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9 The Electroweak Synthesis and Beyond Occasionally in the history of science, a new unifying principle has emerged that joins two separate bodies of knowledge whose connec- tion at some deep level had not previously been recognized. The first great unification in physics was probably Newton's demonstration that gravity acts on the heavenly bodies in the same way that it acts on objects in our own world. Later, in the nineteenth century, Maxwell unified electric and magnetic forces by showing that they are just two different manifestations of a single force-electromagnetism. In our own century, Einstein unified the concepts of space and time surely one of the greatest single intellectual achievements in physics-and of matter and energy, through relativity. After the mid-1930s, the four fundamental forces of nature were considered to be gravitation, electromagnetism' the strong force, and the weak force. In 1967, however, the work of S. Weinberg, A. Salam, and S. Glashow led to a remarkable synthesis of electromagnetism and the weak nuclear force into a single electroweak force. This achieve- ment, one of the triumphs of modern science, has had a profound effect on the development of nuclear physics and particle physics during the last decade. In this chapter we examine a few of the directions in which the electroweak synthesis appears to lead. THE STANDARD MODEL The value of great unifying syntheses comes not only from the ways in which they illuminate the underlying simplicity of nature in a very 160
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THE ELECTROWEAK SYNTHESIS AND BEYOND 161 real sense, they change our view of the world-but also from the predictive power of their logical consequences. Maxwell's unification of electricity and magnetism, for example, required the existence of electromagnetic waves moving through a vacuum with the speed of light, and we know that this requirement is fulfilled. Similarly, the electroweak synthesis already has an impressive list of successful predictions to its credit. One of these is that the weak force should be mediated not only by the exchange of massive charged particles (the W+ and W- bosons) but also by the exchange of a massive neutral particle (the Z° bosom. All three of these particles were discovered at CERN in 1983. Furthermore, the electroweak theory makes detailed predictions about nuclear processes. For exam- ple, the weak-interaction decay of a neutral kaon into a positive muon and a negative muon is permitted by the exchange of a neutral particle, such as the Z°, but this process occurs only very rarely. The electroweak theory explains this result correctly on the basis of subtle effects pertaining to the strange and down quarks. Consideration of this problem led to the postulation of a new type of quark called charm (so named because it made the theory "work like a charmed. The charm quark was subsequently shown to exist-another triumph of the theory. It is because the present theories of the electroweak force and the strong force are so successfill that together then Burp roll the Standard Model. Every known fact about nuclear and particle physics is consistent with the Standard Model. This does not mean, however, that the Standard Model explains everything that we know-far from it! Despite its spectacular successes, physicists are certain that the Standard Model is incomplete. It does not, for example, include the gravitational force; it does not tell us why there are three lepton families; and it does not explain some important conservation laws or their violations. Parity violation, for example, is a dominant charac- teristic of the weak force, yet it must be built into the electroweak theory arbitrarily. Similarly, time-reversal-invariance violation is known to occur, but among several possible ways of incorporating it into the theory, it is not clear which way is correct. As for the conservation laws for certain other properties, such as lepton family number, we do not know whether an underlying symmetry principle is at work or whether the law seems to hold only because present experiments are insufficiently sensitive to detect possible violations of it. The mathematical form of the electroweak theory inspires confi- dence, however, because it is the only known theory of the weak interaction that is renormalizable. In a renormalizable theory, of which ~in_ ~A^_^ ~_] - A _ _' AA~ ~11~ ~C _ ~ _ 4C ~. ~-
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162 NUCLEAR PHYSICS quantum electrodynamics is the archetype, observable quantities can be calculated to apparently any desired degree of accuracy. Quantum chromodynamics (QCD) is also a renormalizable theory, but its math- ematical complexities are so great that reliable QCD calculations are very difficult, except near the limit of asymptotic freedom. PHYSICS WITH NEUTRINO BEAMS The advent of very intense beams of protons at meson factories has opened up the possibility of making neutrinos from the nuclear debris created when these beams are brought to rest in matter. Neutrinos interact only through the weak interaction and can penetrate vast amounts of matter without stopping. However, if copious numbers of neutrinos are present and detectors weighing many tons are used, a few neutrino interactions can be observed. Such experiments permit the study of the weak part of the electroweak force and, by comparison with the much more easily studied electromagnetic part, can test the fundamental unity of the electroweak interaction. An experiment now under way at the Los Alamos National Labo- ratory is designed to measure the scattering of electron neutrinos from electrons in an advanced detector. According to electroweak theory, this scattering can happen in two ways: the neutrino and the electron can exchange a W- boson, thereby also exchanging their identities (the neutrino turns into an electron, and vice versa), or they can exchange a Z° boson and retain their original identities. There is no way an observer can tell which process actually happened in any given scattering, so quantum mechanics predicts that these processes can interfere with each other: the total probability for the event is not just the simple sum of the individual probabilities. Demonstrating this interference and measuring its sign will be a key test of electroweak theory. With even more-intense and more-energetic neutrino beams, such as might be produced by the next generation of accelerators, one can hope to carry out experiments in which neutrinos scatter from nuclei, sometimes leaving them in excited states. Because the nuclear states have specific quantum numbers, experiments of this sort will be able to dissect electroweak theory into its parts, each corresponding to these different quantum numbers. Such tests have never been performed and would provide a far more searching evaluation of electroweak theory than can be made at present.
