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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics 3 The Experimental Opportunities As described in Chapter 2, recent discoveries in particle physics have led to the key scientific challenges that now define the frontiers of research in the field. This chapter looks at experiments that could be done in the coming decade to address these exciting research challenges. Some of the facilities needed to carry out the next generation of experiments are now being built, such as the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN), new experimental facilities at the Japan Proton Accelerator Research Complex (J-PARC), experimental devices designed to measure cosmic microwave background (CMB) radiation, detectors for high-energy particles from cosmic sources, and instruments to detect gravity waves. Other key experimental facilities—such as the proposed International Linear Collider (ILC); enhanced neutrino studies at accelerators, at reactors, and in large underground laboratories; proton decay experiments; and new space-based experiments—are the subject of planning and ongoing research and development. This chapter divides potential experiments into three categories: those using high-energy beams, those using high-intensity beams, and those using particle sources provided by nature. As is the case throughout particle physics, different experiments can address the same questions from different perspectives, revealing the rich interconnections within the field and between particle physics and other fields. The chapter concludes by outlining the increasing importance of international collaboration in particle physics—collaboration that best meets the needs of science and represents the most responsible public policy.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics As the preceding chapter demonstrated, particle physics has entered a special time. The most exciting scientific questions that need to be addressed are clear. The next cohort of experiments needed to address many of those questions are about to begin or are on the scientific horizon. Expert groups of scientists, engineers, and advanced students are available and eager to move this segment of the scientific frontier forward. A goal that has occupied science for centuries—gaining a fuller and deeper understanding of the origins and nature of matter, energy, space, and time—is ready for what may be a revolutionary leap forward. HIGH-ENERGY BEAMS: DIRECT EXPLORATION OF THE TERASCALE Discoveries at the Terascale With experimental study of the Terascale about to begin, physicists are finally gaining the tools needed to address questions that have been asked for decades: Why do the weak interactions look so different from electromagnetism, given that the fundamental equations are so similar? Where do particle masses come from? Does the Standard Model describe them correctly, or do the particle masses come from some more exotic mechanism? Are the forces of nature unified at some high energy scale? With the elementary particles known today, unification does not quite work, but it fails in a way that suggests the missing pieces will be found at the Terascale. Do space and time have additional dimensions? Do they have new quantum dimensions? What is the dark matter of the universe? Can it be produced in the laboratory? The next generation of experiments will answer at least some of these questions. Tools for Exploring the Terascale Particle accelerators recreate the particles and phenomena of the very early universe. When particles collide in accelerators, new particles not readily found in nature can be produced and new interactions can be observed. These new particles and interactions were prominent in the early universe but disappeared as it cooled, leaving only scattered clues about their continuing influence. Understanding the properties of these particles, however, is essential to building a full understanding of the natural world and its evolution. Accelerator experiments are the sole places
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics where these particles and interactions can be studied in a controlled fashion. Other facilities provide crucial information, but high-energy particle accelerators remain the most important single tool available for addressing the scientific challenges facing particle physics. The Tevatron collider at Fermi National Accelerator Laboratory (Fermilab) outside Chicago is currently the highest energy accelerator in the world, and it will remain so for another year or two. The Tevatron collides beams of protons and antiprotons with a total energy of about 2 trillion electron volts (TeV). The luminosity, or intensity, of the particle beams at the Tevatron has steadily increased in the last few years, and continued increases are essential to the success of the Tevatron physics program. Precision measurements and discoveries at the Tevatron have helped to pave the way toward exploration of the Terascale at the LHC; measurement of the W boson mass and the discovery and measurement of top quark properties now help point the way toward the possible discovery of the Higgs particle and even supersymmetry at the Terascale. The program at the Tevatron has two main thrusts: searches for new particles and precise measurements of particle properties. In the latter category, for example, the Tevatron continues to improve knowledge of heavy particles such as the top quark, which was discovered at the Tevatron and whose large mass still places it out of reach of other facilities. In 2007 the LHC at CERN is scheduled to begin accelerating beams of protons to a total energy of 14 TeV, thus exceeding the energy available at Fermilab by a factor of 7. In historical terms, this is a large jump in energy, which is made all the more exciting because so many clues point to the importance of the Terascale (see Box 3-1). With its initial luminosity, the LHC has wide potential for new discoveries. The prospects are so varied as to defy brief summary, but they include possible new elementary particle forces, the first evidence for supersymmetric particles, the discovery of a Higgs particle, and much more. The LHC’s discovery capabilities will grow further when it achieves its full luminosity after a few years of operation. What is the next step beyond the LHC? The advance of science proceeds on many fronts and requires many different kinds of tools. If one kind of tool were the best for all purposes, that tool would be built and then made bigger or better. But the world does not give up all of its secrets that way. In particle physics the obvious needs are for higher energy, more accurate measurements, and the ability to detect new, rare, or elusive processes. Each of these frontiers is best advanced with a different kind of instrument. To make an analogy, in astronomy the largest Earth-based telescopes are capable of detecting the dimmest objects; the Hubble Space Telescope (HST) has a smaller mirror but is able to produce the sharpest pictures; and numerous other
