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1 I~ A +- Over the last two decades our understanding of the fundamental nature of matter has undergone a revolution. The ideas we learned as students have been superseded by concepts at once simpler and more elegant. The more than 100 different kinds of particles identified in the early 1960s are now known to be made up of only a few different kinds of more elementary (simpler) particles called quarks. Seemingly unre- lated particles such as quarks and electrons are found to be related. And two apparently different forces have been shown experimentally and theoretically to be simply different manifestations of the same more fundamental electroweak force. This revolution in elementary-particle physics is the result of the continuing interplay between new theories and new experimental results. Almost all of these experiments have used accelerators, the machines that produce the high-energy particles that are needed to study matter on the smallest scale. These machines have included both traditional accelerators, those in which a beam of high-energy particles strikes a stationary target, and also the newer colliding-beam ma- chines, accelerators in which two beams of high-energy particles collide head on. We are now in a position to look for the answers to yet more basic questions: What determines the properties of the elementary particles? How many different kinds are there? How are they related to each other? Are the other forces in nature also simply different aspects of a 11

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12 ELEMENTARY-PARTICLE PHYSICS single, truly fundamental force? The new colliders can provide the immense energies needed to answer some of these questions, while others can be attacked by more traditional methods. Thus the next several decades offer us the opportunity to continue the remarkable progress of the recent past. This report is about that progress and also about the opportunities for future progress. ELEMENTARY-PARTICLE PHYSICS Elementary-particle physics is the study of the basic nature of matter, of force, of energy, of time, and of space. We seek to discover the simplest constituents of matter, which we call the elementary particles, and we seek to understand the basic forces that operate between them. Above all, we seek the unifying principles and physical laws that will give us a rational and predictive picture of the elementary particles and the basic forces that constitute our world. Elementary particles are very small, much smaller than atoms; hence this is the physics of the very small. The size of the objects studied in this field compared with those studied in other areas of physics is sketched in Figure 1.1. Elementary particles are too minute to see or study directly. We examine them and make new types of particles by colliding particles together at high energies. High energies are needed because the elementary particles are very small and very hard; it takes a great deal of energy to penetrate them or to break them up. Colliding two particles together at high energy and studying the results of that collision is the heart of elementary-particle physics experiments. Thus this field of research is also called high-energy physics. In the last decade, close connections have been established between particle physics and astrophysics. Hence elementary-particle physics is also connected with very-large-scale phenomena. WHAT WE KNOW During the past two decades, particle-physics research has cleared away much of the underbrush that had concealed from us the world of elementary particles. As shown in Figure 1.2, we now know that there are three basic families of elementary particles: the quarks, the leptons, and the force-carrying particles. Some of these particles can only be produced in the laboratory or occur very rarely in nature. But others make up the matter of our everyday world. Thus the atoms that make up all matter consist of electrons moving in orbits around the atomic nucleus. The electron is one of the leptons, and the nucleus consists of protons and neutrons that in turn are made up of quarks.

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INTRODUCTION I 3 ,o2o lo Is lo lo 10 S A) 10-~ .s ~ OMIT AND _ ASTROPHYSICS _ _ _ _ ~ _ GEOPHYSICS E loS _ _ _ _ _ o MEC~AN'CS Of -10 SOLIDS AND ~ FLUIDS PL ASMA O PHYSICS _ _ CONDENSE O _ _ _ _ PHYSICS ATOMIC AND MOLECULAR PHYSICS _ _ T NUCL E AR 1PHIYSICS _ _ ELEMENTARY PARTICLE PHYS ~ CS 2o Dl STANCE TO NEAREST STAR DISTANCE TO SUN RADIUS EARTH HEIGHT OF TALLEST MOUNTAI NS HEIGHT OF PERSON SIZE OF VERY LARGE MOLECULES RADIUS OF HYMOGEN ATOM RADIUS OF PROTON UPPER LIMIT TO RAD I US OF ELECTRON FIGURE 1.1 The different subfields of physics study parts of nature that are very different in size. Elementary-particle physics studies the smallest objects in nature. objects that are smaller than 10-'3 centimeter. Figure 1.2 also shows the four known basic forces. Two of these have been known for hundreds of years: the forces of electromagne- tism and of gravitation. The other two forces were discovered in the twentieth century. One is the strong or nuclear force that holds the atomic nucleus together, and the other is the weak force that operates in many forms of radioactivity. One of the goals of the physicist is to find out if these four forces can be derived from an even more basic, single, unified force. Significant progress has been made in this direction in the last two decades, as we now know that the electro- magnetic and weak forces are two manifestations of a single underlying force. Thus our present knowledge of elementary-particle physics can now be organized in a simple and elegant way. Chapters 2 and 3 describe this present knowledge in some detail.

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14 ELEMENTARY-PARTICLE PHYSICS {a) QUARKS LEPTONS BEAUTY OR BOTTOM QUARK CHARM QUARK STRANGE QUARK UP QUARK -- DOWN QUARK TAU - TAU NEUTRINO MUON NEUTRI NO ELECTRON E LECTRON NEUTRI NO 1 1 1 1 900 1 940 1 980 YEAR DISCOVERED (b) KNOWN BASIC FORCES GRAVITATIONAL FORCE WEAK FORCE STRONG OR NUCLEAR FORCE ELECTROMaGNETIC ., FORCE PARTI CLE CARRYING TH E FORCE ~ NOT \ \DISCOVERED} _ W AND Z PART ICLES GLL'ON PHOTON 1 9X 1 940 1 980 YEAR WHEN THE PARTICLE CARRYING THE FORCE WAS DISCOVERED FIGURE 1.2 (a) History of the discovery of the leptons and quarks. The dashed line means that there was strong indirect evidence for the existence of the particle but that the particle itself had not been directly identified. For example, there was strong indirect evidence for the existence of neutrinos before 1940, but the electron neutrino was not identified directly until the 1950s. After this report was completed. initial evidence was reported for the existence of the sixth or top quark. (b) Each of four basic forces is believed to be carried by different elementary particles. This figure identifies the particles that carry the weak, strong, and electromagnetic forces and shows when these particles were discovered. The gravitational force should also be carried by a particle. called the graviton, but at present there is no indirect or direct evidence for its existence.

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INTRODUCTION 1 5 WHAT WE WANT TO KNOW Now that we have discovered three basic types of elementary particles, we are finally able to attack some of the central questions that have for so long intrigued and baffled the physicist. As an example, what determines the most basic property of a particle its mass? The masses of the different kinds of particles vary enormously: the heaviest known quark has more than ten times the mass of the lighter quarks, while the tau lepton has about 3500 times the mass of the electron. Another important question concerns whether there are more kinds of elementary particles waiting to be discovered. Or, alternatively, can we find a unifying principle that connects the known particles and tells us that there are no more? As already mentioned, we also seek a unifying principle for the basic forces. In Chapter 4 the questions that intrigue and even haunt us are discussed. THE TOOLS OF ELEMENTARY-PARTICLE PHYSICS Elementary-particle physics progresses through a complicated inter- action between experiment and theory. As experimental work pro- duces new data, theory is tested by the data, and theory is used to organize the data. Sometimes a flash of theoretical insight leads to new experiments; at other times an experiment unexpectedly produces surprising new data and upsets currently accepted theories. Thus experiment and theory are two kinds of tools of elementary-particle physics. In Chapters 5 and 6, we emphasize the experimental tools. We do this because the size and complexity of these tools, particularly of the accelerators, is a special quality of this field. But more fundamen- tally, physics is an experimental science, and in the end it is only ex- periments that can tell us if our ideas are right or wrong. Almost all experiments in this field are carried out by using an accelerator to produce high-energy particles, allowing those particles to collide, and then using an apparatus called a detector to find out what has come out of the collision. In the traditional arrangement, a beam of high-energy particles produced by an accelerator strikes a stationary target. Much of the experimental progress in the last two decades has come from such fixed-target experiments. Examples are the demonstration that protons and neutrons are made of quarks, one of the discoveries of the c quark, the discovery of the b quark, and the discovery of the still mysterious CP violation effect. increasingly, however, particle colliders have come to play a dom- inant role in contributing new knowledge to the field. In such acceler-

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16 ELEMENTARY-PARTICLE PHYSICS ators, two beams of particles collide head on, and this produces much higher energy in the collision than is available in fixed-target acceler- ators of the same size. Colliding-beam accelerator technology has been paced by the development of electron-positron colliders and proton- proton colliders. Experiments at electron-positron colliders have given us the co-discovery of the c quark, the discovery of charmed particles, the completely unexpected discovery of the tau lepton' the discovery of jet structure in particle production, and much of the evidence for the idea that the strong or nuclear force is carried by the gluon particle. Experiments at a proton-proton collider studied the details of the . . . . interactions of quarks and gluons. Recently a proton-antiproton collider has begun to contribute substantially to particle physics. The most notable contribution is the discovery in 1983 of the W and Z particles that carry the weak force. Our understanding of the physics of accelerators, together with inventions in accelerator technology, has now reached the point that we can substantially increase the energy reached by colliding-beam accelerators. Higher-energy electron-positron colliders and antiproton- proton colliders are now under construction in the United States, Europe, Japan' and the Soviet Union. The antiproton-proton collider being constructed in the United States will be the highest-energy collider in the world when completed in about 1985. The electron- positron collider under construction in the United States uses a new collider technology, called a linear collider, rather than the conven- tional circular collider. Construction has begun in Europe on an electron-proton collider' something that has never been done before. More information on accelerators is given in Chapter 5. In this Introduction we have not discussed the kinds of elementary- particle research that do not use high-energy physics accelerators. While not by any means the majority of experiments, such experiments are important in our field. Some use lower-energy accelerators in- tended for nuclear-physics studies, and some use cosmic rays. Some are conducted in an ordinary laboratory setting, while others are carried out on mountain tops or deep underground. The highlights of this work are given in Chapters 6 and 7. THE FUTURE TOOLS OF ELEMENTARY-PARTICLE PHYSICS Experimental investigation of some of the fundamental questions in elementary-particle physics requires energies higher than those pro- vided by any accelerators now in operation or under construction anywhere in the world. For this reason the U.S. elementary-particle

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INTROD1JCTION 1 7 physics community is now preparing a proposal for a very-high-energy, superconducting proton-proton collider, the Superconducting Super Collider (SSC). It would be based on the accelerator principles and technology that were developed in connection with the construction of the Tevatron and on other extensive work on superconducting magnets in the United States. This proposed collider would have an effective energy range about 60 times higher than that of any collider now in operation. Not only is the SSC needed to answer some of the questions that we face in elementary-particle physics, but in addition such a large increase in energy will open up new regions of elementary-particle physics to be explored. Since accelerators are at the heart of elementary-particle experimen- tation, there is extensive research and development work on new types of accelerators and higher-energy accelerators. An important part of this work concerns extending the electron-positron linear collider to yet higher energies. It seems quite likely that technology can be developed to build a very-high-energy electron-positron collider. Since the physics that can be done at such a collider is mostly complementary to that which can be done at a proton-proton collider, the elementary- particle physicist would hope to see both types of collider in operation eventually.