimatter, and particle identifica- in everyday words, and in the articles on such topics, tion provide a working base for the study of particle symbols appear and even an occasional equation. physics. Articles such as those on quantum chro- Even these articles, however, are written so that the modynamics, neutrino oscillations, electroweak sym- reader with little physics background can capture a metry breaking, and string theory bring readers to general sense of the topic covered. subjects that fill the conversations of contemporary Several features of the encyclopedia are de- particle physicists. Finally, articles such as those on signed to help the general reader navigate the lan- the cosmological constant and dark energy, super- guage of physics and mathematics included in the symmetry, and unified theories discuss the key top- articles on the more complex topics. A glossary in ics replete with many exciting questions left to be the back of the book provides definitions for terms answered. vii PREFACE Articles also detail the history of particle physics, tion of particle physics and cosmology, astrophysics, including the discovery of specific particles, such as philosophy, culture, and metaphysics. Also included the antiproton and the electron. In addition to the are articles describing the spin-off technologies cre- historical articles, a time line is included to provide ated in the search for particles as well as the fund- an overview of the development of the field of par- ing of this research. ticle physics. This time line of research and devel- A reader’s guide in the beginning of the ency- opment in what is now called particle physics extends clopedia arranges the topics into broad categories back almost three millennia. The time line demon- and thereby helps organize the array of individual strates the commanding grip that the desire to iden- entries into a comprehensive field of study. Addi- tify the basic building blocks of matter has had on tionally, the article on elementary particle physics the minds of past and present scientists. Biographi- provides an overview of the field and its current cal articles of physicists who have made seminal con- questions. tributions to our understanding of the material world complete the encyclopedia’s coverage of the history The authors of the articles contained in this en- of particle physics. The selection of physicists for the cyclopedia work in the top particle physics laborato- biographies was based on the desire to provide a his- ries and are professors at renowned colleges and torical background for the topics presented in this universities. Not only does this encyclopedia provide encyclopedia, and so no living physicist was included. a comprehensive coverage of the field of particle physics, but it also brings together articles from the Since experimentation is a vital part of particle top members of the physics and scientific community. physics, detailed articles discuss the technologies used to discover particles, including current accel- This collection of articles would not have been erator types and subsystems. Articles also profile the possible without the effort of those who contributed, international laboratories that house these acceler- and I thank each of the authors. Jonathan Rosner, ators, describing experiments, both historic and University of Chicago, has responded to personal re- current, conducted at these labs. Articles on case quests I made of him, and I thank him. Also, I am studies are included to provide the reader with grateful to both editors, Jonathan Bagger, Johns more in-depth information as to how these tech- Hopkins University, and Roger H. Stuewer, Univer- nologies contribute to the past and continuing search sity of Minnesota, for their work and advice. Lastly, for particles. the Macmillan editor, Deirdre Graves, has been de- voted in her assistance throughout the project. We, Particle physics both affects and is affected by the editors, thank her. other sciences as well as by the political and philo- sophical environment. Articles discuss the interac- John S. Rigden viii BUILDING BLOCKS OF MATTER INTRODUCTION Physicists distinguish between classical and modern measurements had established that hydrogen was the physics. The classical era began in the Scientific Rev- least massive of the chemical elements, and in 1815 olution of the seventeenth century and extended William Prout proposed that hydrogen was the build- throughout the eighteenth and most of the nine- ing block of all the chemical elements. Prout’s idea teenth centuries. By then there were rumblings had supporters through the nineteenth century, but among some prominent physicists that their subject it was finally discredited with the discovery of isotopes was complete, that no more basic physics remained early in the twentieth century. to be discovered. Then, in 1895, Wilhelm Conrad One of the major themes of twentieth-century Röntgen discovered X rays, and abruptly, although physics, a spectacular period in the history of physics, perhaps unknowingly, the modern era of physics be- has been the continuation, although greatly intensi- gan. During the following year Henri Becquerel dis- fied, of the ancient quest to identify and understand covered radioactivity, and in 1897 the work of several the fundamental constituents of matter. The elec- physicists culminated in the discovery of the electron, tron, discovered in 1897, was the first elementary par- which is generally credited to J. J. Thomson. With ticle, and, after a century that saw “elementary” the first subatomic particle, the electron, to account particles come and go with great profusion, the elec- for, physicists knew that a new era was under way. tron was and remains truly elementary. The idea of basic building blocks of matter is at What makes a particle elementary? Simply put, least 2,600 years old. In the sixth century B.C.E. it contains no parts. The electron has no hidden con- Thales proposed that all things reduced to water, stituents. The electron is elementary. The proton, and, coming out of the Greek-Roman eras and for long considered to be an elementary particle, does centuries to come, the four basic elements were have parts—three quarks. The proton is not ele- thought to be earth, water, fire, and air. The atomic mentary. There are currently twelve elementary par- hypothesis, originating in the fifth century B.C.E., lin- ticles that physicists believe make up the observable gered in the background for centuries until experi- matter throughout the universe: six quarks—up, mental support, through the work of eighteenth- and down, charm, strange, top, and bottom—and six nineteenth-century chemists, brought atoms to the leptons—electron, electron neutrino, muon, muon fore as the basic building blocks of matter. By the neutrino, tau, and tau neutrino—all of which fit early years of the nineteenth century, quantitative nicely into three groups, called generations, each ix