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics BOX 3-1 Particle Detectors In particle physics, experimentation studies collisions of particles that have been accelerated to very high energies. The collisions convert energy to mass, producing new particles or new phenomena associated with fundamental particle interactions through Einstein’s famous equation, E = mc2. Particle physics facilities can be thought of as enormous microscopes that are powerful enough to probe physical processes at extremely small distance scales. In modern particle physics experiments, different types of detector systems surround the collision point. The detectors measure the properties of the passing particles. The LHC, which is scheduled to begin operation in 2007, will produce proton beams seven times more powerful than those at Fermilab. The LHC beams also will reach much greater levels of intensity. In fact, experiments at the LHC will witness something like 1 billion collisions per second. Only 100 collisions per second, at 1 megabyte of data per collision, can be recorded for later analysis. It is a major challenge to design and build the high-speed, radiation-hardened custom electronics that provide the pattern recognition necessary to select potentially interesting collisions. In a colliding-beam experiment, the particles travel out in all directions from the collision point, so the detector is usually as tightly closed as possible (see Figure 3-1-1). Following each FIGURE 3-1-1 An artist’s illustration of a particle collision event. Courtesy of the ATLAS experiment.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics collision, called an event, computers record the data. Each particle type has its own signature in the detector, but the detailed analysis of an event can be very complicated and can sometimes take years and a great deal of scientific creativity and judgment to decipher correctly. The results of these analyses generate the key scientific discoveries. There are two multipurpose experiments at the LHC, the Atoroidal LHC Apparatus (ATLAS) and the Compact Muon Solenoid (CMS). The ATLAS experiment, the larger of the two, is about the size of a five-story building. ATLAS and CMS are the largest collaborative efforts ever attempted in the physical sciences. For example, at present ATLAS has more than 1,800 physicists (including 400 students) participating in the experiment from more than 150 universities and laboratories in 34 countries. The two experiments are similar in concept but different in detail. ATLAS and CMS both have charged-particle tracking to determine particle momentum; calorimetry to measure the energy of electrons, photons, and quark jets; and the ability to identify muons. ATLAS detects muons with a gigantic toroid assembly. CMS detects electrons and photons with its crystal calorimeter. Both experiments can detect short-lived particles with silicon pixel vertex detectors. ATLAS and CMS are poised to make discoveries when the accelerator delivers its first collisions (see Figure 3-1-2). Some interesting facts about CMS are as follows (ATLAS has its own set of fascinating facts): The total mass of CMS is approximately 12,500 tons—double that of ATLAS (even though ATLAS is about eight times the volume of CMS). The CMS silicon tracker comprises approximately 250 square meters of silicon detectors—about the area of a 25-meter-long swimming pool. The silicon pixel detector comprises more than 23 million detector elements in an area of just over 0.5 square meters. These detectors are used to identify short-lived, unstable particles like the bottom quark. The electromagnetic calorimeter (ECAL) is used to detect photons and electrons. It is made of lead tungstate crystals, which are 98 percent metal (by mass) but completely transparent. The 80,000 crystals in the ECAL have a total mass equivalent to that of 24 adult African elephants and are supported by 0.4-millimeter-thick structures made from carbon fiber (in the endcaps) and glass fiber (in the barrel) to a precision of a fraction of a millimeter. The hadronic calorimeter (HCAL) will be used to detect the energy from jets of particles. The brass used for the endcap of the HCAL comes from recycled artillery shells from Russian warships. instruments such as cosmic ray detectors or radio telescopes look at the cosmos in different ways. Astronomy would be greatly impoverished if it had just one or two types of instruments. That is, different instruments can work in different ways to make discoveries that advance science. Three types of instruments also can be identified in particle physics. First there are the proton accelerators, such as the Tevatron and the LHC, which offer the fastest route to the highest energy. They might be compared to very large ground-
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics FIGURE 3-1-2 In the underground tunnel of the LHC, the proton beams are steered in a circle by magnets. The LHC will provide particle collisions for the ATLAS and CMS experiments. Courtesy of CERN. The solenoid magnet, which allows the charge and momentum of particles to be measured, will be the largest solenoid ever built. The maximum magnetic field supplied by the solenoid is 4 tesla—approximately 100,000 times as strong as the magnetic field of Earth. The amount of iron used as the magnet return yoke is roughly equivalent to that used to build the Eiffel Tower in Paris. The energy stored in the CMS magnet when running at 4 tesla could be used to melt 18 tons of solid gold. During one second of CMS running, a data volume equivalent to the data in 10,000 Encyclopedia Britannicas will be recorded. The data rate to be handled by the CMS detector (approximately 500 gigabits per second) is equivalent to the amount of data currently exchanged by the world’s telecommunication networks. (The data rate for ATLAS is similar.) based telescopes. Second are the electron accelerators. At any point in history, the energy that was reachable with electron accelerators—such as those currently operating in California and Japan—has typically been lower than what could be reached with a proton accelerator, but electron collisions offer a much clearer picture of particle properties and interactions. Electron-positron colliders might be compared to HST. Finally, as in astronomy, there are a host of different instruments—nuclear reactors, underground laboratories, tabletop measurements,
