Executive Summary

INTRODUCTION

The Committee on Elementary-Particle Physics was convened by the National Research Council's Board on Physics and Astronomy to assess the field of elementary-particle physics as part of the survey series Physics in a New Era. The committee was charged to make recommendations about the role that the United States should play in research in this field in the next two decades. The members of the committee, all active researchers in the field, brought a diversity of perspectives to bear on this study.

In preparing this report, the committee had two main objectives: (1) to describe the current status of elementary-particle physics and the most important research issues within this domain; (2) to identify the elements of a research program for the next two decades that, given limited resources, represents a wise approach to addressing these issues and maintaining the United States as a leader in the field.

ELEMENTARY-PARTICLE PHYSICS: EXPLAINING THE PHYSICAL WORLD

How it is that our universe came to be so rich and varied? Why are there stars, light, planets, and a hundred different atoms that can be combined into countless molecules? Elementary-particle physicists seek answers to these questions by studying subatomic particles and forces. Although these investigations require sophisticated instruments to reveal phenomena far smaller and more en-



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Executive Summary INTRODUCTION The Committee on Elementary-Particle Physics was convened by the National Research Council's Board on Physics and Astronomy to assess the field of elementary-particle physics as part of the survey series Physics in a New Era. The committee was charged to make recommendations about the role that the United States should play in research in this field in the next two decades. The members of the committee, all active researchers in the field, brought a diversity of perspectives to bear on this study. In preparing this report, the committee had two main objectives: (1) to describe the current status of elementary-particle physics and the most important research issues within this domain; (2) to identify the elements of a research program for the next two decades that, given limited resources, represents a wise approach to addressing these issues and maintaining the United States as a leader in the field. ELEMENTARY-PARTICLE PHYSICS: EXPLAINING THE PHYSICAL WORLD How it is that our universe came to be so rich and varied? Why are there stars, light, planets, and a hundred different atoms that can be combined into countless molecules? Elementary-particle physicists seek answers to these questions by studying subatomic particles and forces. Although these investigations require sophisticated instruments to reveal phenomena far smaller and more en-

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ergetic than we are aware of in daily life, the deep connection between the two realms inspires researchers in elementary-particle physics and lends added significance to their investigations. In fact, the properties and interactions of the elementary particles have much to say about the properties of the world around us. A century ago, the first elementary particle-the electron-was identified. A revolutionary view of the way matter in the universe is put together was provided by experimental evidence that electrons were basic constituents of all atoms and that they carried electricity. The theory of quantum mechanics explained the paradoxical motion of electrons in the atom and the formation of molecules. Eventually, a vast range of phenomena-the stiffness of steel, the way gasoline burns, the colors in the surface of a bubble, the ability of x rays to reveal tumors inside the body-could be accounted for by the quantum mechanical behavior of the electron. This new perspective, a view of the world on a more fundamental scale than the everyday world of our experience, led to a century of spectacular science and technological innovation. Understanding the behavior of the electron and the photon (the quantum of light) has been critically important to the fields of chemistry, materials science, and biology, as well as to the development of modern computing and communication. Particle physicists further zoomed in on the subatomic realm with increasingly powerful instruments. Forces were revealed on the subatomic level that no one had predicted, the best example being the strong nuclear force that holds the atomic nucleus together. Experiments revealed the existence of hundreds of different-and unexpected-particles. Eventually patterns emerged and theories were put together and tested; today, elementary-particle physics provides the basis for understanding an astonishing variety of phenomena-including those in our daily lives-in terms of just a few truly elementary particles and the forces between them. The remarkable state of our understanding of elementary particles, embodied in the present theory called the ''Standard Model," has taken shape over the last 30 years. The Standard Model provides an organizing framework for the known elementary particles. These consist of "matter particles," which are grouped into "families," and "force particles." The first family includes the electron, two kinds of quarks (called "up" and "down"), and a neutrino, a particle released when atomic nuclei undergo radioactive decay. There are two more families consisting of progressively heavier pairs of quarks and a corresponding lepton and neutrino. All normal, tangible matter is made up only of particles from the first family, since the others live for very short times. Why are there three families? This question is one of the great unsolved mysteries of elementary-particle physics. The matter particles exert forces on one another that are understood as resulting from the exchange of the force-carrying particles. Electric and magnetic forces arise when particles exchange photons (the familiar repulsion or attraction

