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2 What Is Elementary-Particle Physics? INTRODUCTION In the literal sense, nothing is simpler than an elementary particle: By definition, a particle is considered to be elementary only if there is no evidence that it is made up of smaller constituents. Yet, identifying the elementary particles, understanding their properties, and studying their interactions are turning out to be the key to illuminating why that most unelementary entity—the entire universe—is the way it is, how it came to be this way, and what its ultimate fate will be. Philosophers through history have speculated on what matter is composed of. About a hundred years ago, atoms were considered elementary, until physicists learned that atoms consisted of electrons orbiting a nucleus. We now know that quarks make up the protons and neutrons inside the nucleus. These particles are infinitesimal. Their scale is less than 10—18 cm—smaller relative to a grain of sand than a grain of sand is to the entire planet. Performing experiments to investigate the physics of elementary particles requires extremely sophisticated instruments and theoretical tools. This chapter relives the ancient quest to find the fundamental constituents of matter, shows how this quest is shaped by relativity and quantum mechanics, introduces the forces (interactions) among particles, shows what we know about how orderly the universe is, and describes the technique of particle collisions that has revealed so much about its inner structure and beauty.
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FUNDAMENTAL CONSTITUENTS OF MATTER For centuries, philosophers have asked, "Are there a small set of fundamental constituents out of which everything is made? Or might it be that we will always find structure within structure, layers within layers like an onion?" Thales of Miletus, who suggested that water was the single fundamental entity from which matter is built, may be the first person recorded to suggest the appealing notion of an ultimate form of matter. The following material briefly sketches how this thinking has progressed to the present time. Earth, Air, Fire, and Water Thales' student Anaximander added earth, fire, and air to water to the list of fundamental building blocks. The important notion that rational principles could explain what was observed was contained in this philosophy. Chemical Elements, the Periodic Table, and Atoms What are earth, air, fire, and water made of? Addressing this question led, by the nineteenth century, to recognition of the familiar chemical elements. The definition adopted then for an "element" was a substance that cannot be decomposed further into simpler substances by ordinary chemical means. Thus, the world consisted of a number of distinct substances (at the time, only about 30 elements were identified; today, there are more than 100). It was discovered that elements combine with other elements according to very simple rules: Two hydrogens plus one oxygen make one water "molecule." The essentially limitless number of chemical "compounds" that are found in nature are then the result of combinations of elements. At the beginning of the nineteenth century, John Dalton proposed an atomic theory of matter: The elements themselves are collections of tiny indestructible atoms characterized by their atomic weights (oxygen atoms weigh 16 times as much as hydrogen atoms). This can be considered the first theory of "elementary particles" having a sound scientific basis. Dmitri Mendeleev arranged a table of the elements in order of increasing atomic weight and thereby made one of the most important discoveries in the history of science: The properties of the elements are "periodic" functions of their atomic weights. For example, in Mendeleev's "periodic table," the metals copper, silver, and gold, which have vastly different atomic weights, all appear in the same column. There were also gaps in this table: Elements not yet discovered that should exist if this atomic theory were correct. Confirmation of Mendeleev's "predictions" then made this scheme the basis of chemical thinking in the late nineteenth century; by the early twentieth century, further experiments established atoms as real physical entities.
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Protons, Neutrons, and the Electron Although atoms were thought to be elementary, they too are composite objects. The atom, as first revealed in experiments by Rutherford, is an electrically neutral object, approximately 10 billionths of a centimeter in diameter. This means that there are about 2 million atoms stretching across the diameter of the period at the end of this sentence. Each atom has a tiny positively charged nucleus, about 10,000 times smaller yet. Negatively charged electrons occupy the space surrounding the nucleus. The mass of the electron is about 2,000 times lighter than the mass of the hydrogen nucleus: the proton. Electrons were the first of the modern elementary particles to be discovered. The mass of most nuclei is about twice the mass of the protons they contain. The additional mass is provided by another particle, the neutron, which has a mass very close to that of the proton but is electrically neutral. Today's Fundamental Constituents It is natural to ask what protons, neutrons, and electrons are made of. With today's particle accelerators, one can "look inside" these objects for an internal structure. The proton and neutron are found to be made up of "quarks." Two quarks with a positive electric charge of 2/3 (of the electron charge), called u quarks, and one quark with a negative electric charge of -1/3, called the d quark, make up the proton. Similarly, the neutron is made up of one u quark and two d quarks. There are many other particles that can be built out of the quarks combined in particular ways; these are called hadrons. Physicists have also tried to see if there is anything smaller inside the electron. Experiments have the sensitivity to detect objects even 10,000 times smaller than the proton itself, but nothing has been found. As far as physicists today know, quarks are also fundamental and are not made of yet smaller constituents. The question is still open experimentally, but theory and experiment are pointing more than ever before to the possibility that we have discovered the "ultimate constituents." The Neutrino and Leptons There is a fundamental particle, called an electron neutrino, that does not combine with other particles in the way that quarks combine to make hadrons, hadrons combine to make nuclei, and electrons and nuclei combine to make atoms. This is a massless or almost massless particle that carries no charge and is, as shown later, a "partner" to the electron, as its name implies. These are the first members of a class of particles different from quarks, which are called leptons: the electron and its associated neutrino.
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Particle "Generations" So far then, a charge 2/3 quark, a charge −1/3 quark, a neutral lepton, and a charge −1 lepton have been discussed. The masses of these particles in units of 109 electron volts (GeV) are shown in the first part of Table 2.1; and they comprise what is called a particle generation or family. A major surprise has been production in the laboratory of extra particle generations. A remarkable feature of nature that has been discovered is that this pattern of particles—two quarks and two leptons of the indicated charges—is repeated and then repeated again. Except for the neutrinos, which perhaps remain massless, the particles of each subsequent generation become heavier, as Table 2.1 shows. These additional generations appear to have nothing to do with "ordinary tangible matter." Yet they were important in the first moments of the universe and have a profound role in our understanding of nature. The masses of the quarks and leptons range from zero, or near zero, for neutrinos to almost 200 times the proton mass for a t quark. Understanding why quarks and leptons exhibit this not quite random progression of masses is an important topic of research in elementary-particle physics. Good experimental evidence exists that there are only three generations. Why this should be so constitutes a major mystery in the field. TABLE 2.1 Particles of All the Generations Particle Mass (GeV) Electric Charge First generation u quark 0.005 +2/3 d quark 0.008 −1/3 electron neutrino 0 0 electron 0.0005 −1 Second generation c quark 1.2 +2/3 s quark 0.175 −1/3 muon neutrino 0 0 muon 0.106 −1 Third generation t quark 175 +2/3 b quark 4.25 −1/3 tau neutrino 0 0 tau 1.781 −1
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TABLE 2.2 Major Operating U.S. Accelerator Laboratories Laboratory Primary Particle Energy (GeV) Location Fermilab Proton 1,000 Batavia, Illinois Stanford Linear Accelerator Electron 50 Palo Alto, California Brookhaven Proton 30 Upton, New York Cornell Electron Accelerator Electron 10 Ithaca. New York RELATIVITY, QUANTUM MECHANICS, AND PARTICLE ACCELERATORS The two major scientific revolutions of the twentieth century—relativity and quantum mechanics—still provide the basic framework for describing elementary-particle physics. Quantum mechanics tells us that particles have wave-like properties. These are not observed for large objects such as billiard or baseballs, but for particles with small masses the wave nature becomes evident. In quantum mechanics, the wavelength of a particle is inversely proportional to its momentum. This means that as the momentum (and therefore the kinetic energy) of a particle increases, its wavelength decreases. This is the reason high particle energies are required to probe small distances and is the prime motivation for use of the high-energy particle beams produced at particle accelerators. Chapter 7 discusses just how these technological marvels work; for now, Table 2.2 lists the major accelerators presently used in particle physics in the United States. With high-energy accelerators, particle physicists can effectively "trade" energy for mass, allowing them to directly produce particles that weigh many times more than the particles being accelerated. This follows from relativity, which says that a particle with mass m that is at rest has an energy E given by the famous equation E = mc2. Thus, if two protons each having an energy of 1,000 GeV can be brought together, it would in principle be possible to produce in such collisions two new particles (at rest) each weighing 1,000 GeV, or about 1,000 times as much as the initial protons. This is the means by which very heavy members of subsequent particle generations were discovered. It is analogous to the collision of two tennis balls to produce a bowling ball. The analogy would be even more accurate if this were the only way to produce and observe a bowling ball! More implications of relativity and quantum mechanics, important for particle physics, can be indicated with a discussion of one of the particles under intense study: the B meson.
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First, the B meson consists of a b quark and an anti-d quark. Antimatter was deduced in the 1930s by attempts to understand how the motion of electrons was defined by quantum mechanics and relativity together. In the equation for the electron developed by P.A.M. Dirac, there appeared an extra solution having opposite charge to the electron; this turned out to correspond to the positron, the antiparticle of the electron. This prediction was a brilliant deduction whereby an entire and formerly hidden sector of the world was uncovered using only mathematics and reason! However, mathematics includes many possibilities that are never realized in nature, and this "antiworld" had to be verified by experiment. Physicists now know that every type of particle has a corresponding antiparticle, a symmetry that effectively doubles the number of types of particles in nature (except for the kinds of particle that are their own antiparticle). Antimatter played a major role in the evolution of the early universe, but as shown in the previous chapter, a key question in particle physics is why the universe today appears to be made of matter only. Second, the B meson is unstable, having a mean life of approximately a trillionth of a second. In fact, most types of particles are unstable: Even in a total vacuum, they spontaneously disintegrate or decay into lighter particles. How does the B meson decay? The mechanism involves the possibility of a B meson directly turning into an important particle that has almost 20 times its energy—the W particle—and the W then rapidly decaying into lighter particles. (The important role of the W is discussed below.) It is the Heisenberg uncertainty principle of quantum mechanics that permits these momentary extreme violations of the conservation of energy. Such processes are called "virtual," since they cannot be directly detected. However, through virtual processes, effects that would otherwise be expected to be seen only at very high energies can be detected (albeit infrequently) at much lower energies. Third, we cannot know exactly when a B particle will decay. Quantum mechanics provides a precise expression for the probability of a decay at a particular time; this probability is all we can know. It is still not known why the world obeys quantum mechanics, but that it does is both beautiful and incontrovertible. Finally, a particle's decay time will depend on its speed. The inner workings of the B particle, as Einstein taught, slow down significantly the faster it travels. This effect ("time dilation") makes it possible for particle physicists to directly study short-lived new particles by extending their lifetimes in the laboratory frame so that they travel further in a detector. An example of this is shown in the cover illustration to this volume. FORCES In addition to identifying elementary particles, physicists try to understand the means by which they interact. At the small scales encountered in high-
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energy physics, these forces, or interactions, can also create and destroy particles. It appears that only four forces are needed to describe the behavior of all matter in the universe. Their characteristics are described below. Gravity The force of gravity is the most familiar one in our everyday experience. A chain of observations and reasoning by Galileo, Brahe, Kepler, and Newton led to the universal law of gravitation. This law describes a force between two masses that is always attractive, proportional to the product of the masses of the two objects, and inversely proportional to the square of the distance between the objects. It accurately describes the motion of falling objects near Earth's surface and, amazingly enough, accurately applies to the motions of celestial bodies. As such, Newton's law of gravitation is a ''universal" law. More than 200 years later, Einstein's theory of general relativity subsumed Newton's law and predicted small deviations, which were confirmed. Acting for billions of years on the galaxies, gravity is the force that has had a profound effect on the structure of the universe, second only to whatever interaction initially produced the matter out of which the universe is made. Electric and Magnetic Forces; Electromagnetism Electricity The electric force accounts for all everyday phenomena that are not gravitational. The muscular processes in our bodies, tension and compression in the structural members of tall skyscrapers, the combustion of chemical fuels to power our society: All are examples of the electric force. Much of modern technology utilizes the electric force: The motion of charges in electronic circuits, television screens, and computer monitors—all rely on the electric force. Unlike gravity, the electric force is both attractive and repulsive. The atom is electrically neutral (with the positive charge of the nucleus balanced by the electrons surrounding it), but the arrangement of its electrons determines its chemical activity. It may be an inert noble gas, it may combine in a regular way to form a crystalline metal or a complex molecule, and complex molecules may combine to form living cells. In all of these cases the electric force is at work. Magnetism Magnetism used to be considered the third macroscopic force. Two bar magnets exert a force on each other that is attractive if the bars are oriented one way and repulsive if one of the bars is reversed.
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One of the most far-reaching phenomena in physics was discovered by Hans Christian Oersted in 1820 when he observed that a magnetic compass needle was deflected when brought near an electrical current. Eleven years later, Michael Faraday and Joseph Henry independently discovered that a moving bar magnet produces an electrical current in a nearby conductor. These experiments strongly suggested that electricity and magnetism were deeply interrelated. A consistent description of both phenomena was first achieved by James Clerk Maxwell. Interestingly, his equations also implied that electromagnetic "radiation" could travel in vacuum. His equations predicted the speed of this radiation, which turned out to be the speed of light. Heinrich Hertz in a classic experiment was able to create this electromagnetic radiation and detect it a distance away. Thus, light itself was understood for the first time to be propagating electromagnetic energy, and the first step was taken in the development of our modern telecommunications industry. This huge industry is just one of many made possible by the application of the theory of electromagnetism as described by Maxwell's equations. Weak and Strong Forces There are two forces that apparently have little significance in everyday life because they operate only at subatomic distances. Nevertheless, they play crucial roles in physical processes important to us. These are the weak force and the strong force. The Weak Force The "weak" force between elementary particles is much weaker than electromagnetic forces. It is a very short-range force, acting only over microscopic distances (10−15 cm). The weak force controls the nuclear fusion reactions by which the Sun and stars shine. Deep within our Sun, the density is so great and the temperature so high that nuclei can overcome the repulsion from the electrical force and release energy by fusing together. Neutrinos created by this weak interaction carry energy out of the star to cool it, controlling its temperature (and consequently that of its surrounding planets). In other reactions, photons carrying electromagnetic energy are emitted. It is of course the photons emitted by the Sun that warm Earth's surface and help to sustain life. Earth also has an internal source of energy, which offsets the heat being radiated back into space from its surface. This is supplied by radioactive decay of heavy nuclei in Earth's interior. One of the key processes in radioactive decay of heavy nuclei is called beta decay—another manifestation of the weak force—whereby a neutron in a nucleus transforms into a proton and the nucleus emits an electron and a neutrino.
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The Strong Force The protons within the atomic nucleus repel each other through the electric force, but the nucleus does not fly apart. It is bound together by another very short-range force, the strong force. The stability of all the matter in our everyday experience comes about through the action of the strong force. It is also the strong force that binds together quarks inside the proton and the neutron. An interesting and important aspect of the strong force between quarks is that the strength of the force increases as the quarks get further apart, somewhat like stretching a rubber band tightly. This explains why quarks are never seen in isolation; they are said to be "confined." A relatively large amount of energy is stored in heavy atomic nuclei, in the form of the energy it takes to hold protons and neutrons together. Heavy nuclei, such as uranium-235, when bombarded by neutrons become unstable and split into lighter products (nuclear fission), releasing a great deal of this energy. In a nuclear reaction, about a million times more energy is released than in a typical chemical reaction such as the burning of carbon. This energy, resulting from the strong force, is the source of nuclear power. What "Transmits" Forces? The four fundamental forces encountered—gravity, electromagnetism, the weak force, and the strong force—underlie all observed phenomena. Over the years, other forces have been hypothesized, but experiments searching for them have so far produced null results. More sensitive experiments in the future might discover still other forces. Physicists understand today that when quantum mechanics and relativity are taken into account, forces actually arise from the exchange of other particles. Physicists then speak of two distinct types of particles: matter particles and particles that carry forces. The electromagnetic force is communicated between particles via exchanges of photons. These photons are the same quanta of energy that are familiar to us as radio waves, light, and x rays. The carriers of the weak force are W and Z bosons, first detected directly in 1983. (The interesting effects of the W boson were first seen in the observation of radioactive decay at the end of the last century, and it took about 85 years for a W boson to be directly produced in the laboratory.) The carriers of the strong force are called gluons. Gluons have not been observed in isolation—they, like quarks, are confined—but direct evidence for their existence is seen routinely in experiments. Also, gluons are present in hadrons and can be considered a constituent of, for example, the proton. The carrier of the gravitational force has been named the graviton, but it has yet to be directly observed, because the gravitational force is exceedingly weak.
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TABLE 2.3 The Four Fundamental Forces Force Force Carrier Approximate Mass of Carrier Gravity Graviton 0 Electromagnetism Photon 0 Weak force W and Z bosons About 90 times the proton mass Strong force Gluon 0 Table 2.3 lists the force carriers. Note that although three of them are massless, the fourth is extremely heavy. Given this disparity, it might seem impossible that they could arise from a single mechanism. Yet, as the next section indicates, physicists are optimistic that all the forces can be described in a "unified" framework. Unification of Forces Further unification is treated in the following chapter. It has been established that two of the forces listed in Table 2.3 (electromagnetic and weak) are indeed unified, and there is compelling evidence that a third (strong) is as well. It may even be that gravity is also subject to unification so there would be in a real sense just one force. LAWS OF NATURE Underlying the electrical, magnetic, gravitational, and other phenomena of particle physics are a number of general principles. Examples of the most fundamental of these principles (often referred to as laws) are the conservation of energy, the conservation of charge, and the law of cause and effect (causality). Physicists believe these laws to be universal and absolute, applying to interactions between the smallest components of matter that have thus far been observed, to interactions between galaxies of stars, and even to the development of the universe itself.
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Our universe is not "random": The behavior of atoms and the laws of quantum mechanics that physicists study here on Earth are found to hold everywhere else. By studying light, radio waves, and other electromagnetic energy from the most distant stars, physicists can determine that the forces and rules discovered on Earth apply equally well there, far away and at much earlier times in the evolution of the universe. As discussed in the next chapter, physicists have realized that the conservation of energy is a direct consequence of the fact that to a good approximation the laws of physics do not depend on time. It did not have to be so, and this observation is of profound significance. PARTICLE COLLISIONS How do particle physicists uncover the phenomena that are described? The large accelerator laboratories have been mentioned already. This section briefly characterizes what goes on at such laboratories where the key study is of collisions of particles; Chapter 6 elaborates more fully. Scattering Experiments Perhaps the archetypal example of probing the subatomic world involved experiments (alluded to earlier) performed by Rutherford from 1909 to 1911. He and his colleagues directed a beam of alpha particles (nuclei of helium), which originated from radioactive decay, at a thin gold foil. The atoms of gold were transparent to the bombarding particles. Occasionally, however, some of the alpha particles were scattered backward from whence they came, as if they had encountered an object with much greater mass (the atomic nucleus) than their own. In a memorable statement after making these observations, Rutherford said, "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you!" The principle of directing beams of subatomic particles to collide with other particles, observing what emerges from these encounters, and interpreting the results via models of their interaction remains the major technique for exploring the physics of elementary particles. Accelerators provided more energetic beams that were then used to study phenomena at much smaller distances than could be done simply by using the particles from natural radioactivity. By this technique, new heavier particles are produced. Also, intense beams of new particles can be made and the ways in which they decay or scatter on other targets can be studied. Colliders For the study of phenomena revealed only at the very highest energies (e.g., production of the very heavy t quark or of W and Z bosons), the technique of "colliding beams" is employed. Here, one beam is directed at another rather than at a fixed target. In collisions of billiard balls, for example, the effective
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collision-energy increase is a factor of four if two balls of the same speed collide head-on as opposed to one of them being at rest. However, because of relativity and the conservation of momentum, the effective energy of the collision of particles aimed at each other is far greater. At the Fermilab Tevatron, collisions have been made between 900-GeV protons and 900-GeV antiprotons. Achieving the same collision energy with a stationary target would require an accelerator with a circumference about 2,000 times as large as Fermilab's, or about 8,000 miles! There are different types of colliders operating today that are important for particle physics. The relatively large mass of protons and antiprotons makes it more efficient to accelerate them to high energies than to accelerate electrons or positrons. However, in the collisions, it is their constituents—quarks and gluons—that interact. Since many subatomic constituents make up the proton and antiproton, no single one carries the full energy of the accelerated particle. For example, in proton-proton collisions, the effective collision energy is about a factor of 10 lower than the full energy of the beam. By increasing the intensity of the beams, it is sometimes possible to study higher-energy processes. In collisions between electrons and positrons, the energy of collision is the full energy to which the particles are raised (since electrons and positrons appear to have no substructure). Such collisions cleanly probe the electromagnetic and weak interactions: They do not create the extraneous debris characteristic of proton collisions and are easier to interpret. Finally, because an electron lacks substructure and behaves in a point-like way, it is a useful probe for exploring the structure of the proton. An electron-proton collider provides information about the structure of a proton that is not available from a proton collider and provides an opportunity to search for hypothesized objects that combine both quark and lepton characteristics. SUMMARY The variety of phenomena that particle physicists have uncovered and are studying has been surveyed in this chapter. Although these phenomena occur at the smallest distance scales, what is observed has relevance in understanding the physics of forces that govern the atom, the energetic processes in cores of stars, and even the structure of the universe. The collisions of high-energy particles have been shown to reveal new and important structure. These collisions re-create the conditions of the universe just after its birth. The laws that are discovered have existed for all time and everywhere in the universe. A small number of forces have been discovered, all of which could arise from a single one. The particles on which these forces act have a mysterious structure; the lighter ones make up our everyday world, whereas the role of the next two generations still represents a major puzzle. The following chapter presents the theoretical framework in which these phenomena are currently understood.
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