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9 Conclusions and Recommendations THE REVOLUTION OF THE PAST TWO DECADES During the past two decades our understanding of the fundamental nature of matter has undergone a revolution. We have found three families of elementary particles: the family of leptons' the family of quarks, and the family of force-carrying particles. The lepton family and the quark family each consist of three generations, with strong similarities between the generations. The four fundamental forces electromagnetic, gravitational, strong, and weak were known earlier, but during the last two decades the electromagnetic and weak forces have been unified theoretically and experimentally. In addition, the particles carrying the strong force, called gluons, and the particles carrying the weak force, the W and Z' have each been discovered. During the same period, the vast family of particles called the hadrons has been shown not to be elementary in themselves, but rather to be made up of quarks. In particular, the quark nature of the proton and neutron, the most common hadrons. has been studied and under- stood in great detail. We have also acquired a good understanding of how quarks behave inside hadrons and of how quarks interact when hadrons are involved in high-energy collisions. 194
CONCE USI ONS A ND RECOMMENDA TI ONS 1 95 HOUr THE REVOLUTION WAS MADE This revolution in elementary-particle physics came about because of three interacting components of particle research: Progress in Accelerators. Particle colliders and higher energy fixed- target accelerators have come into operation during the last two decades. These new facilities provided the high-energy particles that were needed for most of the experimental work involved in this revolu- tion. . Ned Experiments and Ne`~' Experimental Techniques. In physics, new discoveries are made experimentally. and all new ideas and theories must be tested and validated experimentally. Some of the revolution in particle physics occurred because theory predicted the existence of new phenomena or new particles. That is how the W was discovered. Conversely, sometimes experiments led the way. The tan lepton was discovered through an experimental search, and thus the concept of the third generation was introduced. Almost all of these experiments used new experimental techniques such as particle-detecting wire chambers and integrated circuitry. Theoretical Progress. The third necessary component of elementary- particle research is progress in elementary-particle theory. Sometimes that progress is in the form of an elegant mathematical theory, such as the theory that unifies the weak and electromagnetic forces. Sometimes it is in the form of a broad insight; for example, the realization that most,of the properties of hadrons can be directly explained by models of the behavior of quarks inside the hadrons. WHAT WE WANT TO KNOW With this revolution accomplished, we are now led to deeper questions about the basic nature of matter and energy. These questions could not be asked in a sensible way until we had identified the three families of elementary particles and learned how the four basic forces behave. Some of these questions express our dissatisfaction with present theories, which require several dozen unexplained numerical constants. What sets the values of these constants? Are they intercon- nected or independent? Among these constants are the masses of the various elementary particles and the strengths of the various basic forces. The masses of the particles are completely unexplained, because we do not yet understand the origin of mass. Other questions that we need to answer include the following: How
196 ELEMENTARY-PARTICLE PHYSICS . many generations of quarks or leptons are there? Are the quarks and leptons related to each other? If so, how? Can the strong force be unified with the already unified electromagnetic and weak forces? Then there are the questions that are related to our overview of elementary-particle physics. Are the quarks and leptons really elemen- tary? 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, does quantum mechanics apply in all parts of elementary-particle physics? Do we understand the basic nature of space and time? These questions indicate the hopes and opportunities for continued progress in elementary-particle physics during the next several dec- ades. Although the United States has until quite recently been the leading contributor to elementary-particle physics research, gradually other regions, particularly Western Europe and Japan, have substan- tially increased their contributions to this research. This is as it should be, since science is a worldwide endeavor. Elementary-particle physics is a basic science. It interacts with many other areas of physics and astronomy, and it develops, stimulates. and uses many new technolo- gies. In the belief that the United States should maintain a forefront role in particle-physics research, we conclude this report with a set of recommendations for the elementary-particle physics program in the United States. No priority is indicated by the order in which we present these recommendations. This program has as its centerpiece the plan. based on ongoing accelerator development and design work, for the construction in the United States of a very-high-energy, superconduct- ing proton-proton collider, the Superconducting Super Collider (SSC). In arriving at this program the elementary-particle physics commu- nity has had to choose from a number of alternative physics directions and from proposals for building other types of new accelerators and facilities. A great deal of thought, research, and discussion has gone into the program described here. 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
CONCLUSIONS AND RECOMMENDA TIONS 197 at the heart of this field. The diversity in size, in scientific interests, and in styles of experimentation of these research groups are essential for maintaining the creativity in the field. Therefore itch recommend float the strength apace din Fruity of these groups be pre.ser~'ed. 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 (AGS) at the Brookhaven National Laboratory and the 1000-GeV superconducting accelerator. the Tevatron, at the Fermi National Accelerator Laboratory. Cornell University operates the elec- tron-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 primarily devoted to nuclear physics. Experimentation at the four accelerator laboratories requires com- plex detectors that are often major facilities in their own right. The equipment funds for major detectors and the operating funds for the accelerators have been insufficient to allow optimum use. Because accelerator laboratories necessarily have large fixed costs, the produc- tivity of the existing accelerator facilities can be increased considerably by a modest increase in equipment and operating funds. We recom- mend fuller support of existing facilities. RECOMMENDATIONS FOR NEW ACCELERATOR FACILITIES IN THE UNITED STATES The capability of two existing accelerators in the United States is now being extended by adding collider facilities to each of them. A 100-GeV electron-positron collider, using a new linear collider princi- ple, is now being constructed at the Stanford Linear Accelerator Center. The Tevatron at the Fermi National Accelerator Laboratory is being completed so that the superconducting ring can also be operated as a 2-TeV proton-antiproton collider. We recommend continued support for the completion of these reels colliders on their present schedule. In addition, ice recommend that their experimentalfacilities and programs be fully developed. The United States elementary-particle physics community is now carrying out an intensive research. development, and design program intended to lead to a proposal for a very-high-energy, superconducting proton-proton collider, the Superconducting Super Collider (SSC). It
198 ELEMENTARl'-PARTICLE PHYSICS a will be based on the accelerator principles and technology that have been developed at several national laboratories. in particular the extensive experience with superconducting magnet systems that has been gained at the Fermi National Accelerator Laboratory and Brook- haven National Laboratory. The SSC energy would be about 20 times greater than that of the Tevatron collider. This higher energy is needed in the search for heavier particles. to find clues to the question of what generates mass, and to test new theoretical ideas. Our current ideas predict ~ rich world of new phenomena in the energy region that can be explored for the first time by this accelerator. Furthermore. history has shown that the unexpected discoveries made in a new energy regime often prove to be the most exciting and fundamentally important for the future of the field. On its completion this machine will give the United States ~ leading role in elementary-particle physics research. Sitcom the SSC is ~ c~trc~l to to/? fitter `'f c~le~ne''tc~r!~-particl`, pl'ysics r esearch in th`, U''itec! St~`tes lie .stro'?,£~1! r ecom',2c,'~d its e.rpeditio'~s co~zstr'~c- tion. RECOMMENDATIONS FOR ACCELERATOR RESEARCH AND DEVELOPMENT Since accelerators are the heart of most elementary-particle experi- mentation, physicists are continuing research and development work on new types of accelerators. indeed, technological innovation in accelerators has been the driving force in extending the reach of high-energy physics. An important part of this work is concerned with extending the electron-positron linear collider to yet higher energies. One of the purposes of the construction of the Stanford Linear Collider is to serve as a demonstration and first use of such a technology. Advanced accelerator research is also exploring new concepts. based on a variety of technologies. that may provide the basis for even more powerful accelerators perhaps to be built in the next century. Such research also leads to advances in technology for accelerators used in industry, medicine. and other areas of science such as studies based on synchrotron radiation. We recommend strong support for research and development work in accelerator physics and technology. RECOMMENDATIONS FOR THEORETICAL RESEARCH IN PARTICLE PHYSICS Theoretical work in elementary-particle physics has provided the intellectual foundations that motivate and interconnect much experi-
CONCL USI OILS A ND RECOMMENDA TI ONS 1 99 mental research. Theorists working in elementary-particle physics have also played an important role in forging links with other disci- plines, including statistical mechanics. condensed-matter physics. and cosmology. Theoretical physicists make vital contributions to univer- sity research programs and to the education of students who will enter all branches of physics. W`, ret o',~7`e,Zd tl'at the e`-isti'~g' st'o''g' supper ~ fit/ c' h/ `~`c] pI'G#'/'Ci'7? Of tI2c~'reti~al researcl2 in the `'ni`~ersitie~, instit''te.s acid '~c~ti`~al laboratories be c o'`ti''``ecI. A new element of theoretical research is the increasing utilization of computer resources, which has spurred the development and implementation of new computer architectures. This trend will require the evolution of new equipment-funding patterns for theory. RECOMMENDATIONS FOR NONACCELERATOR PHYSICS EXPERIMENTS It is appropriate that some fraction of the particle-physics national program be devoted to experiments and facilities that do not use accelerators. These experiments include the searches for proton decay using large underground detectors, the use of cosmic rays to explore very-high-energy particle interactions, the measurements of the rate of neutrino production by the Sun, and the use of nuclear reactors to study subtle properties of neutrons and neutrinos. There are also diverse experiments searching for evidence of free quarks, magnetic monopoles, and finite neutrino mass. Still other classes of experiments overlap the domain of atomic physics; these include exquisitely precise tests of the quantum theory of electromagnetism, studies of the mixing of the weak and electromagnetic forces in atomic systems, and searches for small violations of fundamental symmetry principles through a variety of different techniques. Many of these are small-scale laboratory experiments. Some provide a means of probing an energy scale inaccessible to present-day accelerators. The value of these experiments is substantial. They `` ill continue to play a vital role that is complementary to accelerator-based research, and bile recommend their continued support. RECOMMENDATIONS FOR INTERNATIONAL COOPERATION IN ELEMENTARY-PARTICLE PHYSICS Our program should be designed to preserve the vigor and creativity of elementary-particle physics in the United States and to maintain and
200 ELEMENTARY-PARTICLE PHYSICS extend international cooperation in the discipline. We recommendfour guidelines for so lit a balanc ed program. First, the continued vitality of American elementary-particle physics requires that there be forefront accelerator facilities in the United States. The use of accelerators developed by other nations provides a needed diversity of experimen- tal opportunities but it does not stimulate our nation's technological base as does the conception. construction, and utilization of innovative facilities at home. The Superconducting Super Collider will be a frontier scientific facility. and the technological advances stimulated and pioneered by its design and construction will serve the more general societal goals as well. Second, the most productive form of cooperation with respect to accelerators is to develop and build complementary facilities that allow particle physics to be studied from different experimental directions. Third, the established forms of international cooperation, including the use of accelerators of one nation by physicists from another nation, should be continued. Fourth, looking beyond the program proposed in this report, there should be further expansion of international collaboration in the planning and building of accelerator facilities. CONCLUSION We believe that the implementation of these recommendations will enable the United States to maintain a competitive forefront position in elementary-particle physics research into the next century. Central to this future is the construction of the SSC, the very-high-energy proton-proton collider using superconducting magnets.