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of two magnets results from one of them emitting photons that the other receives). The strong force that holds quarks together to form protons and neutrons comes from the exchange of gluons. The weak forces that cause radioactive decay are created by massive W and Z particles (the photon and gluon have no mass). These three forces have been successfully described by quantum theories that have remarkably similar structures. Yet crucial questions remain unanswered by the Standard Model. For example, the masses of elementary particles can be established by measurement, but they appear to be arbitrary. There is not a set of rules that allows these masses to be calculated or that explains why the up quark is a bit lighter than the down quark. However, the consequences of these values are profound. Were it the other way around, creation of the heavier elements in the interior of stars —elements essential for the existence of planets and life—would have been dramatically different, leading to a far duller universe. ELEMENTARY-PARTICLE PHYSICS AND SOCIETY Elementary-particle physics is basic research, driven by intellectual excitement and the desire to understand the underlying structure of the universe. Its discoveries illuminate all of science, and the technology developed in the course of this research may ultimately be applied for practical benefit. Synchrotrons were developed to accelerate particles, cause collisions that create new particles, and provide clues about their interactions. A by-product of accelerating particles is the production of intense electromagnetic radiation from the visible part of the spectrum all the way to x rays. Several laboratories now operate synchrotrons purely for the purpose of generating such radiation; they are invaluable for researchers in surface chemistry, materials science and engineering, environmental science, and biology. Biological applications are growing at a rapidly accelerating pace and promise to give new insights into living systems. Devices and techniques developed for elementary-particle physics research are important in several medical imaging techniques. Computer-aided tomography (the CT scan) and positron-emission tomography (the PET scan) use detectors largely developed for particle physics experimentation. Development of the industrial capability to produce large quantities of high-quality superconducting wire, in order to meet the demands of particle accelerators, led directly to the billion-dollar world market in this wire, primarily for use in magnetic resonance imaging (MRI). The World Wide Web, which was developed to enable elementary-particle physicists around the world to share information quickly and easily, now gives every school with a computer access to the largest library of information on the globe.

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These and other offshoots have been immensely valuable and have had a profound impact on other sciences and on our society. Elementary-particle physicists take pleasure in making these contributions for the good of society, but their main goal is to understand the universe: why it looks the way it does today, how it evolved from the earliest moments, and what its ultimate fate will be. The intellectual significance of the field is reflected in the number of Nobel Prizes awarded to elementary-particle physicists, in the illumination that elementary particle physics has provided to other branches of science, and most important, in the new picture it is developing of the way in which fundamental particles and forces shape our world. National support of endeavors such as astronomy and elementary-particle physics is dedicated to the proposition that deepening our knowledge of the world we inhabit increases the pleasure, richness, and value of life. When a nation takes pride in contributing to such explorations, it says something important about itself. KEY RESEARCH OPPORTUNITIES Recent advances in technology, experimental techniques, and theoretical understanding mean that over the next two decades, it will be possible to investigate some of the most compelling issues in particle physics. Questions that once seemed beyond the scope of science now lie within the reach of experiment, and it will be possible to achieve a deeper understanding, not only of elementary particles, but also of the earliest moments in the history of the universe. Determining What Gives Elementary Particles Their Mass One especially important opportunity is to understand what determines the observed—and very disparate—masses of both the force particles and the matter particles. The Standard Model of particle physics invokes a very special kind of particle, called the Higgs (after the theorist who suggested it). Higgs particles (there may be one or more kinds) are unlike any other particles. Their effects are ever present, even in the vacuum: they give all matter particles their mass and account for the large mass of the carriers of the weak force and the masslessness of the carrier of the closely related electromagnetic force. (Physicists refer to this important asymmetry as electroweak symmetry breaking.) Other possible explanations involve new. very strong forces. Only experiments at higher energies than those to which experimenters now have access will conclusively determine which, if any, of these theories is actually realized in nature. One thing is certain: The creation of mass involves some completely new physics that must show up in experiments at sufficiently high-energy. There are very compelling theoretical arguments guaranteeing that this physics will be uncovered by the next generation of experiments.

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Supersymmetry and Strings A long-standing goal of science has been to find the simplicity underlying the wide diversity in nature. Two examples of such achievements in basic physics were the realization that sound and heat could be understood in terms of the motion of atoms, and Maxwell's synthesis of electricity, magnetism, and light, in terms of the electromagnetic field. These unifications of seemingly disparate phenomena resulted in a far deeper, far more useful understanding. Theories have recently been developed that link elementary particles, the very smallest known structures, with one of the grandest questions of all: How did the universe begin? These theories suggest that many of the complexities manifest at lower energies would be greatly simplified under conditions of extremely high-energy. For example, under such conditions the different forces between the particles would be seen as unified, different manifestations of a single underlying force. There is nothing artificial about these energies: They are understood to have prevailed at the time of the "big bang" that initiated the expansion of the universe. If these theories are correct, the simplicity long sought by particle physicists is a fact of cosmic history. Although re-creating these energies lies beyond the reach of existing and planned accelerators, experiments at these accelerators could nonetheless reveal evidence for what is called supersymmetry. Supersymmetry, which is really a very profound statement about the structure of space and time, predicts that for every fundamental particle there should exist another related and as yet undiscovered new particle; conclusive evidence for these additional particles is eagerly sought because supersymmetry shows how the electroweak and strong interactions could be unified. In addition, evidence of supersymmetry would also support another even more comprehensive theory, called string theory. Traditionally, elementary particles have been modeled as points that take up no space at all. This approach leads to some theoretical problems because two particles could (in principle) get extremely close and exert arbitrarily large forces on each other. String theory solves this problem by picturing particles as extremely tiny vibrating loops, with the details of their vibrations determining their properties and interactions. This simple idea, with the aid of recent theoretical developments, leads to a theory that is able to encompass all of the forces of nature in a unified and self-consistent manner, including—for the first time—gravity. Role of Antimatter in the Universe Deep mysteries are connected with the way particles from different families transform into one another. For example, a tiny but startling asymmetry in the behavior of matter and antimatter has been found in certain particle interactions. These particle decays violate what is called CP or time-reversal symmetry. This

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curious phenomenon is crucial to knowing how matter came to predominate in the universe because without it, there would be no stable chunks of matter in the form of stars, planets, and ultimately, human beings to wonder at it all. However, there is as yet no fundamental understanding of this important asymmetry. Experiments in the coming decade are poised to increase greatly our knowledge in this area. Of course, new understanding does not always proceed along a direct path from prediction to discovery. The history of particle physics is full of unexpected experimental results, which have lighted the way to more useful and complete models. Historically, such important surprises have been most probable when experiments are conducted at previously inaccessible energies. ACCELERATORS: INTERNATIONAL INSTRUMENTS OF RESEARCH The Role of Accelerators The scale of elementary particles is so far removed from the human scale that it is almost impossible to comprehend. Elementary-particle physicists today are exploring phenomena on a scale as small as 10−18 m. An object this size compares in length to a meter stick as a meter stick compares to the distance light travels in 100 years. It is amazing that there is a way to study objects this small, but it can be done with particle accelerators, the "microscopes" of particle physics. Accelerators have become so complex and expensive that national, and increasingly international, collaborations are required to design, build, and operate them. When particles of sufficiently high-energy collide, new particles are created out of the energy of the collision. The higher the energy of the collision, the more massive are the particles it can produce. There are strong theoretical arguments that the key to understanding some of the most important issues before elementary-particle physics today is attaining a high rate of collisions in the teraelectron-volt (TeV: 1012 eV) range, today's energy frontier. Research and Development Over the past half century, particle accelerators, particularly machines that bring two beams of particles into head-on collision (colliders), have been increasingly important to elementary-particle physics. Without them, it would have been impossible to obtain most of the data that led to the development of the Standard Model, and the important and beautiful structures of matter at layers below the visible world would have remained hidden. The energy of accelerators has been increased spectacularly over this period, not simply by enlarging or improving a given design but as a result of many innovations and

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technological breakthroughs by engineers and accelerator physicists. Where the exact limits of the current types of machines lie is debated, but it is clear that to move substantially beyond the capabilities of present technologies in a cost effective manner, we must continue to find new ways to accelerate particles. Accelerator development has created a sophisticated technology with uses in other areas of science, chief among which is the synchrotron light source mentioned earlier, and continued development can reasonably be expected to make broader contributions of this kind. THE CURRENT U.S. PROGRAM The scientific strength of the United States in the field of elementary-particle physics is manifest in the quality and influence of the research it carries out. Members of this community, traditionally some 2,000 strong, have played important and leading roles in obtaining incisive experimental results, coaxing innovative technologies into existence, and developing important breakthroughs in theory. One measure of excellence is the fact that many of the best students in the world choose to come to the United States for their graduate training. Another is the leading role that U.S. physicists currently play in preparing and executing experiments aimed at addressing many of the most significant research questions in the field. Research at Accelerator Laboratories The present generation of facilities includes the Stanford Linear Accelerator (SLAC) and the Fermi National Accelerator Laboratory (FNAL). SLAC is a 2-mile-long linear electron accelerator that is capable of accelerating electrons to an energy of 50 GeV ( 109eV), one million times more energy than acquired by electrons in the picture tube of a television set as they travel from the electron gun to the screen. At FNAL, protons are accelerated in the 4-mile circular Tevatron to an energy of 1,000 GeV. This is the most powerful accelerator now in operation, using approximately 1,000 superconducting magnets to steer particles along the desired paths. The United States is also home to the Cornell Electron Storage Ring (CESR), the highest-luminosity collider ever built, an electron-positron collider operating at approximately 5 GeV per beam, and the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory, a 30-GeV proton accelerator with the highest beam intensity in the world available for elementary-particle physics. U.S. researchers are currently conducting experiments at major overseas accelerators, including the Large Electron-Positron collider (LEP) at CERN (the European Laboratory for Particle Physics) near Geneva, the highest-energy electron-positron collider ever constructed: HERA, the world's only electron-proton collider, at the DESY laboratory in Hamburg: and modest-energy electron

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colliders in Beijing and Novosibirsk. They will resume working at Japan's KEK laboratory in the very near future. An important aspect of experiments at all these accelerators is the great complexity and sophistication of the apparatus at the business end of the accelerator. Very high-energy collisions generate a vast profusion of particles. To separate out the interesting events requires complex systems of detectors to trace the paths of the particles, using extremely high-speed electronics to evaluate the events in real time. All of this equipment must have capabilities that far exceed those available commercially. The processing power of the custom high-speed electronics used to untangle the massive bursts of data that cascade out of the detectors compares with the capabilities of the fastest supercomputers. Research with Particles from Space Accelerator-based research is complemented by important studies that detect particles produced in space. One such study addresses the flux of neutrinos emitted from the interior of our Sun as it burns. Over the last two decades, careful measurements with enormous detectors at several independent sites have revealed that the number of neutrinos detected is smaller than the number predicted. Many of the speculations that attempt to explain the discrepancy have significant implications for our picture of elementary-particle physics. A question that has long troubled astrophysicists is the composition of 90% of the mass of the universe, which is invisible—its presence betrayed only by its gravitational effect on the behavior of galaxies. One exciting possibility is that this so-called dark matter, the focus of a second group of studies, consists partly of particles predicted by the theory of supersymmetry. The third study is concerned with the origin of the highest-energy cosmic rays that must originate in cosmic events of extreme violence. These could be produced by some unknown fundamental process, possibly involving relics of the early universe. When they strike Earth's atmosphere, they generate a whole cascade of particles that show up in large arrays of detectors on Earth's surface. Over the next two decades, questions of the greatest importance to understanding the universe at its most fundamental level will at last come within reach of experiment. How elementary particles acquire their mass and whether the known forces are simply manifestations of a single underlying force are two of the most significant issues that must be addressed in comprehending the world around us. It is deep and profound questions such as these that first capture the imaginations of bright young people, whether or not they work in particle physics, leading them to and sustaining them on the challenging and difficult road to a technological education. These are the very bright young people that eventually become our scientists and engineers. The committee believes that these issues are sufficiently compelling that the

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U.S. particle physics community should play a leading role in the international endeavor to conduct research capable of addressing them. If the recommendations in this report are adopted, the United States can be at the forefront of this profound and fascinating intellectual adventure. RECOMMENDATIONS FOR U.S. ELEMENTARY-PARTICLE PHYSICS The committee has developed its recommendations with two goals: (1) to exploit the great opportunities for discovery that lie ahead and (2) to maintain U.S. leadership in the field of elementary-particle physics. These goals require a diverse but focused program. We are poised on the threshold of a new energy frontier, where discoveries are certain to be made and new phenomena are likely to be revealed. This is the TeV mass scale, where both well-established theory and revolutionary ideas predict new physics. First, the remarkable success of the Standard Model ensures that the secret of electroweak symmetry breaking will be revealed at this scale. Second, the exciting idea of supersymmetry, which offers the hope of great insights into unification of all the forces of nature, predicts that a rich array of new particles can be produced. Finally, we will obtain the first glimpse of physics well above the typical mass scale of the Standard Model. In the past, when such a large step has been taken, dramatic experimental surprises have occurred. One might expect that similar revolutionary discoveries will be made at the TeV mass scale. The committee therefore believes that the highest priority is full involvement in TeV mass scale physics at large facilities uniquely suited to this purpose. This involvement includes exploiting the Fermilab Collider (presently the highest-energy facility extant); strong participation in construction of and research at the Large Hadron Collider (LHC) being built in Europe; and taking a leadership role in a future forefront international facility, possibly to be built in the United States. This path has historically provided the most fruitful avenue for uncovering new phenomena. Other problems of great importance to the understanding of elementary particles do not require the highest energies for elucidation. One is understanding rare quark and lepton transitions. Another is the nature of CP violation—a phenomenon that bears on the apparent dominance of matter over antimatter in the universe. There are additional astrophysical questions of great importance that can likely be explained by particle physics dynamics, the most important being the nature of dark matter. A number of the most important findings in the field in the past two decades have been made by experiments studying problems such as these, and facilities presently being upgraded or under construction will allow such studies to continue. The committee believes it is crucial to support a well-targeted program in these areas. Given the limited resources that will be

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available, however, maintaining a proper balance between such efforts and those at the energy frontier will require difficult choices and keen foresight. The committee's recommendations are therefore grouped into two classes: first, those relevant to the energy frontier, and second, one concerning important studies that are best done elsewhere. Both are essential to a balanced program. Before presenting its recommendations concerning experimental initiatives, the committee comments on two subdisciplines of the field that are critical elements of a forefront program: non-facility-specific advanced accelerator R&D, which can lead to extension of the energy frontier, and theoretical physics, which provides the framework that organizes our observations. Advances in elementary-particle physics have historically been tied to advances in accelerator technology. Accelerator research and development is of two general types—efforts targeted at the design and construction of specific facilities and more generic (and forward-looking) R&D targeting completely new methods of acceleration that will be required to support energy frontier facilities decades from now, should the physics demand it. This report contains specific recommendations with regard to the former. It is necessary to maintain an appropriate level of investigation in the latter area to secure the longer-term future of the field. Theoretical work in elementary-particle physics provides the intellectual foundation that motivates and interconnects much of experimental research. The more formal areas of theoretical physics, especially string theory, hold the promise of providing a picture of the universe that accounts for an extremely broad range of observations and phenomena. The committee believes that a healthy level of activity both in formal areas and in the more phenomenological investigations that touch directly on experiments now and in the coming decade should be maintained. 1. Recommendations Concerning the High-Energy Frontier At the present time, the Tevatron at Fermilab and the Large Electron-Positron collider (LEP II) in Geneva are the only machines operating at the energy frontier. In two years, LEP II will be dismantled, leaving the Tevatron alone at this frontier until completion of the LHC in the middle of the next decade. The LHC will dramatically extend the energy reach, pushing beyond the TeV scale, where we know that the physics of electroweak symmetry breaking must appear. However, this report concludes that in the future, another collider will be required to complement or extend the range of the LHC and to explore fully the physics of the TeV scale. These considerations motivate a chronological structure for the committee's recommendations concerning the high-energy frontier.

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1.a. Recommendation on the Fermilab Collider Facility The United States should capitalize on the potential of the Fermilab Collider Facility while it has unique capabilities for investigations of high mass scale physics. The Tevatron collider is the highest-energy accelerator in the world today and will remain so until the LHC era. The recent discovery of the top quark at this facility demonstrates its power to explore physics that is otherwise inaccessible. Its capabilities will be considerably enhanced with the new Main Injector. Although the LHC will be the first machine to extensively explore electroweak symmetry breaking, some of the new particles associated with the TeV scale might exist within the reach of the Tevatron. In particular, the upgraded Tevatron collider facility might discover supersymmetry. This would dramatically enhance our understanding of the universe. 1.b. Recommendation on the Large Hadron Collider The committee enthusiastically endorses U.S. participation in the Large Hadron Collider project as a vital and essential component of the U.S. experimental particle physics program. In the middle of the next decade, the LHC will supersede the Tevatron Collider as the highest-energy machine in the world. U.S. physicists, with their extensive experience at Fermilab and in the research and development toward construction and use of the Superconducting Super Collider (SSC), have established critical roles in the construction of the LHC machine and of the two largest experiments. The resources involved have been established in an agreement reached in 1997 by the Department of Energy, the National Science Foundation, and CERN, the host laboratory. The LHC will systematically explore a new energy regime, the TeV mass scale. LHC experiments will elucidate the mechanism of electroweak symmetry breaking, the central question of elementary-particle physics. The experiments will decisively test the prediction that a rich array of supersymmetric particles appears at this mass scale. If supersymmetry is indeed present at the TeV scale, the LHC will initiate the exploration of a vast new world. The committee is convinced that participation in the enormously exciting physics promised by the LHC is essential for the vitality and continuity of U.S. particle physics. The committee also believes that U.S. participation is vital for the success of the project. 1.c. Recommendations on the Next Generation of Accelerators As this report emphasizes, the committee anticipates that major discoveries will be made at the LHC. These will almost certainly point toward new phenomena that physicists will want to explore using an appropriate new collider.

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Three types of machines have been discussed by the physics community: electron-positron linear colliders, muon colliders, and very large hadron colliders. Each has its unique capabilities and challenges, and each is at a different stage of development. Only the linear collider is far enough along to proceed to a conceptual design, with the engineering details and cost and schedule information appropriate to this stage. The other two options are sufficiently promising that increased research efforts are called for to make more realistic preliminary designs. These steps will put the community in the position to make a decision in the future about starting a new collider construction project with the best information possible. A collider that complements or extends the reach of the LHC will require multiyear and multinational cooperation because of the magnitude of the resources needed. If the United States is to maintain a leadership role in this enterprise, it must participate both in accelerator technology development and in international decisions on the choice of technology and the location of the next facility. Although it is highly desirable to have a forefront facility located within the United States, it is crucial that the United States maintain a technological base sufficient to allow full participation in all aspects of the design, construction, and operation of such a facility, independent of its ultimate location. Recognizing that it is too soon to endorse construction of any new machines, the committee makes recommendations concerning the development of each. 1.c.1. Recommendation on Electron Colliders The committee recommends support of an international effort leading toward a complete design and cost estimate of an electron linear collider that would be able ultimately to reach a center-of-mass energy of 1.5 TeV and a luminosity of 1034 cm−2 s−1. An electron linear collider would contribute important measurements complementary to those from the LHC toward understanding the fundamental physics of the TeV mass scale. In the past, lepton colliders have been essential complements to hadron colliders. For example, W and Z bosons were discovered in a hadron collider, but many of their important properties could be determined only with the electron-positron colliders at LEP and the Stanford Linear Collider (SLC). For the physics of the TeV scale, this complementarity will likely continue to be important. Laboratories in the United States, Japan, and Europe have been engaged for many years in research and development on an electron linear collider operating with an energy of 1 TeV or more. Stanford Linear Accelerator Center (SLAC), with its unique expertise in linear collider technology and the experience gained through the construction and operation of the SLC, is playing a critical leading role in these efforts. Many of the systems required for a second-generation

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linear collider have been or are being demonstrated. The next natural step is a complete design report, accompanied by cost optimization studies and a complete cost estimate. The committee encourages the U.S. linear collider community to work cooperatively with international partners on the development of a common design and possible management structures. The effort to complete an electron linear collider design and optimized cost estimate could be finished early in the next decade. It will then be necessary for the United States, together with the international elementary-particle physics community, to consider a number of factors in deciding whether to propose construction: The physics case for such a collider in light of any new discoveries in the intervening years; The construction and operating costs of the facility, together with the commitments and plans of the nations interested in hosting or participating in a linear collider; and The status of development of muon and hadron colliders. 1.c.2. Recommendation on Muon and Hadron Colliders R&D targeted at developing the technologies for muon and very large hadron colliders should be vigorously pursued. Experiments at the LHC may indicate new physics at energy scales significantly beyond those that it can decisively reach. Extension of the energy frontier beyond the reach afforded by the LHC will require the development of new technologies. A muon collider or a very large hadron collider has the potential for supporting even higher energies and luminosities in the post-LHC era. R&D efforts in both of these areas are in the early stages. Muon collider technology remains to be demonstrated, so the need is to focus on the development and validation of concepts. Present-day hadron collider technology could likely be used to construct a facility with a reach significantly beyond LHC. but the cost would be prohibitive. Hence efforts in this realm should focus on a reduction of cost through the use of advanced technologies. Development of both muon and hadron collider technologies must be pursued in a timely fashion to determine whether they represent technologically and economically viable options for reaching energies beyond those explored with the LHC. 2. Recommendation for Addressing Important Fundamental Physics Problems Below the TeV Mass Scale The committee recommends strong support for a well-targeted program to study the fundamental particle physics that can best be explored with experiments below the TeV scale.

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In its first recommendation, the committee has emphasized the range of important physics questions that are addressed at the TeV scale. It is important to recognize, however, that a number of outstanding fundamental questions can best be studied using other techniques. Foremost among these are the understanding of quark and lepton flavor mixing and of particle-antiparticle asymmetry (CP violation). There are also astrophysical questions of importance to particle physics, such as the nature of dark matter. Experiments studying rare transitions between different families of quarks or leptons are extremely sensitive to new and interesting physics. For example, the 1964 experiment discovering CP violation found new fundamental physics that we are still trying to understand. One of the major themes of experimental particle physics in the next decade will be a systematic study of the interactions that mix the families of quarks and leptons. Experiments in this area include several categories: Decays of the bottom quark. The central question to answer is whether CP violation is explained within the framework of the Standard Model or whether it is due to some new physics. The Standard Model explanation makes specific predictions that can be tested with very large samples of B mesons. Decays of the strange quark. Although CP violation was discovered in the decays of K mesons containing the strange quark, there are still outstanding issues in the CP-violating decays of strange particles. Experiments using extremely intense kaon beams give unique information about CP violation. Neutrino oscillations. Many experiments now give hints that a neutrino of one family can change into one of another family. One of the most important discoveries possible in the next decade would be unambiguous confirmation of any one of these hints. A new era of research in these areas will begin in the next few years as experiments that should decisively answer many of the long-standing questions come on-line. Key U.S. facilities—the Positron-Electron Project II (PEP-II) and the CESR upgrade in addition to the Main Injector—will begin operations in the next few years with greatly enhanced capabilities to address this very important physics. It is important to operate the newly built facilities and fund their critical experiments at the level required to take advantage of the physics opportunities they present. Historically, the U.S. high-energy physics community has phased out programs to accommodate those that are more scientifically desirable, and it should continue to do so. Because of limitations in resources for the field worldwide, in the future, only initiatives that have the most promise for scientific advancement should be undertaken.

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CONCLUSION The recommendations above, if adopted, should maintain U.S. leadership in the field of elementary-particle physics well into the next century. They will allow our scientists to participate in what are likely to be profound and exciting discoveries, discoveries of a nature not seen before.