3
How Are Matter, Space, and Time Unified?

Physicists have long believed that a fundamental, encompassing theory of matter, space, and time must be attainable. The remarkable progress described in Chapter 2 suggests that the opportunity to achieve that unification may be at hand. Realizing that opportunity will involve obtaining information both from high-energy physics laboratory and accelerator experiments and from observations in astronomy and cosmology. This chapter explores the open questions and opportunities for progress in the coming years in exploring the implications of physics beyond the Standard Model for the early universe. Further, it addresses opportunities to use particles from sources outside Earth to reveal physics beyond the Standard Model.

The earliest history of the universe is dominated by physics of the highest energies, so that gaining an understanding of it depends on progress in understanding microscopic physics in these extreme domains. Conversely, the universe, unlike accelerators where experiments are limited by available beam lines and interaction regions, is an ever-open laboratory, one that produces a great range of phenomena that span an incredible energy range and that can be used to probe and extend ideas on microphysics. Some important relics that could have been produced only at these early times may remain today. Astronomical and astrophysical studies add immeasurably and often uniquely to important aspects of particle physics beyond the Standard Model, addressing questions such as these: Do protons decay? Do neutrinos have mass? Is nature supersymmetric? What constitutes the dark energy? Are there additional dimensions of space beyond the familiar three?

The triumph of the Standard Model is based largely on data from particle accelerators of ever-increasing energies, constructed over the past 50 years. Without the Standard Model, it would have been impossible to make with any confidence the very large extrapolations in energy that have yielded insights into the conditions of the early universe.



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3 How Are Matter, Space, and Time Unified? Physicists have long believed that a fundamental, encompassing theory of matter, space, and time must be attainable. The remarkable progress described in Chapter 2 suggests that the opportunity to achieve that unification may be at hand. Realizing that opportunity will involve obtaining information both from high-energy physics laboratory and accelerator experiments and from observations in astronomy and cosmology. This chapter explores the open questions and opportunities for progress in the coming years in exploring the implications of physics beyond the Standard Model for the early universe. Further, it addresses opportunities to use particles from sources outside Earth to reveal physics beyond the Standard Model. The earliest history of the universe is dominated by physics of the highest energies, so that gaining an understanding of it depends on progress in understanding microscopic physics in these extreme domains. Conversely, the universe, unlike accelerators where experiments are limited by available beam lines and interaction regions, is an ever-open laboratory, one that produces a great range of phenomena that span an incredible energy range and that can be used to probe and extend ideas on microphysics. Some important relics that could have been produced only at these early times may remain today. Astronomical and astrophysical studies add immeasurably and often uniquely to important aspects of particle physics beyond the Standard Model, addressing questions such as these: Do protons decay? Do neutrinos have mass? Is nature supersymmetric? What constitutes the dark energy? Are there additional dimensions of space beyond the familiar three? The triumph of the Standard Model is based largely on data from particle accelerators of ever-increasing energies, constructed over the past 50 years. Without the Standard Model, it would have been impossible to make with any confidence the very large extrapolations in energy that have yielded insights into the conditions of the early universe.

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What can be expected from accelerator-based facilities, the center of the traditional high-energy physics effort? The search for the Higgs boson and for supersymmetric partners of the known particles is a primary focus of the programs at the highest-energy accelerators, such as at the Tevatron at Fermilab and the Large Hadron Collider (LHC) at CERN and even at the next large accelerator to be built after the LHC, which will be designed to perform incisive studies of these particles’ properties. Accelerator experiments permit irreplaceable measurements for exploring the Standard Model and beyond, including studies of neutrino masses and the violation charge-parity (CP) symmetry (see Chapter 5, section “Dark Energy”), as well as the creation of an exotic form of matter known as the quark-gluon plasma to mimic an important phase in the early universe. Accelerators are also capable of seeing manifestations of extra dimensions that are macroscopic. This possibility, a recent speculation from string theory, has profound implications for understanding the physics of the very early universe. Experimental signatures include the apparent loss of energy in particle interactions, which, in fact, has gone off into the additional dimensions. Experiments at the Tevatron and the LHC should have significant sensitivity to this exciting possibility. Rather than address ongoing and proposed accelerator programs that are reviewed elsewhere by other responsible scientific groups (laboratory program committees, the NRC, and the DOE/NSF High Energy Physics Advisory Panel and the Nuclear Science Advisory Committee), this committee focuses on identifying additional and complementary opportunities for the use of new techniques and technologies to probe the most fundamental questions at the interface between particle physics and astronomy and astrophysics. This chapter discusses, in turn, experiments seeking signatures of unification, identifying the dark matter, and probing the very foundations of our science. LOOKING FOR SIGNATURES OF UNIFICATION The hypothesis that a single unified theory can account for the three separate forces of the Standard Model is attractive in many ways. Such a theory would organize the quarks and leptons into a simple, beautiful structure and would explain the patterns of charges, which otherwise seem quite arbitrary. And most impressively, by including low-energy supersymmetry, it would account quantitatively for the relative values of the different observed coupling strengths.

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Unified theories predict additional effects that go beyond the Standard Model. In the following subsections the most promising of these new phenomena are discussed. Proton Decay A great cosmological question is how the current preponderance of matter over antimatter in the universe came about. Presumably the abundances of both were equal immediately after the big bang, just as the numbers of negative and positive charges were equal. The subsequent interactions that established the matter-antimatter imbalance at very high energies must also allow proton decay, although at a very low rate given the low energy (mass) of the proton. Unified theories predict that protons are unstable. Early estimates based on the simplest unified theories suggested lifetimes on the order of 1030 years. But those predictions were discounted with the first round of experiments. Today, the predicted lifetime of protons is on the order of 1035 years or less in the most viable models. Experiments currently set lower limits (depending on the mode of decay) of roughly 1032 to 1033 years. Because it would imply the instability of all nuclear matter, the discovery of proton decay would be a historic event that provides a unique window onto some of the most fundamental questions in physics and cosmology. Different unified models make different predictions for the most likely modes of proton decay. Models with supersymmetry, for example, favor decays that include K mesons and neutrinos. Much effort has already been devoted to the search for proton decay, the principal original goal of the Kamiokande and Super-Kamiokande detectors in Japan, the Frejus experiment in Europe, and the Irvine-Michigan-Brookhaven (IMB) and Soudan detectors in the United States. Although no protons were observed to decay in these experiments, the scientists working there made impressive discoveries in neutrino physics. Furthermore, these experiments allowed limits to be defined on proton decay that already rule out the simplest grand unified theories. Clearly, achieving substantial improvements in experimental sensitivity to proton decay will be important to improving our understanding of the early universe. As a bonus, such experiments could also accommodate an extensive neutrino physics program including the study of neutrino properties by detecting neutrino beams from distant accelerators and supernovae in our galaxy and nearby galaxies.

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Neutrino Masses and Neutrino Oscillations As far as physicists know, neutrinos interact only by the weak force, passing through Earth, for example, with ease. Until recently, it was widely believed that neutrinos were also massless, like photons. Despite having properties that render them very elusive, neutrinos can be and have been studied extensively in particle accelerators and nuclear reactors, and they can have major consequences in the cosmos. For example, even though they interact extraordinarily weakly, there was a time in the early universe when even neutrinos were in thermal equilibrium with the high-density, seething plasma of particles and force carriers. At about 1 second after the big bang, the universe became too diffuse to maintain that equilibrium, and neutrinos were free to expand and cool just as the photons of the microwave background did 400,000 years later. Created in numbers comparable to the number of photons (and a billionfold more abundant than protons), neutrinos with a small but nonzero mass of only a few eV/c2 (electron-volts divided by the speed of light squared; in this unit, the electron mass is 511,000) would contribute a significant fraction of the dark matter (though still not enough to allow them to be the seeds of galactic and large-scale structure formation). Neutrinos from weak processes that power the Sun and neutrinos generated in the atmosphere from the decay of secondary particles produced by cosmic rays are providing key information about these elusive particles and their role in the cosmos. A burst of neutrinos was detected on Earth from the explosion of supernova SN1987A, broadly confirming the predictions of supernova models and opening up an astronomical window for the study of a variety of effects beyond the Standard Model. No experiment has directly detected the cosmic neutrino background, but it is likely that the effects of even a 1 eV/ c2 neutrino on structure formation could be seen indirectly by its imprint on the large-scale distribution of matter in the universe. The Sloan Digital Sky Survey, a map of the universe being made from the positions of 1 million galaxies, will soon enable detecting the effect of neutrinos on large-scale structure. In the early universe, neutrinos played a critical role in the formation of elements beyond hydrogen through their ability to transform protons into neutrons and vice versa. The particular pattern of abundances of hydrogen, helium, and lithium nuclei produced depends critically on the rates of neutron production, capture, and decay, which in turn depend on the nature and properties of neutrinos. The predicted abundances have been confirmed spectacularly in studies of the abundances of these elements today.

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We know that there are only three light neutrino types (also called “flavors”)—the electron neutrino, the muon neutrino, and the tau neutrino— named for the particles into which they are transmuted by emission or absorption of a W boson (recall Figure 2.1). The concordance between the predicted and observed cosmic abundances of the light elements would not be nearly as good were there more than these three flavors of neutrinos, and this result from cosmology gave an important early constraint on the number of neutrinos. Subsequently, the number of neutrino flavors was very accurately measured by experiments at the Stanford Linear Accelerator Center’s Linear Collider and CERN’s Large Electron-Positron Collider (LEP). Within the Standard Model, the total number of electron neutrinos and electrons minus the total number of electron antineutrinos and positrons in the universe never changes. Similar lepton-family-number conservation laws apply to the mu and tau families as well. However, physicists have long been alert to the possibility that the lepton-number conservation laws may be only approximate. Indeed, this may be suggested by the fact that similar laws for the conservation of different quark types are known to be only approximate. In a unified theory, it would be natural for quarks and leptons to appear on an equal footing, compelling researchers to think that the conservation of lepton-number really will be violated. A subtle phenomenon that can cause lepton-family-number violation is neutrino oscillation: One flavor of neutrino produced initially may be detected later as another flavor, with a probability that changes as the neutrino moves through space or passes through matter (see diagram in Figure 3.1a and 3.1b). The changes are oscillatory in the sense that the probability of a change in flavor occurring reaches a maximum at a certain distance, diminishes to zero at twice that distance, and so on. The effect can occur only if different neutrinos have different masses. The rate of oscillation depends on the energy of the neutrino, on the mass differences between the various neutrinos, and on the value of a “mixing factor” that controls the process of conversion from one to the other. If the mass differences are tiny, then sensitivity to neutrino oscillations can be achieved only by looking at neutrinos that have traveled a very long distance, since the oscillations are then very gradual, although the oscillation rate can be enhanced for electron-type neutrinos traveling through dense matter, for example in the Sun. The first real hints that neutrinos oscillate came from studies of solar neutrinos. The nuclear reactions that power the Sun produce electron neutrinos. Because they interact so weakly, these neutrinos from the Sun can be detected only in experiments on a heroic scale. For many years the only suitable detector was a gigantic vat of cleaning fluid, mounted and

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FIGURE 3.1 (a) If neutrinos have mass, the different types mix with one another, as indicated in the first diagram. The so-called neutrino mixing parameters correspond to the arrows in each direction. It is because of this mixing that one flavor of neutrino can “disappear” (and reappear as a another flavor that has different detailed interactions) and be noted as a deficit in the detector. (b) Two mass schemes for neutrinos are consistent with measurements of atmospheric and solar neutrinos. The three “mass states” are mixtures of the different neutrino types, as illustrated in the second figure. Note, too, that the solar and atmospheric neutrino experiments determine mass differences and do not set the absolute scale of neutrino masses. Image courtesy of H. Murayama, University of California at Berkeley.

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instrumented by Ray Davis in the Homestake Mine in South Dakota. Davis succeeded in observing electron neutrinos, but at roughly one-third the expected rate. Several later experiments have confirmed this deficit by looking at lower-energy neutrinos, whose rate prediction is less sensitive to details of the model of the Sun. The leading interpretation of these observations is that electron neutrinos emitted from the Sun have partially oscillated into muon or tau neutrinos that cannot be detected using the experiments designed by Davis and his successors. Recent dramatic experimental developments in neutrino oscillations have emerged from the study of neutrinos originating in the atmosphere as by-products of cosmic ray interactions. Since cosmic rays have been carefully studied for many decades, it is possible to predict with considerable confidence the expected relative abundance of the different types of neutrinos so produced. The experiments designed to search for proton decay, the Irvine-Michigan-Brookhaven (IMB) and Kamiokande experiments, observed that the ratio of the number of muon neutrinos to electron neutrinos fell below theoretical expectations. The ratio, naively expected to be 2 (twice as many muon neutrinos as electron neutrinos come from pion decay) is calculable to an accuracy of about 5 percent. It was found to be low by about 40 percent. A recent development, from the Super-Kamiokande detector in Japan (see Figure 3.2), is the observation that the ratio of muon to electron neutrinos depends on the distance that these neutrinos have traveled since their creation. Researchers at Super-Kamiokande see this effect as a modulation of the flux of muon neutrinos as a function of the angle in the sky at which they originate. Muon neutrinos created in Earth’s atmosphere and arriving at the Super-Kamiokande detector having traveled through the Earth’s mass are detected at about one half the rate of those created in the atmosphere directly above the detector. Observation of this dependence on distance from point of creation strongly suggests that the muon neutrinos have oscillated, and, since there is no corresponding angular dependence in the flux of electron neutrinos, the oscillation most likely involves another neutrino, such as the tau neutrino. Even more recently, the Sudbury Neutrino Observatory in Canada (see Figure 3.3) has confirmed that electron-type neutrinos are less than half of the total number of solar neutrinos reaching Earth. Solar neutrino experiments have recently given added evidence for electron neutrino oscillation. The early results giving less-than-expected electron-neutrino flux from the Sun have been confirmed. The Sudbury Neutrino Observatory detector has given an accurate measurement of the electron neutrino flux. The Super-Kamiokande detector in Japan observes a

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FIGURE 3.2 The Super-Kamiokande detector. Here, scientists (in a rubber raft) inspect the large photomultiplier tubes as the enormous tank is filled with water. Image courtesy of the Institute for Cosmic Ray Research, University of Tokyo.

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FIGURE 3.3 The Sudbury Neutrino Observatory (SNO). SNO consists of 1,000 tons of heavy water (made of deuterium and oxygen rather than hydrogen and oxygen) contained in an acrylic vessel (viewed here with a fisheye lens) and surrounded by 9,500 photomultiplier tubes. Neutrino interactions with heavy water reveal separately the number of electron neutrinos and other types. Image courtesy of Lawrence Berkeley National Laboratory. larger total flux, but this detector is sensitive at different levels to all types of neutrinos. The comparison of the two results thus gives a clear indication that neutrinos produced in the Sun as electron-type arrive at Earth as a mixture containing other types, showing that neutrinos have mass and that neutrino oscillation occurs. The combination of the solar and atmospheric results indicates that the mixing angles that characterize the defined-mass neutrinos in terms of the defined-flavor species have a pattern quite different from the equivalent matrix for quarks. Since the initial experiment of Clyde Cowan and Frederick Reines that discovered the neutrino in 1957, reactors and accelerators have been a mainstay of research into neutrino properties. An accelerator-based neutrino oscillation experiment at Los Alamos National Laboratory, Liquid Scintillator Neutrino Detector (LSND), has also found evidence for oscillation between the electron neutrino and the muon neutrino. This experiment found a difference in mass between 0.15 eV/c2 and 1.5 eV/c2, a much larger value than was obtained in other experiments. If there are only three neu-

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trino types, this result and the evidence from atmospheric and solar neutrinos cannot be accommodated simultaneously. Either some additional sterile neutrino is playing a role, or one or more of the results have been misinterpreted. Only additional precise experimental tests can tell. There is now strong evidence that neutrinos have mass. It is important to pursue these studies further. Large neutrino detectors located deep underground can study oscillations from laboratory-produced neutrino beams, as well as look for angular dependence in neutrinos from the atmosphere. These solar and atmospheric neutrino results describe neutrino disappearance effects, i.e., they detect a shortage of the neutrino type produced. More convincing would be an experiment in which an appearance effect is observed, i.e., detection of a type of neutrino not produced at the source. Neutrino oscillation experiments measure only differences between the masses of neutrinos (more precisely, the difference between the squares of their masses), not the actual value of either mass. To determine the mass itself requires a different approach. Direct measurements are limited in precision both by technical capabilities and by the amount of the energy released in the relevant decays producing neutrinos. (The determination of their mass requires the use of low-energy neutrinos: the lower the energy, the better.) Careful studies of the end-point behavior of the spectrum of electrons from tritium beta decay could in principle yield indications of neutrino mass, but the smaller the mass, the more difficult this approach becomes. One means of illuminating some aspects of the neutrino mass scale might be the study of a rare process in which a nucleus decays weakly with the emission of an electron and a positron but with no neutrinos. The predicted rate for this double-beta decay depends on the neutrino mass and also on the relationship of the neutrino to its antiparticle. Among the mysteries remaining to be resolved for the neutrinos, one is whether each neutrino is identical to its own antiparticle (in which case it is called a Majorana particle) or whether, like other massive fermions, such as the electron, it has a distinct antiparticle partner (a Dirac particle). Owing to the weak interaction’s enforcement of opposite handedness for neutrinos and antineutrinos, most direct experimental tests of this question are impossibly difficult. But observation of neutrinoless double-beta decay would demonstrate the Majorana character of neutrinos. No signal has been seen to date for this type of decay, setting a neutrino mass limit of a few tenths of an eV, provided neutrinos are Majorana particles. New double-beta decay experiments using radioactive sources on the scale of tons will be needed to achieve a neutrino mass sensitivity in the range of 0.01 eV/c2. This is the interesting range suggested by the neutrino-oscillation evidence described above.

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Single- and double-beta decay experiments directly probe the mass of the electron neutrino. But the small mass differences that are representative of oscillations forge links among various masses. When these mass differences are known, to measure any one neutrino mass is to measure them all. The probable values of the neutrino masses indicated by the oscillation experiments are very small, far smaller than the analogues for any other leptons or quarks. The occurrence of neutrino oscillations is the only known phenomenon in particle physics that is not accounted for by the Standard Model in its minimal form. What might this mean? In grand unified theories, the Standard Model describes only the most accessible part of a larger theory, so it is not complete. The extra particles in a complete theory might be very heavy, so that their effects, on neutrino masses in particular, will be small. Remarkably, by analyzing these extensions of the Standard Model, theorists predicted neutrino masses of roughly the right magnitude before they were observed. Thus the recent experimental discoveries about neutrinos suggest that these bold ideas may be on the right track, and further experimental tests might help refine or refute them. Very-High-Energy Cosmic Rays Several serious ideas related to unification and unknown forces, including cosmic strings and dark-matter decays and annihilation (discussed below), would result in signatures in the high-energy cosmic rays detected at Earth. Gamma-ray bursts and ultrahigh-energy cosmic rays have been observed, but their origins are not well understood (see Chapter 7, sections “Understanding the Destiny of the Universe” and “Exploring the Unification of the Forces from Underground”). Further, cosmic rays provide the highest-energy particle beams observable on Earth and hence can be used to probe physics inaccessible at accelerator laboratories. Modern cosmic ray detectors, using sensitive phototubes deployed on a large scale, measure the huge, energetic showers created by very-high-energy primary particles either at Earth’s surface or in the atmosphere. The same technologies can be applied on a much larger scale. Space-based versions of such detectors have been proposed. UNIFICATION AND THE IDENTITY OF DARK MATTER The amount of matter in the universe is an essential cosmological parameter. Evidence has accumulated for the existence of a large amount of exotic “dark matter”—almost 10 times the amount of ordinary matter (see

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Chapter 5). According to the current paradigm for structure formation in the universe, ordinary matter falls into clumps of dark matter. The dark matter has been detected through its gravitational effect on the motion of stars and, more recently, through its gravitational lensing of light from more distant galaxies. This matter, whatever it is, interacts very weakly with photons. A major puzzle is what dark matter is made of. Neutrinos are the only candidate of all the known particles. But they cannot constitute all of the dark matter; with their small masses and velocities near the speed of light, they would not have been gravitationally trapped in density fluctuations in the early universe. Alternative candidates are needed to account for the “cold” (i.e., massive, slowly moving) dark matter that seems to govern structure formation. Remarkably, some compelling ideas in particle physics both predict the existence of particles that could make up this dark matter and suggest ways of detecting them. The simplest implementations of supersymmetry suggest a new, electrically neutral, stable particle type that interacts very weakly—the neutralino. It is thought to have a mass in the range of 100 GeV/c2. Despite varying estimates of the neutralino’s abundance from production in the early stages of the big bang, the amount required for dark matter is easily accommodated. Several promising ways to look for neutralinos are discussed in Chapter 5. Another hypothetical particle that could be a significant component of dark matter is the axion, which was introduced into particle physics to solve a deficiency in the Standard Model. Although the Standard Model is generally a reliable guide to the interactions that can occur in nature, it fails to explain why the strong force does not violate matter-antimatter symmetry, technically known as charge-parity (CP) symmetry. One suggestion introduces an additional, but slightly broken symmetry, into the theory; a general consequence of adding such a symmetry is the prediction of an additional low-mass and very weakly interacting particle, the axion. Fortunately, the idea of an axion is testable. If axions exist, they would have been produced abundantly during the big bang and could quite naturally provide the required dark matter. It is possible to carry out an experiment sensitive to the cosmic axion background using large electromagnetic cavities embedded in strong magnetic fields (see Chapter 5). Many additional dark matter candidates are suggested by other theories (some more speculative than others), but the neutralino and the axion stand out because they are motivated by important concepts in particle physics, and their properties are well characterized and predictable.