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THE ELECTROWEAK SYNTHESIS AND BEYOND 163 TESTING THE GRAND UNIFIED THEORIES With two powerful theories of nuclear matter at our disposal the electroweak theory and QC~the scientific imperative is obvious: we must try to unify the electroweak and strong forces within a Grand Unified Theory that would include them both in one self-consistent mathematical framework. In the previous unifications, the main diffi- culty was in constructing a viable theory having all the required properties. Now, however, we are faced with an unprecedented and most peculiar problem: there is already a glut of Grand Unified Theories, which turn out to be rather easy to construct. Each reduces correctly to QCD and electroweak theory at low (terrestrial) energies; the catch is that at cosmological energies, such as must have existed briefly after the big bang, they predict a bewildering variety of phenomena that are as bizarre as they are different. These differences between contending Grand Unified Theories be- come evident only at particle energies estimated to be about 1O'5 GeV, which is hopelessly beyond the reach of any currently conceivable terrestrial accelerator and far above even the energies of cosmic rays. How, then, can such stupendous energies possibly be achieved so that the correct Grand Unified Theory can be recognized from among the welter of alternatives? The answer may lie in the Heisenberg uncer- tainty principle, which allows a particle of any arbitrary energy to emerge out of a vacuum as a virtual particle, as long as it disappears back into the vacuum within a certain time, i.e., as long as its lifetime falls within a prescribed limit. The higher the energy, the shorter the allowed lifetime. Thus, ultrahigh-energy virtual particles can enable us- if we are clever enough to study interactions that would other- wise be inaccessible. A virtual particle of mass 1O~s GeV would have some astounding properties, even by the standards of particle physics. In terms of conventional units, its free mass would be about 10-9 gram (equivalent to 10'4 carbon atoms, or about the mass of a typical bacterium!), and it might exist for a fleeting 10-39 second, long enough for it to move only 10-~6 of a nucleon diameter at the speed of light. This incredibly brief virtual existence of such a supermassive unification particle means that any effect it may have in a laboratory experiment will be extremely tiny. Experimentalists may have to sift through staggering numbers of nuclear events to find the precious few that reveal the signature of a unification particle. Nevertheless, a number of technically feasible experiments have been designed that bear on the unification of the strong and electroweak forces. A few of these experiments are
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164 NUCLEAR PHYSICS described in the following sections; some of them are already in progress, while others await the construction of specialized new accelerators. Time-Reversal-Invariance Violation The origin of time-reversal-invariance violation is unknown. At present, the only known instance of this phenomenon is in the decay of neutral K mesons (kaons). A neutral kaon and its antikaon are exactly alike except for the quantum number called strangeness, which is related to the strong interaction. The weak interaction does not respect strangeness and "mixes" the pure kaon and its pure antikaon; the two kaons that are actually observed can be thought of (roughly) as two different hybrids of the pure kaon states. Now that tentative Grand Unified Theories are available, it appears to be possible to incorporate time-reversal-invariance violation into their framework, based on certain details of the decay properties of these kaons. Experiments to measure the neutral kaon decay precisely and to search for evidence of time-reversal-invariance violation in another possible decay mode may be crucial in finding the correct way to account for the violation in the context of grand unification. However, kaon beams 10 to 100 times more intense than those currently available will be needed for these experiments. The Electric Dipole Moment of the Neutron Finding a second example of time-reversal-invariance violation would be a major event in physics. Such an example might conceivably be found in the neutron if it can be shown to have an electric dipole moment. An electrically neutral particle can possess a measurable electric dipole moment (internal separation of positive and negative charge) only if both parity and time-reversal invariance are violated. Very sensitive experiments have been carried out over the past three decades to try to measure the electric dipole moment of the neutron. When a neutron is between the poles of a magnet, the interaction with the neutron's intrinsic magnetism produces two possible energy levels, depending on whether the neutron's axis is aligned parallel or antiparal- lel to the applied magnetic field. An observable change from one level to the other can be induced by bathing the neutrons in an oscillating radio-frequency field having just the right frequency; a representative value is 60 megahertz (60 million cycles per second) in a strong magnet. The principle is just the same as in the nuclear-magnetic-resonance
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THE ELECTROWEAK S YNTHESIS AND BEYOND 165 equipment routinely used by chemists to detect protons in molecules. However, a beam of free protons is not suitable for the electric dipole moment search, because protons are charged and would be deflected out of the magnetic field. Neutrons, on the other hand, are uncharged and can be obtained as a slow-moving beam; the experimental sensi- tivity is thus enhanced because of the increased length of time that they remain in the magnetic field. In the experiment, a strong electric field is applied simultaneously with the magnetic field. If the neutron has an electric dipole moment, the energy added by the electric interaction will slightly shift the difference between the neutron's energy levels in the magnetic field. Current experiments are sensitive to shifts as small as 0.001 hertz. With the present sensitivities, no electric dipole moment has yet been observed in the neutron. If the neutron does have an electric dipole moment, it must be smaller than that which would be due to a positive electron and a negative electron separated by only 6 x 10-25 cm (roughly 10-~ times the radius of the neutron). Thus, if a neutron were expanded to the size of the Earth, the "bulge" of electric charge in one hemisphere represented by this maximum value of the dipole moment would be only about the thickness of a human hair! This infinitesimal limit has ruled out a number of theories that predict an observably large moment, leaving only theories that predict either an extremely small moment or no observable time-reversal-invariance violations outside the kaon system. To increase further the sensitivity of the experiments, very-slow- moving (cold) neutrons will be needed, because they will remain longer in the magnetic field of the detector, allowing a more sharply defined measurement. Present experiments have reached the limits imposed by the two major reactor facilities (in France and the Soviet Union) that produce cold neutrons. Further progress will require specialized tech- niques, such as spallation neutron sources and cold moderators at accelerators. Rare Muon and Kaon Decays According to the quark model, the six quark flavors fall into three distinct families of two each. It has been known for many years that the weak interaction "mixes" the quark families, so that a quark from one family can change into a quark from another. The lambda hyperon (quark structure ads), for example, has a rare decay mode in which it transforms to a proton (uadD, an electron, and an antineutrino; this
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166 NUCLEAR PHYSICS decay mode evidently requires a strange quark from one family to become an up quark from another. It is interesting, but not necessarily significant, that leptons also come in three families of two each, and many Grand Unified Theories allow mixing between lepton families, in analogy with the mixing between quark families. Such mixing would, in turn, allow the occur- rence of decay modes in which lepton family number was not con- served- for instance, the decay of a muon into an electron and a gamma ray (see Figure 9.11. The observation of this decay would be both an indication of such mixing and a much-needed signpost pointing toward the correct Grand Unified Theory. Intensive effort at all three of the world's meson factories the Los Alamos Meson Physics Facility, the Tri-University Meson Facility (Vancouver, British Columbia), and the Swiss Institute of Nuclear Research (Villigen) has been put into the search for the electron mode of muon decay. The lowest limit to date, established at Los Alamos, shows that this mode occurs no more frequently than once in every 6 x 109 muon decays. This is a very small limit, but a more-intense muon source would allow even lower limits (greater experimental sensitivity) to be achieved. Failure to see one distinctive electron-mode decay in every 1Ois muon decays might eliminate all but a few of the currently conceived Grand Unified Theories from further consideration. Rare decays of kaons offer a cornucopia of opportunities for looking at the electroweak synthesis and beyond. Present theory predicts that a positive kaon should decay into a positive pion and a neutrino- antineutrino pair somewhere between 1 and 30 times in every 10~° kaon decays. Agreement of experiment with this prediction would confirm the number of quark families, including the existence of the hitherto unobserved top quark, and would even provide a value for the latter's mass. Experiments to search for this decay are planned for existing accelerators and will require large detectors and long measurement times. If the decay probability is significantly less than one event in 10~°, then its detection is out of reach at present. Accelerators capable of producing kaon or muon beams of far greater intensity are needed for the study of electroweak interactions through rare decay modes. Together, the theories of the electroweak and strong interactions explain most of what we know about atomic nuclei. Those things that we know but are unable to explain as well as many of the innumerable things that we do not yet know at all may have their origins in levels of understanding that can arise only from a grand unification of these two interactions. Direct tests of grand unification are at present
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THE ELECTROWEAK SYNTHESIS AND BEYOND 167 FIGURE 9.1 The Crystal Box spectrometer, an advanced particle and radiation detector currently under construction at the Los Alamos Meson Physics Facility. Consisting of several hundred specially shaped sodium iodide crystals with associated electronics packages, it will be used in searching for the decay of muons to electrons and gamma rays. (Courtesy of the Los Alamos National Laboratory.)
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168 NUCLEAR PHYSICS impossible, of course, because no conceivable accelerator could even approach the necessary 10~5-GeV energies. Instead, the current emphasis is on extremely rare but profoundly significant processes that can be observed at accessible energies. In addition to high experimental selectivity and sensitivity, this search requires the maximum possible beam intensities, in order to produce the huge numbers of events among which the occasional rare ones may be found. These invaluable bits of information from nuclear physics may ultimately prove essential for weaving together our fragmentary knowledge into a Grand Unified Theory of the fundamental interactions. a
Representative terms from entire chapter: