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What Is Elementary-Particle Physics? Elementary-particle physics deals with questions first recorded by the philosophers of classical Greece. What is the basic nature of the material world around us? What are the simplest, the most elementary, kinds of matter? What are the basic forces that operate in our material world? Although these are very old questions' it was not until about four centuries ago that scientists began to make progress in trying to answer them. Some of the first answers came with the discovery of certain of the basic forces in nature: the gravitational force, the electrical force, and the magnetic force. It was not until the middle of the nineteenth century that it was discovered that the electric and magnetic forces are in fact two different aspects of the force that we now call electromag- netism. Progress in the study of the basic nature of matter itself also came slowly. Indeed, it was not until the last decade of the nineteenth century that the first of the particles that we now call elementary was discovered; this was the electron. In the next six decades only a few more kinds of truly elementary particles were discovered: the moon, the neutrinos, and the photon. It is just in the last two decades that tremendous progress has been made in our field that we have been able to understand the families of elementary particles and have been able to get for the first time a full view of the basic nature of matter. This chapter is devoted to introducing the fundamental ideas of 18

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WHAT IS ELEMENTARY-PARTIC~E PHYSICS? 19 particle physics that have been developed over the last 50 years. We will attempt to present these ideas in a way that does not require a previous knowledge of high-energy physics nor of mathematics. Chap- ter 3 will explore our present picture in somewhat greater detail and will describe in particular how these ideas have been developed and verified over the last two decades. WHAT IS AN ELEMENTARY PARTICLE? We call a piece of matter an elementary particle when it has no other kinds of particles inside of it and no subparts that can be identified. We think of an elementary particle as occupying no room in space; indeed, we often think of it as a point particle. How do we know whether a particle is elementary? We know only by experimenting with it to see if it can be broken up or by studying it to determine if it has an internal structure or parts. This is illustrated in Figure 2.1. We know that molecules are not elementary because they can be broken up into atoms by chemical reactions or by heating or by other means. Nor are atoms elementary: they can be broken up into electrons and nuclei by bombarding the atom with other atoms or with light rays. Nor is the nucleus elementary: by bombarding nuclei with high-energy particles or with high-energy light rays called gamma rays, the nucleus can also be broken up into protons and neutrons. For about 50 years physicists considered the neutron and proton to be elementary, but in the last two decades we have found that these particles themselves are made up of yet simpler particles called quarks. That is, protons and neutrons have other particles inside of them, hence they are not elementary. However' we have no evidence as yet that the neutron and proton can actually be broken up into these individual quarks; this is a subtle point and is discussed later. What about the electron, the other constituent part of the atom? Despite all of our experiments and all of our probing of the electron, we have not succeeded in breaking up an electron, and we cannot find any evidence that electrons have internal parts or structure. This is why we call the electron an elementary particle. How Many Kinds of Elementary Particles Are There? How many different kinds of elementary particles are there in the universe? If some physicist succeeds in breaking up an electron next year, what has happened to its claimed elementary nature? More gener- ally, how will we ever know if a particle is truly elementary? Will there

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20 ELEMENTARY-PARTICLE PHYSICS PART1 CLE Large Molecule SIZE AND STRUCTURE t ~ , o to Cry 10 7cm~ ~ Eoch Circle Am_ ReDresents an Atom Atom About 10-8 cm~ ~ two Nucleus ./ - . Nucleus Several x 10- 1334 I Proton or ~ Neutron Proton about 10 ':Ouarks | Less than Quark -16 10 cm ENERGY REQUI RED 1/10 of an eV to a few eV Electron Moving Around Nucleus a few eV a few MeV Ito IOOGoV ( (;eV = 109 eV ) iLess than Electron {T10~86cm T more thon I DO GeV FIGURE 2.1 Many basic objects in nature are made up of yet simpler objects. For example, molecules are made up of atoms, and atoms are made up of electrons moving around a nucleus. To the best of our present knowledge, the elementary particles. electrons and quarks, are not made up of simpler particles. It requires larger energies to investigate the size and structure of the smaller particles. At the right side of the figure are shown the energies required to study the structure of the various objects. The smaller the object, the greater the energy required. ever be an end to the sequence of particles within particles within particles . . .? In Chapter 3 we describe the present research on these questions. In this section we present a historical perspective. Figure 2.2 sketches the history of our progress in understanding the number of kinds of elementary particles. The classical Greeks posited just four basic elements: earth' air, fire, and water. In subsequent