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EXAMINING THE FOUNDATIONS OF UNIFICATION Searching for Violation of Basic Symmetries The universe around us is made of matter, not antimatter. To explain the observed difference in the amounts of antimatter and matter seen today requires, in addition to the baryon-number changing processes discussed above in this chapter, violation of CP symmetry. There are well-established laboratory manifestations of CP violation, seen in the decays of the neutral K meson, or kaon. But very little is known about the fundamental nature of this important phenomenon. Is the pattern of CP violation consistent with that of the Standard Model of particle physics? The search for new sources of CP violation is important. It appears that there must be at least one new source since the magnitude of the CP violation allowed by the Standard Model appears to be far smaller than that needed in the very early universe to account for the dominance of matter over antimatter. Evidence of CP violation in the neutrino sector could lead to a quite different model for the development of the matter-antimatter asymmetry of the present universe. There is much still to be learned in this area. Important new studies of CP nonconservation in B decays have recently yielded first results, showing a definite CP-violating effect in one channel, consistent with that predicted by the Standard Model. An ongoing program studying the many additional modes is needed, as are additional experiments sensitive to other B decays or to very rare kaon decays. Some of the hypothesized sources of CP violation beyond the Standard Model predict electric dipole moments of elementary particles such as the neutron and the electron, which could one day be detectable in ambitious experiments. (A symmetry principle called time-reversal invariance, or T symmetry, holds that the laws of physics should be the same when time is run backwards. An electric dipole moment would be a violation of T symmetry.) Many unification models, especially those incorporating low-energy supersymmetry, predict an additional and quite different sort of T violation that could be visible through its very tiny effects on ordinary matter. In response to an applied electric field, the macroscopic material would generate, by T violation, a small magnetic field (or, conversely, an applied magnetic field could generate a small electric field). Modern precision spectroscopic techniques provide sensitive tools with which to look for such effects.

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In all field theories, T violation and CP violation are intimately connected, since such theories incorporate an overall prediction of a combined CPT symmetry that must be exact. However, the higher the energy, the less string theory looks like a field theory. Thus, the search for violations of CPT symmetry is a potential test for the validity of string theory. Probing Unification with Gravitation Experiments After more than 300 years, Newton’s law of gravitation remains experimentally valid in and around Earth (at least up to the tiny corrections resulting from general relativity). It states that the net force between two uncharged objects is proportional to mass and independent of internal composition (the equivalence principle) and decreases as the inverse square of the separation. Strangely, high-precision tests of Newton’s basic law on laboratory scales may provide important probes of unification. The axion is but one of several hypothetical very light, very weakly coupled particles suggested to resolve issues in particle physics. Others are familons, dilatons, and moduli fields. (A proper explanation of these possibilities would take this discussion far afield.) One way to be sensitive to light particles such as the axion is to detect the forces they generate. Since an inverse relationship exists between the mass of a particle and the range of the associated force, such light particles could generate new forces on macroscopic scales of microns and larger. These forces would appear as deviations from Newton’s inverse-square law of gravity. Also, since these putative particles could interact differently with different kinds of material, they could result in testable violations of the principle of equivalence. The violation of the equivalence principle is a generic prediction of string theory, although the level of the violation is not currently predictable. To address the speculation that nature contains extra spatial dimensions, possibly some of macroscopic size, it is necessary to explain why we experience only three spatial dimensions. According to one explanation, the ordinary particles we are made of are confined to three-dimensional structures (“branes”) that exist within the larger space, while the graviton is not so confined. This arrangement would also modify the behavior of the gravitational force at distances comparable to the size of the extra dimension. Discovery of deviations from Newton’s gravity at any distance scale would revolutionize knowledge of the physical world. Tests of the principle of equivalence in the laboratory and using the Moon have reached the level of parts in 1013 and could be improved by another order of magnitude,

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while a space experiment could yield improvement by a factor of 105. At scales of 1 mm or less, sensitive laboratory inverse-square law experiments that are clever variations on the original one by Henry Cavendish are under way, with the goal of probing the force between bodies in the submillimeter range (while excluding the dominating effects of electromagnetic forces). See Figure 3.4 for an experimental design. Are the “Constants” Constant? Modern theories of particle physics suggest that some or all of the quantities regarded as constants of nature are in reality associated with dynamical fields that change. The axion field is an excellent example; familons, dilatons, and moduli fields are other examples. In string theory, as FIGURE 3.4 Extra dimensions that are open to gravitational interactions may explain gravity’s apparent weakness in our four dimensions. If those dimensions are “curled up” on the scale of fractions of a millimeter, it may be possible to observe a strengthening of the gravitational force at very short distances. The highly sensitive short-range torsion balance shown here is used to detect this effect. An aluminum ring was suspended by a fine tungsten wire above a disk that rotated once every 2 hours. The ring and disk both had 103/8-inch-diameter holes bored into them. The gravitational force between the “missing mass” in the two sets of 10 holes produced a twist on the ring that varied back and forth 10 times for each revolution of the disk. The disk’s twist was measured by bouncing a laser beam off one of two mirrors mounted above the ring. So far, extra dimensions of extent greater than 0.2 mm have been ruled out by experiments such as this one. Image courtesy of E. Adelberger, B. Heckel, and E. Swanson, University of Washington.

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currently understood, it appears that all “constants” are in principle dynamical. Modern precision spectroscopic techniques can be used to search for the evolution of the electromagnetic coupling with great sensitivity, by looking at the spectra of distant, and hence ancient, stars. The mass of the photon is strictly zero in the Standard Model. It is severely constrained by astronomical observations of electromagnetic fields at distances of 1020 meters from their source, providing an impressive limit of about 10−33 of the electron mass. Speculative ideas about the quantum structure of space allow the speed of light to vary with photon energy. This concept is testable by monitoring the arrival times of gamma rays of different energies in gamma-ray bursts from distant sources, probing a fundamental property of light in a new regime. Monitoring the arrival times of neutrinos from astrophysical sources such as supernovae also provides a means of directly probing neutrino masses (especially those of muon and tau neutrinos, which are much less accessible in the laboratory). Unfortunately, supernovae are rare events, one per 30 years or so within our galaxy, so such measurements cannot be scheduled; rather, experiments must be prepared to catch a supernova whenever it happens. NEW OPPORTUNITIES All of the research fields discussed above span, in one way or another, the boundary between particle physics and the physics of the universe. In recent years it has been the physics at this boundary that presents and probes ideas at the limits of the knowledge of matter and of space-time. It will take the concerted efforts of astrophysicists and particle physicists to mine this rich area for all that can be learned from it. The discussion in this chapter can be summarized by posing four of the crosscutting fundamental science questions for the new century outlined in this report. Are Protons Unstable? The discovery of proton decay and improved understanding of CP violation would provide evidence for unification and help to answer the question of why matter in the universe dominates antimatter. Large-volume detectors with greater sensitivity could dramatically improve limits on the proton lifetime, and further laboratory and accelerator tests of CP violation could distinguish among competing models of unification.

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What Are the Masses of the Neutrinos? There is strong evidence that neutrinos have a mass and that oscillations occur among the various neutrino flavors. Several opportunities are ripe for experimental progress. The needed measurements or observations include confirming various effects of neutrino oscillations and identifying the neutrino species involved in each, measuring the values of the mixing parameters responsible for the observed solar neutrino abundances, and measuring the values of the neutrino masses themselves. Answers to these problems are within reach. Much more difficult and subtle issues remain, such as the particle-antiparticle properties of neutrinos and possible CP-symmetry violations in their transitions. New global-scale investigations in the planning stages should culminate in precise results describing these elusive fundamental particles. What Is Dark Matter? Well-founded ideas from unification and particle physics suggest interesting candidates for dark matter, such as neutralinos and axions, with calculable properties. Do these particles exist? Are any of them the actual dark matter observed astronomically? Initial experiments to detect these particles have been mounted, but more sensitivite searches will be needed to detect or rule out these candidates. Are There Extra Dimensions? Attempts to unify space, time, and matter beyond the Standard Model and general relativity introduce additional interactions and extra space-time dimensions. Tests of the strength of gravity at short range, experiments at particle accelerators, and tests of the principle of equivalence can probe for such signatures of unification.