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics space-based observations, and more—each of which elicits entirely different kinds of information. Science is full of uncertainty, and new discoveries from the LHC or elsewhere might change the picture. But as of today, the substantial majority of particle physicists in the United States, Europe, and Japan do not advocate that the next step in particle physics should be a larger facility of the same type as the Tevatron and the LHC. Rather, the dominant view—increasingly so in recent years—has been that the next step should be to push the frontier of clarity and sensitivity with a TeV-class electron-positron collider, the ILC. The initial phase of the ILC is envisioned to have a total energy of 500 GeV, with the possibility of a subsequent increase in the energy to 1 TeV.1 The ILC can make many important discoveries that are beyond the reach of the LHC, even though LHC energies will allow the production of particle states up to around 5 TeV. It can provide detailed information about phenomena that the LHC can only glimpse. These may include phenomena predicted in the Standard Model but not yet observed, such as the Higgs particle. They may include phenomena that are already observed but difficult to study fully at proton colliders, such as the top quark. Or they may include entirely new phenomena that emerge at the LHC, including supersymmetry, large extra dimensions, new particle forces, and more. The LHC can see farther (higher in energy) into the Terascale but with relatively blurry vision, while the ILC can see more clearly but not directly into the higher regions of the Terascale (see Figure 3-1). The advantage of the ILC is that it collides electrons, which are simpler and easier to understand than the protons used at the Tevatron and the LHC. Protons can be accelerated more cheaply and easily, but electrons typically give more detailed information. In that respect, building the ILC will be like launching a telescope above Earth’s atmosphere. Historically, the energy reach of hadron colliders has been greater than that of electron colliders, while the ability to extract the details of collisions has been better with electron colliders than with hadron colliders. (For more discussion on this topic, see Box 3-2.) Most previous electron colliders accelerated the beams in circular orbits, allowing the beams to be reused again and again. Energy is lost in 1 For a full description of the internationally agreed-upon general parameters for the ILC, please see International Linear Collider Steering Committee, Parameters Subcommittee, Parameters for the Linear Collider, September 2003; the report is available online at <http://www.fnal.gov/directorate/icfa/LC_parameters.pdf>. For the baseline configuration design of the ILC, please see <http://www.linearcollider.org/wiki/doku.php?id=bcd:bcd_home>.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics FIGURE 3-1 As depicted in this artist’s montage, while both the LHC (left) and ILC (right) will collide particles at Terascale energies, the character of the interactions will be quite different. For the LHC, protons (containing various elementary quarks) will collide; at the ILC, pointlike electrons (and positrons) will collide. Courtesy of CERN and DESY Hamburg. each orbit of the electrons, however, and the energy loss increases dramatically as the energy of the beam is increased. For this reason, it is impractical to reach Terascale energies with a circular electron collider. To reach such energies in electron collisions requires the challenging new technology of a linear collider. An early accelerator of this type, the SLC, operated at the SLAC laboratory in California in the early 1990s and proved to be an important milestone in establishing the feasibility of a linear accelerator; the project also led to some of the most precise tests yet of the Standard Model (see Figure 3-2). Building on this experience and using novel technology, physicists today are proposing to build a large-scale version of an electron-positron linear collider—possibly 30 km long—that can explore the Terascale. The LHC, with the high energy of its collisions, and the ILC, with the extremely precise measurements possible at an electron-positron collider, can combine to provide the necessary tools to explore the Terascale. Taken together, discoveries at the LHC and ILC could uncover the much anticipated mysteries of this new domain of nature.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics BOX 3-2 Collisions of Different Types of Particles: Electrons vs. Protons For a physicist, the electron is about as simple as a particle can be. It is called a “point particle,” and it obeys the simplest laws that are allowed by the principles of relativity and quantum mechanics. Electrons have been smashed together at huge energies in accelerators and probed in ultraprecise tabletop experiments to measure their magnetic and electric properties. The results fit with the current understanding of the electron as a relativistic and quantum mechanical point particle. The proton, by comparison, is not simple (see Figure 3-2-1). It is composed of simpler objects called quarks and gluons. The equations governing quarks and gluons have been known for 30 years, but they are so complex that even with modern supercomputers, physicists are still struggling to understand how quarks and gluons behave. Electrons and protons, and their antimatter counterparts (the positron and antiproton), are the most easily accelerated particles. But they have contrasting virtues for experiments: Protons can be accelerated more easily than electrons to higher energies. Because proton accelerators can reach higher energies, they have been able to directly produce and discover heavy particles, including the W and Z particles and the top quark. The great advantage of electrons is that they are point particles. Collisions involving electrons are much easier to understand and interpret. As a result, many discoveries have been made first with protons, and often the most precise measurements are made with electrons. For example, the direct evidence for quarks was demonstrated in electron-proton scattering experiments in the 1960s at SLAC. Proton-proton scattering had reached higher energies, but the results were too complicated to reveal the existence of quarks. More recently, many of the high-precision tests of the Standard Model have come from collisions involving electrons. Physics at the Terascale Discovering the Higgs Particle According to the Standard Model, the difference between the weak interactions and electromagnetism is related to the origin of the masses of most elementary particles through the unusual behavior of a new particle called the Higgs particle. Whether this hypothesis is correct is not known experimentally. All that is
