Revealing the Hidden Nature of Space and Time Charting the Course for Elementary Particle Physics (2006) / Chapter Skim
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3. The Experimental Opportunities
3. The Experimental Opportunities
Pages 56-100
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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)
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· What is the dark matter of the universe? Can it be produced in the laboratory?
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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.
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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.
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: · 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)
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TH E EX P E R I M E N T A L O P P O R T U N I T I E S 61 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.
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These may include phenomena predicted in the Standard Model but not yet observed, such as the Higgs particle. They may include phe nomena that are already observed but difficult to study fully at proton colliders, such as the top quark.
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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)
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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 interac tions and electromagnetism is related to the origin of the masses of most elemen tary particles through the unusual behavior of a new particle called the Higgs particle.
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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.
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The high energy of the LHC will enable it to produce and detect Higgs particles if the Standard Model is correct, but the com plexity of proton interactions limits the information about these particles obtain able from the LHC. The ILC will be able to zoom in on the Higgs particle and measure its proper ties and to measure multiple Higgs particle interactions with high precision.
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The greater clarity and precision of the ILC will likely be even more important if the Standard Model theory of these phenomena is incomplete or incorrect. Even such a basic property of the Higgs particle as its spin cannot be easily measured at the LHC.
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These new particles are called superpartners and may well provide the explanation for dark matter (see Figure 3-4)
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Space may have extra dimensions beyond the three that we experience in everyday life. These are new dimensions that would be unlike the quantum dimensions of supersymmetry; they would be more akin to ordinary dimensions, like the ones seen in everyday life except smaller.
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These extra dimensions, if they exist, must be small, simply because they have not yet been detected. Discovering such extra dimensions requires the high energy of particle accelerators.
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Dark Matter One of the great surprises in astronomy is that matter of the sort familiar to us -- atoms and molecules, electrons, protons, and neutrons -- makes up only about
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The capability of the ILC to change the collision energy of the electrons is thus crucial to this type of measurement. Courtesy of the American Linear Collider Physics Group.
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What is dark matter? Calculations suggest that it consists of Terascale particles, though these guesses require physics beyond the Standard Model.
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The LHC and ILC colliders could deter mine the mass of an individual dark matter particle. For example, agreement between satellite and collider measurements might imply that supersymmetric particles known as neutralinos are the dark matter.
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Taking advantage of this, physicists using the LHC, and especially the ILC, will be able to explore the validity of the Standard Model at energies even higher than can be reached with today's technologies. Toward the Terascale Soon the LHC will begin the exploration of the Terascale, and a proposed linear collider would extend this exploration into unknown realms and add new insights to those discoveries.
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Discover which particles are travelers in extra dimensions and determine their locations within them. Missing energy from a weakly Discover its identity as dark matter.
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If the LHC sees phenomena that are inexplicable within the Standard Model, such as the particles associated with supersymmetry, studies of B meson decays could reveal some of their properties. A relative of the B meson, called the Bs meson, has been produced in sufficient quantity for detailed studies at hadron colliders.
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· The ILC could study the properties of the lightest superpartner with great precision to determine whether it makes up some or all of the dark matter. · The LHC and the ILC will also address many questions about extra dimensions.
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Courtesy of the ILC Global Design Effort. FIGURE 3-3-2 Artist's conception of the ILC accelerator structure in the underground tunnel; the cutaway view shows the interior of the superconducting cavities.
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Therefore, sensitivity to discovering these unseen but indirectly involved particles is greatest when particle physicists have very accurate knowledge of the Standard Model prediction for a specific experiment. In particular, where the new contribution makes possible a decay that was predicted to be extremely rare (or even absolutely forbidden)
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n1 FIGURE 3-5-1 Schematic depiction of how the neutrinos fit into the new version of the Standard Model along with their charged lepton partners, the electron (e) , muon (µ)
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The detailed properties of neutrino oscillations are important to understanding how Standard Model particles interact and the properties of galaxies and the universe. Courtesy of Paul Nienaber and Andrew Finn, BooNE Collaboration.
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The proposed NOA experiment at Fermilab would not only be sensitive to q13 but may also be able to use the interaction of neutrinos with Earth to learn whether neutrinos masses are ordered in a way reminiscent of quarks and the charged leptons. The ordering of the neutrino masses could be a critical clue for understanding what the structure of the constituents of the Standard Model reveals about the underlying physics.
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Sensitive experiments using the beta decay of tritium have been carried out for many years. Using the most ambitious experi ment so far conceived, an international collaboration is mounting an experiment in Germany called KATRIN, which is designed to be sensitive to the distortion from a neutrino with a mass less than 1 eV.
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-- Direct detection of dark matter particles in the Milky Way passing through Earth, -- Direct production of dark matter particles in accelerators, -- Detection of gamma rays from dark matter particle annihilations in the cores of galaxies, in dark matter clumps, and in the sun and Earth, -- Improved observations of dwarf galaxies and small-scale structure to study cluster ing of dark matter and to test alternative models for dark matter, and -- Measurement of the CMB temperature anisotropy and large-scale structure of the CMB to search for new particles that may contribute to a portion of the dark matter. · Testing cosmological models and probing new physics.
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If dark matter is composed of weakly interacting elementary particles, as many astrophysicists and particle physicists believe, then, as Earth passes through a cloud of dark matter in its path around the sun, some of these particles can easily pass through the atmosphere and thousands of feet of rock to reach a detector deep underground. As they travel through the detector, it is ex pected that some will occasionally scatter off an atomic nucleus, causing the nucleus to recoil with the energy of a few tens of thousands of electron volts.
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Observing the dark matter coming from the cosmos and producing dark matter in a particle accelerator (assuming that a particle is responsible for the dark matter) will combine to shed light on this mystery.
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and will have an initial suite of experiments that could include biological observations, dark matter ex periments, a double beta decay experiment, and searches for solar neutrinos. In addition, the laboratory might contain a large cavern that would be suitable for a proton decay experiment.
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researchers have participated in efforts abroad. Global Activity in Particle Physics Some examples of the formal mechanisms the particle physics community has used to carry out international collaborations of various kinds are listed in Box 3-7.
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. This working group of IUPAP was established in 1976 to facilitate international collaboration in the construc tion and use of accelerators for high-energy physics.
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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.
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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)
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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.
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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 col laboration in particle physics.
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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.
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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 par ticle 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 Labora tory, Los Alamos National Laboratory, SLAC, Thomas Jefferson National Accel erator Facility, and others have all contributed to scientific and technological ad vances in particle physics.
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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)
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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)
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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.
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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 More over, it is a deeply human endeavor that involves some of the most world's most talented scientists, engineers, and students. 12NAS, 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)
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