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Copyright © 1986 National Academy of Sciences
All right reserved.
Elementary-particle physics, the science of the ultimate constituents of matter and the interactions among them, has undergone a remarkable development during the past two decades. A host of new experimental results made accessible by a new generation of particle accelerators and the accompanying rapid convergence of theoretical ideas has brought to the subject a new coherence and has raised new possibilities and set new goals for understanding nature. The progress in particle physics has been more dramatic and more thoroughgoing than could have been imagined at the time of the 1972 survey of physics, Physics in Perspective (National Academy of Sciences, Washington, D.C., 1972). Many of the important issues identified in that report have been addressed, and many of the opportunities foreseen there have been realized. As a result, we are led to pose new and more fundamental questions and to conceive new instruments that will enable us to explore these questions.
Elementary-particle physics is the study of the basic nature of matter, energy, space, and time. Elementary-particle physicists seek the fundamental constituents of matter and the forces that govern their behavior. In common with all physicists, they seek the unifyingprinciples and physical laws that determine the material world around us.
The atom, the atomic nucleus, and the elementary particles of which they are composed are too small to be seen or studied directly. Throughout this century, physicists have devised ever more sophisticated detection devices to observe the traces of these particles and their constituents. At the same time, they have developed increasingly energetic beams of particles to probe deeply into the structure of matter. Early examples are x-rays to probe the electronic structure of the atom and radioactive sources to study the atomic nucleus. Some of the constituents of ordinary matter, notably electrons and protons, are quite stable and easily manipulated in electric and magnetic fields. They can therefore be accelerated to high energies and used as probes to reach the very small distance scale of the fundamental constituents. The colliding of high-energy particles and the analysis of collision products is at the heart of experimental particle physics. For this reason the field is often called high-energy physics.
THE REVOLUTION IN PARTICLE PHYSICS
Thirty years ago, ordinary matter was thought to consist of protons, neutrons, and electrons. Experiments were under way to probe the structure of these particles and to study the forces that bind them together into nuclei and atoms. In the course of these experiments, physicists discovered more than a hundred new particles, called hadrons, which had many similarities to the proton and the neutron. None of these particles seemed more elementary than any other, and there was little understanding of the mechanisms by which they interacted with one another.
Since that time, a radically new and simple picture has emerged as a result of many crucial discoveries and theoretical insights. It is now clear that the proton, the neutron, and other hadrons are not elementary. Instead, they are composite systems made up of much smaller particles called quarks, much as an atom is a composite system made up of electrons and a nucleus. Five kinds of quarks have been established, and initial experimental evidence for a sixth species has been reported.
Unlike the neutron and the proton, the electron has survived the revolution intact as an elementary constituent of matter, structureless and indivisible. However, we now know that there are six kinds of electronlike particles called leptons. According to our present understanding, then, ordinary matter is composed of quarks and leptons.
An important difference between quarks and leptons is that a formidable interaction, known as the strong force, binds quarks together into hadrons but does not influence leptons. Both quarks and leptons are acted upon by the three other fundamental forces: the electromagnetic force, the weak force responsible for certain radioactive decays, and the gravitational force.
Over the past two decades, great progress has been made in understanding the nature of the strong, weak, and electromagnetic forces. A unified theory of the weak and electromagnetic forces now exists. Its predictions have been dramatically verified by many experiments, culminating in the discovery of the W and Z particles in 1983. These carriers of the weak force are analogous to the photon, the carrier of the electromagnetic interaction, whose existence was established in the 1920s. In addition, there is indirect but persuasive evidence for particles called gluons, the carriers of the strong force. Strong, weak, and electromagnetic interactions all are described by similar mathematical theories called gauge theories. At this time, the role played by the gravitational force in elementary-particle physics is unclear. We have not been able to measure directly any effect of gravity on the collisions of elementary particles.
With the identification of quarks and leptons as elementary particles, and the emergence of gauge theories as descriptions of the fundamental interactions, we possess today a coherent point of view and a single language appropriate for the description of all subnuclear phenomena. This development has made particle physics a much more unified subject, and it has also helped us to perceive common interests with other specialties. One important by-product of recent developments in elementary-particle physics has been a recognition of the close connection between this field and the study of the early evolution of the universe from its beginning in a tremendously energetic, primordial explosion called the big bang. Particle physics provides important insights into the processes and conditions that prevailed in the early universe, and deductions from the current state of the universe can in turn give us information about particle processes at energies that are too high to be produced in the laboratory, energies that existed only in the first instants after the primordial explosion.
WHAT WE WANT TO KNOW
Developments in elementary-particle physics during the past decade have brought us to a new level of understanding of physical laws. This new level of understanding is often called the standard model of elementary-particle physics. As usual, the attainment of a new level of understanding refocuses attention on old problems that have refused to go away and raises new questions that could not have been asked before. The quark model of hadrons and the gauge theories of the strong, weak, and electromagnetic interactions organize our present knowledge and provide a setting for going beyond what we now know.
Although the standard model provides a framework for describing elementary particles and their interactions, it is incomplete and inadequate in many respects. We still do not understand what determines the basic properties of quarks and leptons, such as their masses. Nor do we understand fully how the differences between the massless electromagnetic force carrier, the photon, and the massive carriers of the weak force, the W and Z particles, arise. Existing methods for dealing with these questions involve the introduction of many unexplained numerical constants into the theory-a situation that many physicists find arbitrary and thus unsatisfying. Physicists are actively seeking more complete and fundamental answers to these questions.
Another set of questions goes beyond the existing synthesis. For example, how many kinds of quark and lepton are there? How are the quarks and leptons related, if they are related? How can the strong force be unified with the already unified electromagnetic and weak forces?
Then there are questions related to our overview of elementary-particle physics. Are the quarks and leptons really elementary? Are there yet other types of forces and elementary particles? Can gravitation be treated quantum mechanically, as are the other forces, and can it be unified with them? More generally, will quantum mechanics continue to apply as we probe smaller and smaller distances? Do we understand the basic nature of space and time?
THE TOOLS OF ELEMENTARY-PARTICLE PHYSICS
Elementary-particle physics progresses through a complicated interaction between experiment and theory. As experimental work produces new data, theory is tested by the data, and theory is used to organize the data. Sometimes theoretical insight leads to new experiments; sometimes an experiment produces surprising new data that upset currently accepted theories. Patient accumulation of data may lead to paradoxes that cannot be resolved without major revision of theoretical ideas. And sometimes experimenters may seek new entities, such as free quarks or magnetic monopoles, which do not fit known patterns. In the end, physics is an experimental science and it is only experiment and observation that can tell us if we are right or wrong.
Most experiments in our field are carried out by the use of accelerators, which produce beams of high-energy particles. These beam particles collide either with a stationary target (a "fixed-target" experiment) or with another beam of particles. Accelerators in which two beams of particles collide are called colliders. Either in fixed-target experiments or in colliders, the results of the collisions are recorded with devices, often complex, called particle detectors. Accelerators and particle detectors are the main tools of elementary-particle physics. Through the years invention, research, and development have led to major innovations and vast improvement in the technology of accelerators and detectors. In turn, these tools are fundamental to experimental progress in our field.
The fixed-target experiments of the past two decades have contributed much to our knowledge. Examples of these experimental results are the demonstration that neutrons and protons are composed of quarks, one of the two simultaneous discoveries of the fourth (or charmed) quark, the discovery of the fifth (or bottom) quark, and the discovery of the violation of what were thought to be fundamental symmetries in time and space. Fixed-target experiments have accumulated a large body of data that has led to the systematic understanding of the interactions of hadrons.
Experiments utilizing colliders have become increasingly prominent because more of the beam energy is available to the fundamental collision processes. The extension of colliding-beam accelerator technology was led by the development of electron-positron and proton-proton colliders and by other basic advances in that technology. Experiments at electron-positron colliders have given us the shared discovery of the charmed quark; the discovery of the unexpected new "relative" of the electron-the tau lepton; the discovery of intense jets of hadrons; and much of the evidence for the theory that the strong force is mediated by the gluon particle. Recently the development of the proton-antiproton collider contributed substantially to particle physics by making possible the discovery of the carriers of the weak force-the W and Z particles. This development confirms an expanded future role for proton-proton and proton-antiproton colliders.
Most of the discoveries described above were made possible only through the building of new high-energy particle accelerators. This is most evident in the discoveries of the new massive particles, such as the W, the Z, the heavy quarks, and the new lepton. Higher-energy accelerators in the future will similarly open up the possibility of discovering new fundamental particles of still higher mass.
Progress in elementary-particle physics also depends on studying rare or unusual collisions. Therefore it is important to have very intense beams of particles to produce the rare events within a back ground of less-interesting phenomena. Thus, both intensity and energy are critical parameters of high-energy accelerators.
THE FUTURE OF ELEMENTARY-PARTICLE PHYSICS IN THE UNITED STATES
Elementary-particle physics is perhaps the most basic of the sciences; it interacts with many other areas of physics and astronomy; it develops, stimulates, and uses new technologies. Two decades ago the United States was the dominant force in elementary-particle research. Gradually other regions, particularly Western Europe and Japan, have increased their elementary-particle physics programs until together they equal or exceed the U.S. program in personnel, financial support, and scientific accomplishment. This is as it should be, since science is a worldwide endeavor. International participation leads to innovation in accelerator and detector technology, to an interchange of ideas, and to a more rapid pace of discovery. Indeed, many of the most important recent discoveries have been made in Europe. This report includes recommendations for the future U.S. program in this field that are intended to exploit the scientific opportunities before us and to permit us to maintain a competitive role in the forefront of this science.
The program for the future of the field embodied in our recommendations has emerged from an intense discussion within the community of elementary-particle physicists. During the past 3 years physics study groups and federal advisory panels have considered several different initiatives for new facilities. They have also considered the balance between support of existing facilities and construction of new facilities. Ultimately the choice was determined by the belief that new phenomena that are crucial to the understanding of fundamental problems will be discovered in the tera-electron-volt (TeV) mass range. This region cannot be reached either by existing accelerators or by the accelerators now under construction. The successful conclusion of the long and difficult development of superconducting magnet technology makes a large new machine a feasible and timely choice. Our recommendations form a plan that has as its keystone the construction of a very-high-energy superconducting proton-proton collider, the Superconducting Super Collider (SSC).
RECOMMENDATIONS FOR UNIVERSITY-BASED RESEARCH GROUPS AND USE OF EXISTING FACILITIES IN THE UNITED STATES
The community of elementary-particle physicists in the United States consists of about 2400 scientists, including graduate students, based in nearly 100 universities and 6 national laboratories. They work together in groups frequently involving several institutions. It is their experiments, their calculations, their theories, their creativity that are at the heart of this field. The diversity in size, in scientific interests, and in styles of experimentation of these research groups are essential to maintaining the creativity in the field. Therefore we recommend that the strength and diversity of these groups be preserved.
Most elementary-particle physics experiments in the United States are carried out at four accelerator laboratories. Two fixed-target proton accelerators are now operating: the 30-GeV Alternating Gradient Synchrotron at the Brookhaven National Laboratory and the 1000-GeV superconducting accelerator, the Tevatron, at the Fermi National Accelerator Laboratory. Cornell University operates the electron-positron collider CESR. The Stanford Linear Accelerator Center operates a 33-GeV fixed-target electron accelerator, which also serves as the injector for two electron-positron colliders, SPEAR and PEP. In addition, some elementary-particle physics experiments are carried out at medium-energy accelerators that are devoted primarily to nuclear physics.
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