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics FIGURE 3-2-1 A proton consists mainly of three quarks, but it also contains gluons and other quarks and antiquarks, which makes it a very complex object. This artist’s conception illustrates the nonelementary nature of the proton. Here the artist imagined cutting open a proton to see the material inside, including quarks (the three large balls), gluons (wiggly lines), and extra quark-antiquark pairs (the small balls that come in pairs). known for sure, based on extrapolating from what has already been observed, is that at Terascale energies, either a Higgs particle will emerge or the Standard Model will become inconsistent and a new mechanism will be needed. If the Standard Model is correct, the LHC will discover the Higgs particle. But its ability to test the Standard Model theory of the Higgs particle will be limited. Is the Higgs particle really responsible for particle masses? Have Higgs particle interactions hidden the weak interactions from our everyday experience, as the Standard Model claims? Is there just one Higgs particle, or several? Answering these
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics FIGURE 3-2 A 30-year history of electron colliders around the world indicates the increasing energy of collision. The colored bars represent the chief operating periods of the named accelerators. The red region in the upper right corner surrounded by a dashed line represents a proposed scenario for the ILC. Figure content courtesy of M. Tigner, Cornell University, and R.N. Cahn, Lawrence Berkeley National Laboratory. questions requires measuring the interactions of Higgs particles in a more precise way than can be done at the LHC. The high energy of the LHC will enable it to produce and detect Higgs particles if the Standard Model is correct, but the complexity of proton interactions limits the information about these particles obtainable from the LHC. The ILC will be able to zoom in on the Higgs particle and measure its properties and to measure multiple Higgs particle interactions with high precision. The ILC will be sensitive to subtle modifications of the behavior of the Higgs particle resulting from unknown physics at much higher energies, perhaps even from exotic new physics such as extra dimensions of space and time (see Figure 3-3). Of course, it is possible that the Standard Model theory of weak interaction symmetry breaking and particle masses is incorrect, or not entirely correct. Perhaps instead of a Higgs particle there is a more exotic mechanism behind these phenomena—possibly something that physicists have not even thought of yet. Or perhaps something exists that is somewhat like a Higgs particle but the Standard
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics BOX 3-7 Existing Mechanisms to Promote International Cooperation Over the years the particle physics community has used a number of mechanisms for international discussion and planning. Current fora include: The International Union of Pure and Applied Physics (IUPAP). This organization, chartered in 1933, is a member of the International Council for Science (ICSU, formerly known as the International Council for Scientific Unions). IUPAP is a nongovernmental union whose mission is to coordinate international activity in physics. It works through subject-area commissions and standing working groups or committees that are tasked with international coordination for more specific areas of physics. The International Committee on Future Accelerators (ICFA) . This working group of IUPAP was established in 1976 to facilitate international collaboration in the construction and use of accelerators for high-energy physics. It has taken an active role in developing plans for the ILC. The Particle and Nuclear Astrophysics and Gravitation International Committee (PANAGIC) . Created by IUPAP in 1999, this working group is charged with the coordination of non-accelerator-based international projects. PANAGIC has established two subpanels relevant to particle physics, one on high-energy neutrino astrophysics and one on gravity waves. The Global Science Forum (GSF) . Created by the Organisation for Economic Co-operation and Development (OECD), GSF is an organization of senior science policy officials from member countries. They meet twice yearly and discuss large science projects, including those in particle physics. GSF created a special group, the Consultative Group on High Energy Physics, which issued a report in June 2002 that contained a roadmap for high-energy physics extending to beyond 2020. Issues highlighted in the report include the legal structures, financial arrangements, governance, and roles of the host nations and laboratories for accelerator facilities. The Funding Agencies for the Linear Collider (FALC) . An informal group formed in 2003, FALC brings together representatives of the principal governmental agencies that fund research programs in particle physics. U.S. representation to FALC includes the NSF and DOE’s Office of Science. Perhaps the most important international collaboration in particle physics is the CERN laboratory in Geneva, which is a long-term cooperative effort of many European countries.6 The construction programs for the detectors at the LHC, 6 CERN member states are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, The Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland, and the United Kingdom. Member states make a contribution to the capital and operating costs of CERN programs and are represented in the CERN Council, which is responsible for all important decisions about the organization and its activities. The United States is not a member but is granted observer status. Observer status allows nonmember states to attend Council meetings and to receive Council documents without taking part in the decision-making procedures.