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Introduction

This report assesses the field of elementary-particle physics. The Committee on Elementary-Particle Physics of the National Research Council's Board on Physics and Astronomy was assembled to review what has been learned, to identify research priorities for the next two decades, and to describe the instruments and infrastructure needed to carry them out. This chapter introduces the main themes of the report.

The universe is constructed with remarkable economy. Galaxies and hummingbirds, computers and the neurons firing in our brains as we read this sentence—everything in the tangible world is built from about a hundred different kinds of atoms. Every atom, in turn, is a combination of just three different constituents: u quarks and d quarks (which in different combinations form protons and neutrons) and electrons. Up to the resolution of current experiments, no internal parts have been detected in quarks and leptons, so they are called elementary particles.

Although elementary particles are infinitesimal—smaller relative to a grain of sand than a grain of sand is to the entire Earth—the consequences of their properties are enormous. If, for example, the electron were much heavier, the universe would have evolved entirely differently: No atoms would exist, and the universe would now consist solely of electrically neutral particles. No stars would shine; no people would be around to wonder at the universe's origin or ultimate fate.

The richness of the phenomena in our universe, even biological systems, stems from the physical principles that operate on the scale of elementary par-



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1 Introduction This report assesses the field of elementary-particle physics. The Committee on Elementary-Particle Physics of the National Research Council's Board on Physics and Astronomy was assembled to review what has been learned, to identify research priorities for the next two decades, and to describe the instruments and infrastructure needed to carry them out. This chapter introduces the main themes of the report. The universe is constructed with remarkable economy. Galaxies and hummingbirds, computers and the neurons firing in our brains as we read this sentence—everything in the tangible world is built from about a hundred different kinds of atoms. Every atom, in turn, is a combination of just three different constituents: u quarks and d quarks (which in different combinations form protons and neutrons) and electrons. Up to the resolution of current experiments, no internal parts have been detected in quarks and leptons, so they are called elementary particles. Although elementary particles are infinitesimal—smaller relative to a grain of sand than a grain of sand is to the entire Earth—the consequences of their properties are enormous. If, for example, the electron were much heavier, the universe would have evolved entirely differently: No atoms would exist, and the universe would now consist solely of electrically neutral particles. No stars would shine; no people would be around to wonder at the universe's origin or ultimate fate. The richness of the phenomena in our universe, even biological systems, stems from the physical principles that operate on the scale of elementary par-

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ticles. Investigating these particles is, in effect, deciphering the genetic code for the universe: why it is the way it is and how it came to be that way. The goal of elementary-particle physics is to understand the world around us by identifying the elementary particles, understanding their properties, and learning how they interact. Researchers proceed toward this goal along two avenues: (1) by conducting experiments and (2) by trying to determine the physical principles that account for the phenomena they observe—what theoretical physicist Richard Feynman called ''the patterns [in] the phenomena of nature [that are] not apparent to the eye, but only to the eye of analysis." The dialog between experimenters and theorists shapes the research priorities of the field: Experimental research is often guided by theoretical predictions; about as often, phenomena will turn up in experimental data that no one expected to find, and theorists endeavor to account for them. Investigating phenomena on this almost unimaginably minute scale requires the most powerful microscopes ever built: devices known as particle accelerators. In a particle accelerator, beams of subatomic particles are boosted to nearly the speed of light and then brought into collision with either a stationary target or another beam of accelerated particles coming head-on. In these collisions, remarkably, matter is actually created. The particles that emerge from the collision point, like sparks radiating out from microscopic exploding fireworks, are not contained within the original colliding particles. They are created out of the energy of the collision according to the rules of relativistic quantum mechanics. The higher the energy of the collision, the heavier are the particles it can create. Such particles, although fundamental, are often ephemeral, existing only briefly before transforming themselves into more stable particles. High-energy accelerators thus provide elementary-particle physicists with the opportunity to study phenomena that they could otherwise not observe on Earth. Today's accelerators can collide particles with such high energies that, on a very small scale, they replicate the conditions prevailing when the universe was only a fraction of a second old and enable physicists to study the kinds of particles that long ago shaped the evolution of the universe, before the cosmos cooled off too much for these particles to continue to be produced. If accelerators function as microscopes, then the eyes and brains that see and record the phenomena that accelerators reveal are detectors. In essence, detectors are devices that surround the collision point to capture enough information about the particles produced to deduce their properties: Are they electrically charged? Are they light or relatively massive? How long do they exist before being transformed into other kinds of particles? Over the past hundred years, advances in experimental instrumentation and technique have revealed subatomic phenomena that scientists in earlier centuries had no idea existed. These phenomena, in turn, have led to discoveries of physical principles that are crucial for understanding how the universe is put together.

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In addition, these increasingly sophisticated ventures in both experiment and theory have opened profitable new avenues in many fields. Last year, for example, marked the one-hundredth anniversary of the first discovery of an elementary particle. In 1897, British physicist J.J. Thomson concluded that in experiments with cathode-ray tubes, he had seen negatively charged constituents of atoms. Thomson called these entities "corpuscles"; other physicists referred to them by the name that stuck: "electrons." Around the same time, in an enormously fruitful period of research into the nature of matter and energy, x rays and radioactivity were also discovered. When classical concepts of physics proved incapable of explaining these phenomena, quantum mechanics was developed. From this work emerged nothing less than a radically new picture of nature, which in turn had dramatic consequences for other branches of science and for technology. Physicist John Bardeen noted that "quantum theory opened up the possibility of understanding the properties of solids from their atomic and electronic structure," which led him and his colleagues at Bell Laboratories to the invention of the transistor and related devices. The revolution in electronics that followed brought new applications in computers, medical electronics, industrial controls, and communications that would have been impractical or impossible with the vacuum tubes that transistors replaced. Quantum mechanics also turned out to be essential for understanding basic chemistry, the properties of materials, molecular biology, and many other aspects of the physical world. Today's experiments in elementary-particle physics can investigate phenomena 1012 times smaller than Thomson's. Yet, thanks to many ingenious advances in instrumentation, the actual scale of the largest modern experiments is only 10,000 times greater. Most of the experiments are conducted at a few large accelerator laboratories in the United States, Europe, and Asia, although some researchers obtain data from other sources, such as very high-energy cosmic rays from outer space. Many different investigations can be conducted using a single large detector; although the detector project is frequently referred to as one "experiment," in essence a large detector is comparable to a whole laboratory in other fields of science. A decade or more can be spent on a detector's design, construction, use, and improvement. Almost all elementary-particle physics detectors are designed, built, and operated by groups that involve more than one institution; a typical group includes university faculty members and their students, accelerator laboratory staff members, postdoctoral researchers, engineers, and technicians. Collaborations range in size from 30 to more than 1,000 people; most are now international. Creative problem solving is called for at almost every stage of both the experimental and the theoretical sides of elementary-particle physics. Elementary-particle physicists are intimately involved in the design and construction of their tools. Graduate students and postdoctoral researchers have the opportunity to master—and help develop—new approaches to integrated circuit design and

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fabrication, new algorithms and software, new techniques in precision engineering. Young researchers frequently devote most of their waking hours to their work; as in medical school, this total immersion is an important and valuable aspect of their training. Their experience in creatively solving novel problems, working with sophisticated technologies, discerning patterns hidden in massive data sets, and collaborating on large, complex projects is invaluable, whether they remain in elementary-particle physics or go into other fields, as more than half of them now do. Modern elementary-particle physics experiments have enabled physicists to test theories that predict the behavior of elementary particles under extreme conditions, which sheds light on how the universe itself behaved in its earliest moments of existence. These cosmological insights, in turn, have brought physics to the point where the specific questions that are next on the elementary-particle physics agenda have the potential to illuminate some profound general questions—such as, What is matter? and What is force?—questions that only a few decades ago would have belonged to the realm of philosophy, rather than to experimental science. Physicists expect that the next generation of experiments, which will be conducted with more powerful instruments than ever before, will reveal new phenomena crucial for understanding the origins of essential quantities, such as the mass of the fundamental particles, that at present can only be measured but not explained. Although many of the specific questions that are ripe for investigation require fairly technical discussions to explain, the more general issues can be appreciated without specialized knowledge. These issues include the following: Why are there three generations of elementary particles, and what accounts for their seemingly arbitrary progression of masses? In addition to u and d quarks and the electron, one more elementary particle plays a role in our everyday lives: the electron neutrino. These four particles form a complete generation whose interactions can be described with precision by the theory that is now universally accepted by elementary-particle physicists. However, for reasons particle physicists do not yet fully understand, nature has been generous. There are two more complete generations of elementary particles, each analogous to the first one. They obey exactly the same principles as the particles in the first generation. They differ only in the masses of the analogues of the electron and the u and d quarks. For example, one of the electron's counterparts has 206 times the mass of the electron; the other has a mass 3,640 times the electron's. In a recent experimental triumph, the heaviest particle to be observed in the third generation, the t quark, weighed in with a mass about 60,000 times greater than its counterpart, the u quark. Theorists have proposed specific physical processes to explain the origin of these particles' masses; these ideas will be tested in the next generation of experiments.

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Can Einstein's dream of unifying the known forces be realized? Phenomena governed by the strong, weak, and electromagnetic forces can be described by a unified mathematical theory, but for many years no one could see how to include gravity in the description. Today, however, one of the most exciting areas of theoretical physics is an approach that would unify all four forces. It is called string theory, and some believe it represents a scientific revolution on the scale of quantum mechanics. Experiments at existing and planned accelerators will search for phenomena that are expected if string theory is correct, such as the existence of supersymmetric particles and the Higgs boson. Why is there apparently more matter than antimatter? Every known type of particle has an antiparticle counterpart, with the same mass and opposite electric charge. (Neutral particles either are their own antiparticles [e.g., the photon and the neutral pion], or have distinct antiparticles [e.g., the neutron and the antineutron].) When a particle and its antiparticle come close together, they are annihilated. Generally, whenever matter is created, an equal amount of antimatter is also created, so one would expect matter and antimatter to have been present in equal amounts in the early universe. If that were true, however, the universe should now be an excruciatingly dull place, since almost all pairs of matter and antimatter particles would have had more than enough time to encounter and annihilate each other. Why is there so much matter around—in the form of galaxies, solar systems, planets, and people? Of course, as in any branch of science, serendipity and unforeseen developments are bound to play a key role in shaping the course of this work. Just as the Hubble Space Telescope is used to study many different phenomena, not all of which were even known when it was being built, particle accelerators and detectors are used to investigate issues that are recognized or become amenable to experiment only after the instruments are running. Elementary-particle theorist Steven Weinberg observed recently that physicists frequently "do not know in advance what are the right questions to ask, and we often do not find out until we are close to an answer." Whatever future research in elementary particle physics reveals about the world around us, one thing is certain: It will inspire awe for the intrinsic beauty of the fundamental principles that shape our universe. The following chapters report on the field of elementary-particle physics in a way that we think is accessible to readers without scientific backgrounds. Chapters 2, 3, and 4 present a comprehensive picture of the scientific status of the field today and how it reached this point. Chapters 5, 6, and 7 describe the research objectives and instruments for the next two decades. Chapters 8, 9, and 10 describe the structure of the field and how it relates to other branches of physics and technology and to society at large. Finally, Chapter 11 presents the committee's conclusions concerning the health of elementary-particle physics and its recommendations for the future.