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WHATIS ELEMENTARY-PARTICLE PHYSICS? 21 100 o 10 c, - y 1 1 1 1 1 1 1 _ .m .~ c ~ at ~ _ LL ~ _ J 1/ 1 / 1 1 1, 1 1 1 1 000 0 1000 1500 Be 800 1 900 1 950 1 980 1 990 AD FIGURE 2.2 Mankind has always tried to explain the world as made up of a limited number of different kinds of basic matter. Until a thousand years ago, most people believed that the basic types of matter were earth, air, fire, and water. About 1900 the basic types of matter were thought to be the almost 100 different chemical elements. At present we believe there are about a dozen types of basic matter, namely the leptons and the quarks. centuries philosophers and alchemists added aether (to include the heavens), mercury, sulfur, salt, and so on. Already we see a simple picture (albeit a wrong one) beginning to expand. In 1661 Boyle defined the concept of a chemical element, and by 1789 Lavoisier had compiled a list of 33 known elements. At this point, a modern particle physicist might have questioned whether these elements were truly elementary. But the list grew steadily, doubling before Mendeleev found a convinc- ing way to classify them into smaller related families in 1868. By 1914 the number of elements had reached 85. Then revolutionary new developments in physics led to a much simpler picture of matter. Discovery of the electron, the proton, and the tiny dense nucleus of the atom gave rise to the atomic model. Each chemical element consisted of unique atoms, defined by a specific number of electrons surrounding a nucleus made of protons. Thus all matter seemed to be made of only two kinds of constituents, the proton and the electron. A dramatic reduction indeed, from 85 elements to 2 particles.

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22 EfEMENTARY-PARTICLE PHYSICS The neutron was discovered in 1932, providing a more satisfactory picture of the nucleus as a combination of neutrons and protons and increasing the number of fundamental particles to three. In the same year, the positron or antielectron was also discovered. The positron was followed by the muon, the pion, and the first strange particles, all found in cosmic rays. These particles were the first in a long sequence of particles that were unnecessary in the sense that they were not needed as constituents of ordinary matter. Indeed, these particles pre- sented a problem: why did they exist at all, and how were they related to each other? By the 1950s, particle accelerators began to produce hordes of new particles, and their numbers grew in a way quite similar to the number of chemical elements in the nineteenth century (see Figure 2.2~. As before, scientists (now physicists) tried to find patterns in the data that might indicate some underlying simplicity. In 1964 it was proposed that the rapidly growing number of strongly interacting particles (called hadrons) could all be explained as simple combinations of smaller constituents called quarks. There should be three such quarks, and these together with the four known leptons (electron, muon, and their associated neutrinos) would be the seven basic constituents of matter, including the exotic new forms produced only in accelerators. At about the same time, more detailed study of the properties of hadrons, mainly the absence of certain decay processes, caused theorists to suspect the existence of a fourth kind of quark. Such speculation increased with the observation of a new type of force, the weak neutral force. This so-called c or charmed quark was in fact discovered in 1974, as a constituent of a very striking new kind of particle known as the Jib. The next year, a new lepton called the T (taut was discovered, together with indirect evidence for an associated neutrino vat. In 1976, more charmed particles were discovered, and in 1977 a fifth quark, the b or bottom quark, was discovered. Thus the number of fundamental constituents of matter has now grown to 11, and if the expected t or top quark is found it will be 12. Is this the final roll call of the elementary particles, or will more be found and the situation once again become complicated? We do not know the answer to that question. Physics, like all the sciences, is based on experimental knowledge. At any given time, all we can do is assemble the full body of our experimental knowledge and try to explain it with a rational and perhaps even elegant theory. If we can explain all of our experimental knowledge with a theory that regards only a certain set of particles as elementary, then that must be sufficient.

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WHAT IS ELEMENTARY-PARTICLE PHYSICS:' 23 The Size of Elementary Particles As one proceeds down through the sequence of molecule, atom, nucleus, proton, and neutron, and finally quark, the size of the particles gets smaller and smaller. Let us begin with atoms, whose size is of the order of 10-8 centimeter (0.00000001 centimeter). This one-hundred millionth of a centimeter is very small by everyday standards. Mole- cules are larger, their size depending in a rough way on the number of atoms in the molecule. Molecules containing hundreds of atoms, such as organic molecules, can be examined by electron microscopy, and thus can almost be seen in the ordinary sense of that word. But once we go below the atomic level to nuclei, there is no way to look at these particles with any sort of microscope. The nuclei consist of neutrons and protons packed rather closely together. The proton and neutron are each about 10-~3 centimeter in size, about 1/100,000 the size of an atom. Nuclei are a few times bigger than a neutron or proton, depending on how many of these particles they contain. But the nuclei are still not much bigger than lot centimeter. The sizes of nuclei, neutrons, and protons are too small to be found by looking directly at the particles; they must be measured by indirect methods. When we come to an elementary particle such as a quark or an electron, we go to a yet smaller scale. By indirect means the sizes of quarks and electrons are known to be less than 10-'6 centimeterless than 1/1000 the size of a neutron or proton! Indeed we have no evidence that these particles have any size at all. Thus the scale of elementary-particle physics is distances of 1o-~3 centimeter and smaller. Elementary-particle physics in its search for the simplest forms of matter has become the physics of the very small. Elementary Particles and High Energy At first it seems puzzling that elementary-particle physics, the physics of the very small, is also called high-energy physics. The term high-energy refers to the energies of the particles used to produce particle reactions. By high energy we mean that the kinetic energy (energy of motion) of a particle is much higher than its rest mass energy. Why do we need to carry out our particle reactions with high- energy particles? There are two reasons for this. First, as Einstein discovered, kinetic energy can be converted into mass, and mass can be converted into kinetic energy. The equation for the conversion is the famous E = met, where E is the kinetic energy

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24 ELEMENTARY-PARTICLE PHYSICS that can be converted into mass m, and c is the velocity of light. Since we want to produce new particles, and particularly new massive particles, in the reactions that we carry out, we need a large kinetic energy E to make a large mass m. The second reason for needing high-energy particles is that, as we have already said, we cannot directly see the size of a particle nor directly see if it has internal structure or parts. We must investigate the particle's size and structure by bombarding it with other particles. And the deeper we wish to penetrate into a particle, the higher must be the energy of the bombarding particles. The famous Heisenberg uncertainty principle also leads to the conclusion that the investigation of small distances requires high energies. If we wish to measure small distances precisely, then there must be a large uncertainty in the momentum associated with that measurement. A large uncertainty in momentum can only be accom- modated by a large initial momentum. And large momentum means large energy. The principal way in which we give high energy to a particle is to accelerate it through an electric field. Thus accelerators are simply machines that have strong electric fields and that guide the particles through those electric fields. (Chapter 5 discusses accelerators and the basic principles of their operation.) This leads to a convenient unit, the electron volt (eV), for measuring both energy and mass. An electron volt is the energy acquired by an electron or proton passing through an electric potential with a total voltage of 1 volt. As we shall see, the electron volt is a rather small unit of energy or mass, so the elementary- particle physicist uses larger units: MeV - 1046 eV = 1 million electron volts GeV = 10~9 eV = 1 billion electron volts TeV = 10+'2 eV = 1 trillion electron volts The significance of these energy units can be appreciated by looking at some particle masses expressed in electron volts: 1. The electron mass is about 0.5 MeV. 2. The proton mass is about 1 GeV. 3. The heaviest known particle, the Z, has a mass of about 100 GeV = 0.1 TeV. 4. New kinds of fundamental particles are predicted by some theories to lie in the still higher mass range of 0.1-2.0 TeV. In Figure 2.1 we have indicated the range of energies needed to study each type of particle. For the elementary particles shown in the figure,

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WHA T IS ELEMENTAR Y-PARTICLE PHYSICS? 25 the quark and the electron, the highest energies are needed. In Chapter 5 we describe how the energies of accelerators are related to experi- mental studies of the elementary particles. THE KNOWN BASIC FORCES AND FUNDAMENTAL PARTICLES The Four Basic Forces One of the great triumphs of physics has been understanding that all -the multitudinous phenomena of the material world operate through just four basic forces. We have already mentioned two of these forces: the gravitational and the electromagnetic. Two more were discovered in this century. One is the nuclear or strong force, which holds the nucleus together and also holds the proton and neutron together. The last force to be discovered is called the weak force; we shall describe its behavior below. Table 2.1 gives some comparative properties of the four forces. The gravitational force is important in our everyday lives and in astronom- ical phenomena because of the immense mass of the planets and stars. But the gravitational force exerted by one elementary particle is very small compared with the three other forces that can be exerted by that particle. The electromagnetic forces between elementary particles follow the same laws as the electromagnetic forces that are used in modern technology, such as in motors, generators, and electronic equipment. The elementary particles simply act as small bundles of electric charge and small magnets. The strongest of the four forces is the nuclear force. However, the nuclear force is not felt directly in everyday phenomena, since it does not extend beyond a distance of about 1o-~3 centimeter from the elementary particle. This distance is about the same as the size of an individual neutron or proton, and thus it determines the size of atomic nuclei. Since atoms and molecules are at least 100,000 times larger, they do not feel the nuclear force. But at distances less than 10-'3 centimeter the nuclear force is powerful, much more powerful than the electromagnetic force. This is why it is also called the strong force. Finally we return to the weak force. The distance over which this force acts is also small less than about 10-'6 centimeter and it is much less powerful than the strong force. Yet the weak force is not negligible. In a certain sense it is more pervasive than the strong force. Some elementary particles such as the electron are not affected by the

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26 ELEMENTARY-PARTICLE PHYSICS TABLE 2.1 The Four Basic Forces Type of Force Behavior over distance Gravitational Weak Electro- Strong or magnetic Nuclear Limited to less Extends to Limited to less than about very large than about 10~'3 1~'6cm distances cm l~2 Strength relative to strong force at a distance of 10~'3 cm Time for a typical small-mass hadron to decay via these forces Extends to very large distances 1~38 Particle that carries Not the force discovered 1~' s W+, W~, Photon and Z; intermediate bosons Mass of particle Not known About 90 GeV O 1~2os l~23s Gluon. The gluon has been identified indirectly but it has not been, and perhaps cannot be, isolated. Assumed O strong force but are affected by the weak force. The radioactive decay of the neutron and of nuclei, as well as the decays of many of the ele- mentary particles, occur through the weak force. Since the 1920s physicists have speculated about the possibility that different forces can be unified into one general theory. That is, are the seemingly different forces simply different manifestations of one gen- eral force? First thoughts were about unifying the gravitational and electromagnetic forces; that has not been done, and we do not know if it can be done. But within the last 15 years, a significant unification of the electromagnetic and weak forces has been made and has been verified experimentally. In Chapter 3 the state of current research on force unification is discussed. The Known Families of Elementary Particles At present, all our observations in particle physics can be explained by the existence of the four basic forces and by the existence of three families of elementary particles. These families are the leptons, the quarks, and the force-carrying particles.

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WHATIS ELEMENTARY-PARTICLE PHYSICS? 27 THE FORCE-CARRYING PARTICLES We turn first to this family of elementary particles. It is a basic prin- ciple of quantum mechanics that a force has a dual nature: it can be transmitted through a wave or through a particle. The clearest example is the electromagnetic force, which can be treated in some situations as being carried by an electromagnetic wave (radio waves or light waves, for example) and in other situations as being carried by a particle (the photon). The question then arises whether the other forces also obey quantum mechanics in this sense and thus can be thought of as being carried by particles. Table 2.1 summarizes our present knowledge. The weak force is indeed carried by particles: the W+, W-, and Z intermediate bosons have recently been discovered. We believe that the strong force is also carried by particles called gluons, but here the evidence is indirect. Unlike the photon, W+, W~, and Z, the gluon has not been isolated. Finally, the particle conjectured to carry the grav- itational force has been called the graviton, but such a particle has not yet been discovered, and there is no experimental evidence for its existence. Because of the feebleness of the gravitational interaction among elementary particles, its detection would be extraordinarily difficult. THE LEPTONS ties: The lepton family of elementary particles is defined by two proper- 1. Leptons are affected by the gravitational, electromagnetic, and weak forces but not by the strong force. 2. Leptons must be either created or destroyed in particle-anti- particle pairs; the total number of leptons (number of leptons minus number of antileptons) is conserved in all processes to the best of our knowledge. Figure 2.3 shows the six known leptons. They come in pairs, each pair consisting of one charged lepton and one neutral lepton. The neutral lepton is called a neutrino. Each pair is called a generation, and in each generation the mass of the neutrino is much less than the mass of the charged lepton. In the last few years there has been speculation, but as yet no evidence, that the proton might very rarely decay to a lepton plus hadrons. If that turns out to be true, the total number of leptons would not be conserved in this process.

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28 ELEMENTARY-PARTICLE PHYSICS Generation Particle Charge Mass elect ron (e ~ - I 0.5 I MeV electron neutrino (ye) O less than 50 eV 2*_:muon(~) | muon neutrino (v,,) 106 MeV=0.106 GeV O less than 0.5 MeV 3 ~ tau (T) tau neutri no* (via) -1 1784 MeV = 1.784 GeV O less than 160 MeV=0.160GeV *indirect evidence FIGURE ~.3 The six known leptons are arranged in pairs. The members of a pair interact only with each other. For example. the electron and electron neutrino interact with each other but not with the muon, the muon neutrino, the tau, or the tau neutrino. There is indirect evidence for the tau neutrino; it has not been directly detected. The questions that we now face are profound. Are there more genera- tions of leptons? What sets the mass of the leptons, and the difference in masses between generations? And of course the ultimate question: are the leptons really elementary? THE QUARKS The quark family of elementary particles (Figure 2.4) is also defined by two properties: Quarks are affected by all four basic forces. Because they are affected by the strong force, quarks act very differently from the leptons in many situations. In particular, it is either impossible or very difficult to isolate quarks, whereas leptons can easily be isolated. 2. Quarks, like leptons, cannot be singly created or destroyed to the best of our knowledge. Therefore the number of quarks, like the number of leptons, is conserved in every physical process. A very peculiar property of the quarks is that they have electric charges of 2/3 or 1/3 of the unit of electric charge carried by the electron and the proton. All other particles, elementary or not, have either zero or integral charges. Like the leptons, the quarks fall into pairs called generations. Each pair has a +2/3 unit charge quark and a -1/3 unit charge quark.

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WHA T. IS E' EMENTAR Y-PARTICLE PH YSICS? 37 l / Axis FIGURE 2.9 An example of fourfold symmetry in a four-bladed windmill. Its axis of rotation, marked by the central black dot, is perpendicular to the paper. The picture is not changed by a 90 rotation about that axis. words, it looks the same from all sides. Invariant, a word that occurs frequently in physics, means unchanged. Physical theories can have symmetries of a similar kind, but what remains invariant or unchanged after a transformation is not a pattern or an object but the mathematical structure of the laws of the theory itself. Physicists now agree that symmetries play a central role in our understanding of nature. The twin concepts of symmetry and invariance can be important in limiting the equations and theories that are applied to a phenomenon. Consider the force of the Earth's gravity on a person walking on the Earth's surface, and use the good approximation that the Earth is a sphere. Then without knowing anything about the laws of gravitational force, we can make two statements from just the arguments that a sphere is symmetric about its center for any rotation and that the gravitational force must be invariant to any such rotation. First, the size of the force must be the same, no matter where the person walks

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38 ELEMENTARY-PARTICLE PHYSICS , ~ at// \ \\\ /~/7 7 4/1 ~ - ~ ~/1 1- ~ \\\S L I I _ _ ~ )~N TV 1 1 1 I 1 1 OR I I __~ 1 1R 111 ~ I I I ~ if/ \\ ~ _ _ ~ \\\\\ 1- ~ 111/// \\ \ _ ~ a\\\ \ ii~ ,, ~~ ORIGINAL SPHERE GLOBAL SYMMETRY TRANSFORMaTION (o) ( b) t\~\~ ~ LOCAL SYMMETRY TRA NSFORMATION (C) FIGURE 2.10 The ideas of global and local symmetry can be illustrated by a sphere marked with lines of longitude and latitude. When the sphere is simply rotated about its axis the shapes of the lines are not changed; that is called a global symmetry transformation. If the surface of the sphere is distorted as one might do with a sphere made out of rubber, such that the lines of longitude and latitude are twisted, that is a local symmetry transformation. _. _~ on the Earth. Second, the force must point directly toward the Earth's center or directly away. It cannot point in any other direction' east for example, since that direction is not invariant to a rotation. But this is as far as this symmetry argument can go; it cannot tell us whether the force is up or down or its strength. To know that, we need first ex- periment and observation then a theory with explicit equations. Physicists use other symmetry and invariance ideas in much the same way, to provide some general information and to limit the range of equations and theories that can apply. This is particularly important in particle physics where the basic objects, the elementary particles, are relatively simple and have many kinds of symmetries. The symmetries of physical theories are of two types, called global and local. The distinction between them may be illustrated by consid- ering an ideal spherical balloon [Figure 2.10(a)] marked with a system of latitude and longitude coordinates so that the positions of all points on the surface can be identified. A global symmetry is exhibited if the sphere is rotated about some axis [Figure 2.10(b)~. In geographical terms, the rotation depicted is equivalent to displacing the prime meridian from Greenwich, England, to Alexandria, Egypt. This rota- tion is a symmetry operation because the form of the sphere remains unchanged. It is called a global symmetry because the locations of all the points on the surface are changed by the same angular displacement in longitude. Local symmetry is a more demanding statement. It requires that the

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WHA T IS EN EMENTAR Y-PARTICLE PHYSICS? 39 balloon maintain its shape even if the points on the surface are displaced independently [Figure 2.10(c)~. A local symmetry operation stretches the balloon and therefore introduces forces between points. Each of the fundamental forces is now thought to arise from a similar requirement that a law of Nature be invariant under local symmetry transformation. Because the earliest attempts to construct interactions from symmetries dealt with invariance under a change of scale or gauge, the resulting theories are called gauge theories. The symmetries we have discussed so far are known as continuous symmetries, because they may be built up from infinitesimal motions. Another important class of symmetries of physical laws is made up of discrete, or discontinuous, transformations. Of these the most familiar in everyday experience is left-nght or mirror symmetry, which is mani- fested by many objects in our environment. Many microscopic physical processes are invariant under time reversal; a film of the event, run backwards, would also correspond to an allowable event. Similarly, in many situations the replacement of all particles by their antiparticles leads to no change in the physical outcome. As an illustration, the light emitted by an antineon lamp would be indistinguishable from the light emitted by a conventional neon lamp. Symmetry Breaking It may happen that the laws of physics embody a certain symmetry, but some of their consequences do not manifest that symmetry. An example will show how this may come about. Above a certain critical temperature, the individual microscopic magnets that make up an iron ferromagnet are oriented randomly. This reflects the invariance of the laws of electromagnetism under rotations, which is to say that there is no preferred direction in space. When the iron is cooled below the critical temperature, the micromagnets tend to align themselves along some randomly chosen direction. The randomness of this direction is attributable to the rotational invariance of electromagnetism. Once the micromagnets have frozen along a certain direction, the ferromagnet does not display rotational invariance, because a specific direction has been singled out. Thus the symmetry of the laws of electromagnetism has been hidden. In elementary-particle physics, the most striking case of symmetry hiding occurs in the theory of weak and electromagnetic interactions. There the equations of the quantum theory possess a local gauge symmetry, but the observed particles such as electrons do not display this symmetry in their masses.

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40 ELEMENTARY-PARTICLE PHYSICS EXPERIMENTS, ACCELERATORS, AND PARTICLE DETECTORS Experimental Methods in Elementary-Particle Physics The purpose of experiments in elementary-particle physics is to study the behavior of the forces that act on the particles and to look for new types of particles and forces. But few of these studies and searches can be carried out using the apparatus found in the usual physics lab- oratory. For example, elementary particles are too small to be seen using a visible light microscope or even an electron microscope. Furthermore, many elementary particles have short lifetimes; they simply do not exist for a long enough time to be studied directly. A final example is that the search for new particles usually requires that other particles collide together at high energies to produce the new particles. The primary experimental method in elementary-particle physics in- volves the collision of two particles at high energy and the subsequent study of the particles that come out of such a collision. We are interested in the kinds of particles that come out of the collisions, how many there are, the energies of the particles, and their directions of motion. In this section we give an overview of how such experiments are done. Experiments at Fixed-Target Accelerators The basic concept of an elementary-particle experiment using an accelerator is shown in Figure 2.1 1. A beam of protons is accelerated to high energy by a proton accelerator. The beam of protons leaves the accelerator and passes into a mass of material called a target, which is fixed in position. The collisions occur between the protons in the beam and the material in the target. Hence this is called a fixed-target accelerator, and the experiment is called a fixed-target experiment. The simplest material to use for the target is hydrogen, because the hydrogen atom consists of a single electron moving around the single proton that forms the nucleus of the hydrogen atom. Most of the time the protons in the high-energy beam will pass right through the hydrogen target without striking anything, but occasionally one of the protons in the beam will hit either a proton or an electron in the hydrogen. We restrict our attention here to the case when a proton in the beam hits a proton in the hydrogen atom. Then we have a proton-proton collision. As discussed earlier in this chapter in the sec- tion on Collisions and Decays and sketched in Figure 2.6, one of the

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WHA T IS ELEMENTAR Y-PARTICLE PHYSICS? 41 (a) . ~ Beam of High Energy Protons\ l I Proton Accelerator TV=~: (b) To ~ '~ / arget' A-/ Particle Detectors Proton in To rget A_ Path of High Energy Proton ~ n Beam 4 Particles Produced In Collision FIGURE 2.1 1 In a fixed-target experiment a beam of high-energy particles, for example protons' is produced by an accelerator. The beam of particles interacts with the target producing new particles. The particles are detected and their properties studied using an apparatus called a particle detector. In (a) the entire experiment is sketched. In (b) the interaction of the particle itself is shown: a proton in the beam interacts with a proton in the target and produces four particles. things that can happen is that two protons can simply come out of the collision again. But sometimes many other particleshadrons and led tonscan come out of the proton-proton collision. In order to determine what has happened, we need an apparatus that can detect the particles coming out of the collision. Such an apparatus is called a paIticle detector (see Figure 2.111. Particle detectors cannot see particles directly, but they can determine their energies and directions of motion and the nature of the particles. How this is done is described below. Thus the three basic elements of experiments at fixed-target accelerators are the accelerator, the target, and the particle detector. We next describe each of these elements in more detail. Fixed-Target Accelerators The particles accelerated must be stable and have electric charge, hence either protons or electrons are used. The acceleration process

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42 ELEMENTARY-PARTICLE PHYSICS begins with these particles at rest, and gradually gives them more and more energy until they are moving with speeds close to the speed of light and have high energy. The particles are given the energy by the force of electric fields acting on their charge. Since there is a limit to how strong an electric field we can make, higher energies require larger accelerators. High-energy accelerators are large and expensive machines. Thus few are built, and these are used as intensively as possible. For example, in the United States there are only two high-energy proton accelerators. The Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory has a maximum energy of about 30 GeV and has been in operation since 1960. The Tevatron at Fermi National Laboratory, a circular accelerator with a diameter of 2 kilometers, has just gone into operation; it is the first large accelerator in the world to use supercon- ducting magnets, and it is designed to reach an energy of 1000 GeV. Also in the United States is the 3-kilometer-long electron acceler- ator at the Stanford Linear Accelerator Center. (The complementary uses of the different energy ranges and particle beams are described in Chapter 5.) In addition, the United States has lower-energy proton and electron accelerators that are used primarily for nuclear-physics re- search. Targets We have already described how hydrogen can be used as a target for the beam of particles coming out of an accelerator. Other materials can also be used as targets. For example, deuterium is often used. In deuterium (heavy hydrogen) the nucleus consists of a proton plus a neutron; hence one can study collisions between the protons or electrons coming out of the accelerator and the neutron in the target. Another example is provided by neutrino experiments, which often require a dense target such as iron. Particle Detectors for Charged Particles Not only charged particles, such as protons or charged plans or electrons, but also neutral particles, such as neutrons and photons, can come out of a collision. Charged means that the particle has positive or negative electrical charge, as opposed to a neutron or photon, which have no electrical charge. No particle can be seen directly, but as a charged particle passes through any kind of material, it breaks up the atoms and molecules in that material. The technical term is that it

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WHA T IS ELEMENTAR Y-PAR TI CLE PH YSI CS ? 43 . . ionizes the material. And through that ionization the path of the charged particle can be determined. The bubble chamber provides the classic example. The liquid in a bubble chamber is heated above its boiling point, but it is prevented from boiling by high pressure in the chamber. If that pressure is released for a short time and then reapplied, the liquid still does not boil. However, if a charged particle passes through the chamber while the pressure is released, the resulting ionization leads to the formation of a string of bubbles along the path of a particle. This string of bubbles can be photographed, as shown in Figure 2.12, to produce a picture of the tracks or paths taken by the charged particles in their passage through the chamber. Ionization produced by a charged particle is used in other ways by other types of particle detectors. In a drift chamber, for example, the charged particle ionizes a gas, and the electrical effect of that ionization is used to determine the particle path. In a scintillator, the ionization produces visible light that is detected by a phototube. Some particle detectors, such as Cerenkov radiation detectors, do not use ionization. Chapter 6 describes particle detectors in detail, including a discussion of how neutral particles are detected. Secondary Particle Beams The primary beam produced in an accelerator is always either protons or electrons, because stable and charged particles must be used for the acceleration process. Once the primary beam of protons or electrons leaves the accelerator, it is often used to produce secondary beams of other kinds of particles. Figure 2.13 provides an example in which the primary proton beam from a proton accelerator is used to produce a secondary beam of charged pions. This is done in a production target in which the protons interact with the target material to produce the pions. The beam of plans then passes into a bubble chamber; in this example the chamber liquid is hydrogen. The pions finally interact with the electrons and protons in the hydrogen, those being the collisions that are being studied. Other examples of second- ary particle beams are neutrino beams, muon beams, and photon beams. Particle Colliders In many elementary-particle physics experiments it is important to have very-high-energy collisions. Therefore through the years acceler-

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44 ELEMENTAR Y-PARTICLE PHYSICS FIGURE 2.12 An example of a photograph of charged-particle tracks in a bubble chamber. Two sprays of particles emerge from the two vertex points at which they were created. The upper vertex is the point at which a neutral charmed meson decayed into four charged particles: Do ~ K+7~+~r-~-. The decay distance was 9 millimeters, which corresponds to an unusually long lifetime for this particle of 5.5 x 10-'~ second. The photograph is from the SLAC Hybrid Facility Photon Collaboration.

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WHATIS ELEMENTARY-PARTICLE PHYSICS? 45 P ro ton Acce le ra for Bubble Prod uct ion Chamber ~ L_=~==-~ P roto n Charged Pion Be a m Ben m FIGURE 2.13 In many accelerator experiments the primary particle beam from the accelerator is used to produce a secondary beam, and experiments are carried out with the secondary beam. For example, a proton accelerator can be used to produce a beam of charged pions through the interaction of its primary beam with a production target. The secondary beam of plans is then used for experiments. ator builders have put higher and higher energy accelerators into operation: our phrase is ''pushing the energy frontier." But in fixed- target experiments the useful energy for the collision does not increase nearly so fast as the energy of the primary beam increases. Hence in fixed-target accelerators it becomes increasingly expensive to keep pushing the energy frontier. The alternative is to collide two beams of particles moving in opposite directions, as shown in Figure 2.14. In this case the useful energy is actually the sum of the energy of each of the two beams (if the two beam energies are equal). Particle colliders now produce the highest useful energy of any of our machines. In particle colliders both beams must consist of stable, charged particles; the choice in practice has been restricted to protons and electrons and to their antiparticles antiprotons and positrons. The most common form of collider uses opposing beams of electrons and positrons. This is because the collision of an electron and a positron is often relatively simple to understand. On the other hand, the highest- energy collisions are at present obtained with protons colliding with antiprotons. In Chapter 5, the section titled Accelerators We Are Using and Building describes the world's particle colliders; here we give a few examples. Operating electron-positron colliders range in energy from a few GeV to 45 GeV. The Stanford Linear Collider under construction in the United States will yield 100 to 140 GeV in energy, and the LEP electron-positron collider being constructed at the CERN laboratory in Europe can eventually reach over 200 GeV. CERN is now operating a proton-antiproton collider with a total energy of over 500 GeV, and the Fermi National Accelerator Laboratory in the United States has a 2000-GeV proton-antiproton collider under construction. The elemen- tary-particle physics community in the United States is now discussing

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46 ED EMENTAR Y-PARTICLE PHYSICS ( o ) Col I is ion (b) Occurs Here\ Collision Occurs Here SO Target FIXED TARGET COLLI D I NG BEAMS FIGURE 2.14 (a) In fixed-target experiments. a beam of high-energy particles collides with particles at rest in a target. (b) In colliding-beam experiments, two beams of high-energy particles collide head-on. Colliding-beam experiments allow the experi- menter to reach much higher effective energies when studying the interactions of particles. the possibility of the construction of a proton-proton collider to reach 40,000 GeV. Experiments at Particle Colliders Since there is no fixed target in a particle collider, the particle de- tector must look directly at the region where the opposing beams of particles collide. Figure 2.15 shows how this is done in a circular collider where the beams of particles move in opposite directions ~ BEaMS ~ r or cO~LlDE ~ HERE ~ ~ -_ ~ .~. ~ ~ FIGURE 2.15 In the simplest form of colliding-beam facilities, two beams of particles rotate in the same direction in circles that are tangent at just one point. The beams collide at that point. 1

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WHA T IS ELEMENTAR Y-PARTICLE PHYSICS? 47 Path of Neutral Decay K Meson Poi n t F--~--~- Vacuum Pipe Charged Pion Particle Detector Path of ``' Charged Pion 1 FIGURE 2.16 Sometimes the decay of a particle is of interest. The sketch shows how the decay of a neutral K meson into two charged plans is studied. This is one of the crucial experiments in the study of CP violation. around two circles! In this simple example the beams collide at just one point. In a real collider, the beams would be arranged to collide at several different points, providing the opportunity to carry out several experiments at once. The Decays of Particles Until now we have discussed the most common form of experiment in which the collision of two particles is studied. Sometimes, however, we study the decay of a single particle. Figure 2.16 illustrates this by an experiment that studies the decay of a neutral K meson to two charged pions. Experiments in Elementary-Particle Physics Without Accelerators A large variety of experiments in elementary-particle physics is carried out without using accelerators. Some of the experiments use particles from fission reactors or from cosmic rays. Others look for new particles, such as free quarks or magnetic monopoles, in ordinary matter. Still others study with great precision the properties of the stable or almost stable particles, testing, for example, the equality of the size of the electric charge of the electron and the proton. In Chapter 6, the section on Facilities and Detectors for Experiments Not Using Accelerators takes up this subject.