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics along with the accelerator itself, also are examples of successes in international collaboration, with the United States and other non-CERN members contributing both financial and intellectual resources. The significant U.S. participation in the LHC project exemplifies some of the elements of a new era of global programs in particle physics. During discussions about the high cost of excavating the tunnel for the Large Electron Positron Collider (LEP) at CERN, European researchers chose to examine possible future-generation accelerators to replace LEP at the same site. In 1985 the CERN Long-Range Planning Committee recommended installing a multi-TeV facility in the LEP tunnel after the completion of that program. In late 1991, the CERN Council agreed in a unanimous decision that the LHC was “the right machine for the further significant advance in the field of high energy physics research and for the future of CERN.”7 When Congress terminated the construction of the Superconducting Super Collider (SSC) in 1993, the particle physics community and DOE recognized that the best practical opportunity to explore the Terascale within the next 10 to 20 years would be at the CERN-based LHC. At the request of DOE, HEPAP convened a panel to develop a new long-range plan for U.S. particle physics. It recommended that the United States participate in both the LHC experimental program and the construction of the LHC accelerator through significant contributions of in-kind components and cash for purchases of critical items in the United States. The particle physics community, DOE, and NSF strongly supported these recommendations. In early 1996, CERN’s director general led a delegation to Washington to begin negotiations concerning a U.S. role in the LHC project. Around that time, CERN reached agreements for contributions to the LHC from Japan, India, Russia, and Canada, and NSF began to fund some LHC-related activities. The administration requested funds for strong U.S. participation in the LHC in its FY1997 budget; Congress then appropriated funds for both NSF and DOE to provide the U.S. contributions to the LHC. A very important step in this process was taken when Congress authorized DOE to enter into a formal agreement with CERN on behalf of the United States. U.S. officials signed the agreement with CERN in December 1997, promising to contribute $531 million to the LHC project over about 10 years. That investment is now nearly complete. This process of national initiative followed by international negotiation and agreement (resulting in a significant multiyear commitment from the United States) to invest in a facility abroad was an important achievement for the U.S. particle physics program and the U.S. government. 7 CERN Press Release, PR12.93, December 17, 1993.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics The CMS and ATLAS detectors being built for the LHC each have around 2,000 collaborators from all regions of the world. U.S. researchers make up about 20-25 percent of each detector collaboration, and the number of U.S. researchers is growing. By 2007, more than half of all U.S. experimental particle physicists are expected to be working at the LHC. The overwhelming reason for this shift is the planned conclusion of the U.S.-based experiments at SLAC, Cornell, and Fermilab. Many of the scientists in the university groups and laboratories that participated in the research program of these experiments are now transferring their efforts to the LHC. The model used by particle physicists to fund, build, and perform science with particle detectors has been and continues to be successful even at the largest scales. Among recent projects, the J-PARC multiprogram accelerator complex was approved by the government of Japan, including an accelerator neutrino experiment, after which international involvement was welcomed. Significant non-Japanese funds (80 percent) have been raised to pay for one of the detectors at the facility. In general, if the science is exciting, scientists from around the world will want to join those efforts and will raise modest funds to participate. The director of KEK has said that if it is approved by the government, the new proton decay experiment HyperK will require international funds to move planning forward. Accelerators around the world have thus far been built based on decisions made by a single country or laboratory; the exception has been the largest projects at CERN, such as the LHC (the CERN Council includes scientific and government representatives from each of the member states). The SLAC B factory accelerator was a U.S. presidential initiative, Fermilab’s Tevatron was a U.S. decision, and constructing the SSC was a U.S. decision by President Reagan. The largest accelerator project to be successfully completed in the United States, the Spallation Neutron Source at Oak Ridge National Laboratory (with a cost exceeding $1.4 billion), was an internal U.S. decision. As is customary with DOE acceleratorbased facilities, access will be open to scientists from around the world. DESY used a different model for the HERA accelerator: The plan was to build components of the accelerator in several countries as in-kind contributions to be assembled at the main facility. Although DESY had hoped for substantial contributions, the final non-German fraction was 15 percent. Even the Euro-XFEL, a $1 billion project just under way and being hosted at DESY, was approved by the German government, after which contributions amounting to 50 percent of the cost were sought from Europe. This approach appears to have been successful because of the strong support from the user community for this facility. Europe, through CERN, recently took the next step in formalizing its regional planning activities. A group has been established through the initiative of the CERN Council to develop a strategy that addresses the main thrusts of particle
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics physics in Europe, both accelerator-based and non-accelerator-based, including R&D for novel accelerator and detector technologies. The strategy is designed to address collaboration between the European laboratories, coordinated European participation in world projects, the visibility of the field, and knowledge transfer beyond the field. Since CERN is an international organization, its Council is composed of government representatives. Thus, approval by the CERN Council invokes the treaty relationship between each government and the CERN organization, creating a binding agreement among the individual governments. The opportunities for international collaboration in particle physics and the challenges posed have never been greater. More rigorous international prioritization of new particle physics research opportunities and greater leveraging of international funding could have great benefits as particles physicists seek to answer the exciting questions now before the field. Such benefits, however, can only be realized through genuine cooperation both among scientists and among the government agencies sponsoring their work. The most extensive current example of international collaboration is the set of activities that surround the planning and R&D phases for the proposed ILC. The International Linear Collider Particle physics research communities around the world have declared that the ILC is the highest priority project after the LHC.8 The ILC promises to provide answers to a host of the most important questions in particle physics. It is clearly of a scale where decisions on design, funding, and operation must be international from the start. (See Appendix A for additional analysis of the path forward.) The committee felt strongly that, if possible, the ILC should be located near an existing particle physics laboratory to take advantage of existing resources and talent.9 Past experience with the SSC, as well as current experience with the LHC, shows the advantages of undertaking new projects with existing facilities and talent. As the only laboratory devoted primarily to particle physics, Fermilab is an obvious candidate site. It is attractive as a potential site for the ILC because of its existing laboratory and physical plant infrastructure. Like CERN in Europe, Fermilab has a critical nucleus of accelerator expertise that could play a significant role in the ILC. Fermilab has successfully built, operated, and upgraded the 8 Among 28 large-scale facilities across all of the physical sciences, DOE’s Office of Science deemed U.S. participation in the ILC the highest priority initiative for the mid-term planning horizon. 9 This sense is supported by a number of other reports considering possible site selection criteria for the ILC.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics Tevatron, one of the most sophisticated accelerators in the world. In collaboration with DESY in Germany and other laboratories, Fermilab also has developed expertise with superconducting radiofrequency technology, the choice for the ILC. Fermilab must provide the leadership necessary to mobilize a coalition of U.S.based resources and facilitate U.S. participation in the ILC. The ILC has been an international effort from its inception and should continue to be pursued as a global venture. In 2005 the U.S. effort in ILC R&D was budgeted for $25 million; other regions of the world have invested much more. For instance, European governments invested more than $50 million in 2005. Integrated over about 5 years, the Japanese and European investments in ILC R&D total at least several hundred million dollars. A critical element of any U.S. strategy to move forward with the ILC beyond the initial R&D phase coordinated by the Global Design Effort (GDE) will be the formation of an entity capable of negotiating both scientific and financial matters with the other expected regional partners. At present, the association between the U.S. program (through DOE and NSF) and the GDE is only informal (for more on the GDE see Appendix A). Moving forward on the ILC will demand new mechanisms of cooperation and agreement among the research agencies of many nations. Several such agencies have begun to discuss the ILC project at an international level through the FALC group, an informal body composed of representatives from relevant funding agencies from Canada, France, Germany, Italy, Japan, the United Kingdom, the United States, and CERN. Formed in 2003, FALC provides a forum to discuss funding issues, policy strategies, and progress toward designing an ILC. As this effort moves forward, the decision-making process will be complex and will require simultaneous discussions at the scientific level and at various governmental levels that transcend the FALC group. Experience with other international joint ventures (such as ITER and the LHC) demonstrates the potential for success in sophisticated international agreements of this kind. A PATH FORWARD Over the next 15 years, today’s international collaboration, already extensive, will need to intensify to effectively address the challenges on the scientific frontier. The committee believes that particle physics should evolve into a truly global collaboration that allows the particle physics community to leverage its resources, prevent duplication of effort, and provide additional opportunities for particle physicists throughout the world. This prioritization process could lead to a new model for international collaboration in particle physics. For example, each country or region could special-
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics ize to some extent in programs sited in their country or region and then play a relatively smaller role in programs sited elsewhere. Such an evolution would be in keeping with the framework proposed in Allocating Federal Funds for Science and Technology.10 Among the report’s recommendations, two are particularly noteworthy: The President and Congress should ensure that the [federal science and technology] budget is sufficient to allow the United States to achieve preeminence in a select number of fields and to perform at a world-class level in the other major fields. The United States should pursue international cooperation to share costs, to tap into the world’s best science and technology, and to meet national goals. Both goals would be met if the United States were to participate in a worldwide effort to plan particle physics research from a global perspective. Furthermore, the ILC could serve as the model for a global program, since the early planning has already started from a global perspective rather than from the perspective of an individual country. This planning process could ultimately be expanded into many other areas of particle physics. While meeting these goals would serve the interests of particle physics and fulfill the public policy objective of using resources in the most efficient manner, success can only be achieved through multilateral agreements between governments and/or government agencies, not unilaterally. This is a challenging task but one that must be done given the environment the committee believes will evolve over the next 15 years. The tools of particle physics have evolved significantly over the past 50 years. Originally particle physics was a small field; individual scientists could construct particle accelerators (first tabletop and then room-sized cyclotrons) and detectors (plates of film) in their own laboratories. As the science drove accelerators to higher energies, the scale of projects continued to expand. In the modern era, the most recently designed and constructed machines require literally hundreds of scientists and engineers. Partly because of the demands for high performance and partly because of the eclectic nature of the investigations, particle physics projects in the United States are constructed (and then operated) with sizable involvement of scientists and engineers, more so than in some other fields, such as magnetic fusion. The planning process for particle physics in the United States historically has 10 NRC, Allocating Federal Funds for Science and Technology, Washington, D.C.: The National Academies Press, 1995, pp. 14, 16.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics involved more than one government agency. Broad involvement of the particle physics community has been achieved by creating a variety of advisory committees, such as HEPAP and its subcommittees, which advise DOE and NSF; program advisory committees at the major laboratories; and National Research Council committees that periodically review the field from a broader perspective. Nearly all of the larger national laboratories have had an important program in particle physics, which is a tribute to the broad appeal and importance of particle physics to the physical sciences. Argonne National Laboratory, Brookhaven National Laboratory, Cornell Laboratory for Elementary Particle Physics, Fermilab, Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory, Los Alamos National Laboratory, SLAC, Thomas Jefferson National Accelerator Facility, and others have all contributed to scientific and technological advances in particle physics. Different laboratories have pursued different initiatives by developing machines capable of new investigations, whether involving higher energies, higher intensities, or beams of different particles. A strong and healthy national program was maintained through intense but healthy competition (for both resources and personnel) among the variety of different projects. This situation is changing, however. As Fermilab becomes the only laboratory devoted entirely to particle physics, the system of planning and coordinating efforts will have to evolve as well. Approved projects are subject to ongoing external reviews by experts from the broader community, at least during their construction phase. Large projects that overlap the interests of more than one agency need a planning and review process that is effective and not duplicative. New initiatives need to be able to bubble up within the field, but large-scale efforts need a coordinated decision process to establish their overall priority, a process that is national rather than based on a single laboratory or government agency. The astrophysics community has achieved this goal with a structured decadal review process. In particle physics, HEPAP has been the leading source of advice to the U.S. government, and its recently established P5 subcommittee offers program review and coordination at a higher level than the laboratory program committees for larger ventures, although this mechanism is new and has not yet been effectively deployed. The advisory apparatus has been evolving, and the emerging structure of tactical subfield-specific scientific assessment panels (such as in neutrino physics or dark energy) feeding into P5 and HEPAP for the formulation of strategic guidance is a step in the right direction. The challenge for federal agencies is to continue to get the needed community input but to avoid creating an overlapping and possibly contradictory set of advisory groups and panels. This requires some interagency coordination and works best when there is a stable, long-term planning process that the community understands and accepts as authoritative.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics No description of developments in particle physics would be complete without acknowledging that, as in any area of science, not all experiments have achieved their goals. Some experimental disappointments inevitably accompany the exploration of the unknown and are a part of the process responsible for scientific progress. Other experiments failed to find what they were looking for but instead found other very important results. Within the U.S. program, the biggest disappointment was the collapse, in the early 1990s, of the SSC program. This accelerator was designed to access extremely high energies, substantially higher even than the energies that will be reached at the LHC when it begins operation. The cancellation of the SSC was a severe blow to U.S. scientific leadership and to progress in particle physics (see Box 3-8).11 A number of lessons were learned from this difficult and costly experience about effective ways to proceed with large scientific projects involving international partnerships. First, effective international partnerships require the meaningful participation of all parties from planning and design through conduct of the experiments. Second, very detailed design parameters are essential before starting construction and before announcing any cost estimates. Third, effective simulation models are needed (and have since been developed) to help provide more reliable and robust cost estimates and performance expectations. Fourth, effective, integrated management that takes advantage of existing resources and infrastructure is critically important. These hard-won lessons are being implemented in studies surrounding the proposed ILC. OPPORTUNITIES AHEAD The different tools of particle physicists—high-energy accelerators, intense particle beams, and ground- and space-based observations of the universe—will all be necessary to take the next steps in answering the fundamental questions of particle physics. New physics at the Terascale will be revealed and studied at the LHC and the ILC. Neutrino beams can yield further insights into the properties of many other particles. And a full understanding of dark matter and dark energy will require the tools of particle astrophysics. 11 At the same time, Europe, through CERN, was able to move ahead with a set of objectives articulated (informally) much earlier. The usual pattern is that new accelerators stand on the shoulders of their predecessors. At the time of construction of the underground tunnels for CERN’s LEP in the early 1980s, then director general John Adams had a vision for a natural progression from LEP to an advanced proton collider in the same tunnel, such as the LHC, that would make use of the existing infrastructure.
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics BOX 3-8 A Brief History of the Superconducting Super Collider The idea for a colliding proton-proton accelerator with energy of 20 TeV per beam was first discussed at a series of workshops held in 1978 and 1979 by ICFA. Plans for the collider were discussed extensively at a summer study sponsored by the American Physical Society in 1982 in Snowmass, Colorado. Even then, the project was recognized as a multi-billion-dollar undertaking that would require substantial international collaboration. In 1983, after several subsequent workshops, HEPAP recommended that DOE seek “immediate initiation of a multi-TeV high-luminosity proton-proton collider.”1 In 1984 DOE approved the establishment of the Central Design Group for the SSC under the management of the University Research Association (URA), a consortium of universities that also manages Fermilab. By 1986 the design group, based largely at the Lawrence Berkeley National Laboratory campus, had produced a conceptual design with a price tag of more than $4 billion. DOE recommended moving forward with the project, and in January 1987 the Reagan administration made the project a national initiative. The selection of a site near the town of Waxahachie, Texas, was announced on November 10, 1988. Even at that point, several of the tensions that would later become critical factors in the cancellation of the project were apparent. Proposals for the SSC from the administration posited significant financial contributions from other countries. But parts of the administration and several powerful senators saw the SSC as primarily a U.S. undertaking designed to reestablish national supremacy in high-energy physics. As a result, international collaboration was not part of the project from the beginning and was pursued only after Congress had already committed to the project. The management of the project also was becoming controversial. DOE officials had doubts that physicists could manage a project the size of the SSC. Responding to these doubts, the URA’s proposal to build the SSC featured partnerships with industrial firms that had experience in managing large construction projects. This unusual management scheme contributed to dissatisfaction among the members of the Central Design Group, many of whom declined to continue working on the project. Increases in the estimated cost of the SSC were another source of concern. After the selection of the Texas site, DOE submitted a revised cost estimate to Congress of $5.9 billion in early 1989. However, work was under way at that time to incorporate into the design several additional features felt to be necessary, such as a more powerful proton injector ring and better superconducting dipole magnets. These and other modifications added more than $2 billion to the cost, yielding a revised estimate of $8.25 billion in February 1991. In key votes in 1989, 1990, and 1991, both the House and Senate supported the SSC. But misgivings about the project were growing. The Europeans were working on plans to build their own proton-proton collider at CERN. The Japanese reportedly were willing to contribute to the construction costs of the collider, but they wanted a personal request from either President Bush or newly elected President Clinton, which, for various reasons, never came. The project also was being criticized by other scientists, including some physicists, who saw its funding undermining support for other areas of research. In June 1993 the House voted 280 to 150 to terminate the SSC project. The Senate continued to support the project and prevailed in conference to have funding included in the DOE appropriations bill. Then, on October 19, 1993, the House rejected the entire appropriations bill by a vote of 282 to 143. Support for the SSC subsequently collapsed. Congress directed that the $640 million appropriated for the project in 1994 be used to terminate the project. After
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics expenditures of approximately $2 billion, the contracts for the superconducting magnets were canceled, the entrances to the 15 miles of tunnel already dug were blocked with rock, and the employees of the SSC laboratory began looking for new jobs. Foreign assistance for the SSC had been expected to come not as cash that could be spent within the United States but as in-kind contributions—chiefly furnished materials and manufactured items such as superconducting magnets, cryogenic systems, computers, or other electronic components. Projected international cooperation did not materialize, which meant that the entire cost of the project would have to be borne by U.S. taxpayers. The proponents of the SSC had argued that many countries were eager to participate and contribute financially if only Congress would demonstrate good faith by funding the SSC more fully—a classic chicken-and-egg problem. By 1992, however, India was the only nation to pledge any support for the SSC project—a total of $50 million, or about half of 1 percent of the projected total cost. The European Community, which had been planning its own supercollider (which became the LHC), was not a realistic source of funding for a U.S. project, many contended. Japan had been expected to be a major contributor, but the Japanese government resisted pressures by the U.S. government to become one. Some contend that Japan, which may have been willing to commit up to $1 billion, was reluctant to proceed until more formal government-to-government agreements to provide a framework for cooperation were worked out. According to an editorial in Science, “In its quest for big bucks for the particle accelerator, the United States appears to have ignored the golden rule for getting major contributions from Japan: links must be built at ground level before an official approach for funds.”2 The cancellation of the SSC not only was a severe blow to the U.S. program in particle physics and U.S. scientific leadership, but it also delayed progress in particle physics by postponing direct exploration of the Terascale with a proton collider. The LHC now being built at CERN shares many of the scientific goals of the original SSC, but it has a higher particle intensity and a lower energy. The proposed ILC would differ significantly from both the SSC and the LHC by employing colliding electrons to probe the Terascale. The ILC proposes using a technical approach and management structure entirely different from that for the SSC (see details in the text). 1HEPAP Report to the Department of Energy and the National Science Foundation, 1983. 2Science, April 5, 1991, p. 25. The strong attraction of Terascale physics is underscored by the convergence of interests from distinct scientific areas. From cosmology, there is growing interest in dark matter and dark energy. From particle physics, there is great interest in supersymmetry, in the origins of mass, and in Einstein’s dream that all the forces can be unified. This convergence is what makes the Terascale so compelling. The intersection of scientific interests is often a signal that major new discoveries are
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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics on the horizon. Thus, the committee feels that explorations of the Terascale have enormous scientific potential. Addressing these scientific challenges can be part of a national commitment to renew the country’s portfolio in basic research to “maintain the flow of new ideas that fuel the economy, provide security, and enhance the quality of life.”12 Moreover, it is a deeply human endeavor that involves some of the most world’s most talented scientists, engineers, and students. 12 NAS, NAE, and IOM, Rising Above the Gathering Storm: Energizing and Employing America for a Brighter Economic Future, Washington, D.C.: The National Academies Press, 2005 (Prepublication), p. 20.
Representative terms from entire chapter: