III. ELEMENTARY PARTICLE PHYSICS TODAY A. INTRODUCTION High-energy physics is the search for elementary particles and basic laws of nature. What are the smallest building blocks out of which protons, neutrons, atoms, and all matter are made? Do such elementary particles exist?; and if so, what are they? This search to unveil the elementary constituents of matter, along with the forces that link them, involves distances thousands of times smaller than nuclear sizes, about one ten trillionth of a centimeter, or 10-13cm. Accelerators must have very large energies to probe nature at such small distances. The ultimate goal of this quest is a view of the underlying first principles that govern our entire physical universe. In recent years, we have realized a strong and growing synergism between the physics of short distances and the properties and large- scale structure of the universe. This development reflects the unity of science as explored on both the high-energy and particle astrophysics frontiers. With this connection, we are now addressing some of the most basic questions one can ask: How did our physical universe begin? How did it evolve to its present state? What will be its final fate? Over the past several decades, experimental discoveries and theoretical insights have significantly advanced our understanding of the elementary particles and their forces. We now know that electrons, protons, and neutrons make up the visible matter all around us, but only the electron appears to be a point-like elementary particle. Protons and neutrons are bound states of more basic constituents, the up and down quarks. Those quarks are permanently bound or confined by what are called strong interactions or forces. The strong interactions are governed by a fundamental theory of quarks and gluons known as quantum chromodynamics (QCD). The gluons mediate the strong force that binds the quarks into protons and neutrons. QCD is an elegant theory that, in principle, is capable of explaining all observed strong interaction physics. On another front, two forces previously thought to be distinct, electromagnetism and the weak force that governs radioactive decay, are now properly described by a unified electroweak theory. This theory correctly predicted weak neutral currents as well as the observed properties of W and Z bosons, the carriers of the weak force and partners of the photon. The combination of QCD and the electroweak model provides a beautiful description of all known elementary particles down to distances of order 10-16 cm. The theory of strong and electroweak interactions can be unambiguously tested by comparing its predictions with precision measurements. Remarkably, a wealth of experimental data has been confronted at a high level of sensitivity, without any clear signal of disagreement or inconsistency. Those impressive successes have earned the theory its title as the "Standard Model," a label that describes its acceptance as a proven standard against which future experimental findings and alternative theories must be compared. Its discovery should be viewed as one of the great scientific triumphs of the twentieth century. Despite the successes of the Standard Model, it is believed not to be the final word. That conviction is based primarily on dissatisfaction with the electroweak sector which exhibits a number of shortcomings and leaves unanswered some basic questions: Why are there so many elementary particles and why do they have their observed pattern of masses? What is the origin of mass? Why and how is the symmetry between electromagnetism and weak interactions broken? Why is matter-antimatter symmetry broken and what does it have to do with the observed predominance of matter in our universe? Speculations abound, but physics is an experimental science, and only with new data will we be able to properly address these problems and uncover whatever new surprises lie ahead. B. THE STANDARD MODEL As an outline of the Standard Model, we have illustrated in Table A its spectrum of elementary particles, along with some of their basic properties [including their electric charge, their spin, and their mass, expressed in units of one billion electron volts (GeV), which is roughly the mass of a proton]. The fermions are grouped into three generations with remarkably similar features. Indeed, the masses of the quarks and leptons represent the only significant difference between the generations. The first generation contains the constituents of ordinary matter. The second and third include heavy unstable elementary particles, which can only be studied in high-energy processes. Indeed, a remarkable feature of the theory is that the elementary constituents can transform into one another according to well-defined rules. It now appears that elementary particles are fundamental but not immutable, in contrast to the views of many early Greek philosophers. The neutrinos are massless in the minimal Standard Model. Although this prediction is consistent with experiments to date, there are some tantalizing hints of tiny neutrino masses from solar and atmospheric experiments. (The sun and upper atmosphere are copious sources of neutrinos.) Should nonzero neutrino masses be established, they could be accommodated into theory, but they would likely be a signal of new physics. In fact, many attempts to synthesize the strong and electroweak forces into a grand unified theory naturally predict very small neutrino masses. Table A: Elementary Particles and Their Properties The study of the top quark and its properties represents an exciting frontier for particle physics. Ongoing experiments at the Fermi National Accelerator Laboratory (Fermilab) Tevatron have recently produced the first direct evidence for the top quark, and indicate that its mass is 174 +- 17 GeV, making it much heavier than any other known elementary particle. Why is the top quark so heavy? This question highlights the broader question of why nature chose to repeat the fermion generation structure three times and endow quarks and leptons with their observed pattern of masses. Understanding the mass spectrum of elementary particles is an outstanding problem for high-energy physics. Perhaps the very large top quark mass, relative to all the other quarks, holds the key to solving that problem. Quarks and leptons interact by exchanging spin-one particles known as gauge bosons. The best known gauge boson is the photon that mediates electromagnetism. Its electroweak partners, the W and Z bosons, mediate the weak forces. The large masses of the W and Z in Table A stand in sharp contrast to the masslessness of the photon. The masses of the electroweak gauge bosons indicate the degree by which the symmetries of nature are broken. At very short-distances or high energies, the W, Z, and photon have similar properties and the symmetries among them are manifest. At large distances, the symmetry is broken and the photon is preeminent. As a result, electromagnetism controls most of the physics and chemistry of everyday life. The massless gluons of QCD mediate the strong interactions. Quantum chromodynamics has no free parameters; it is capable in principle of predicting the masses of all hadrons (i.e. the proton, neutron, rho meson, etc.) as well as nuclear properties and scattering cross- sections. It is the fundamental theory that underlies the more phenomenological models appropriate for nuclear physics. In fact, low-energy particle physics is hard to distinguish from nuclear physics, and cross-disciplinary collaborations have helped to address common questions. Calculations in QCD from first principle are extremely difficult because its interaction between quarks and gluons is so strong. Nevertheless, using techniques borrowed from condensed matter physics, theorists are tackling some of these problems with the world's most powerful computers. Now that a complete theory of strong interactions appears to be in hand, the challenge is to fully explore and understand its dynamical properties and subtle features. Who knows what surprises it may yet hold? C. ELECTROWEAK SYMMETRY BREAKING In contrast to quantum chromodynamics, the description of electroweak processes in the Standard Model has many arbitrary or free parameters. Most stem from the breaking of the underlying symmetry between electromagnetism and the weak interactions. This symmetry breaking provides mass for the W and Z, but leaves the photon massless. In the minimal Standard Model, electroweak symmetry is broken by the Higgs mechanism. This idea has its roots in condensed matter physics, where it was introduced in connection with the Landau-Ginsberg theory of superconductivity. In this scheme, a particle's mass depends on its interactions with the Higgs field, a medium that permeates all of space and time. The W and Z masses result from their couplings to this field. The photon and gluon have no such couplings, so they remain massless. Quark and lepton masses are determined by the strength of their couplings to the Higgs field. These couplings also determine the extent to which quarks can mix between generations. Even charge parity (CP) violation--a fundamental asymmetry between matter and antimatter that may be responsible for matter dominance and our place in the universe--is generated by couplings to the Higgs. Unfortunately, we do not understand the origin of these couplings, so they must be determined phenomenologically by experiments. Current theoretical models can accommodate a top quark mass 340,000 times that of the electron and the small degree of CP violation seen in nature, but we cannot explain them. D. THE HIGGS PARTICLE A testable prediction of the minimal Standard Model is the existence of a neutral spin-zero elementary particle H called the Higgs boson, associated with the Higgs field. The Higgs boson mass is, however, not predicted. The lower bound in Table 1 is determined by experimental searches and the upper bound is based on theoretical arguments. If the H is too heavy, it is unlikely to exist as an elementary particle. Instead it is more likely to be replaced by a new set of strongly interacting dynamics. At present, there is no experimental evidence in favor of a Higgs particle, nor is there any against. Finding the Higgs boson, or whatever takes its place, is crucial for understanding and going beyond the physics of the Standard Model. Although introducing a Higgs field provides a simple mechanism for electroweak symmetry breaking, we really do not understand at a deep level why this phenomenon occurs. In fact, the Higgs mechanism with its concomitant spin-zero Higgs boson has a variety of theoretical shortcomings. The model on which it is based is unstable against quantum corrections when embedded in a theory of gravity or grand unified theory. In addition, although the simplest Higgs model can accommodate all known particle masses, mixings, and even CP violation, it does not explain their origin. Even though our knowledge of electroweak symmetry breaking is incomplete, the mass values of the W and Z bosons identify the energy scale where this phenomenon becomes manifest. Irrespective of what is the precise agent that causes the symmetry breakdown, we believe that the physics which underlies it will be uncovered when we will be able to thoroughly probe matter at this energy scale. Through experimentation at much higher energies than those presently available we hope that a truly fundamental understanding of electroweak symmetry breakdown will emerge that will elucidate the origin of mass through additional symmetry, new dynamics, or by some as yet unknown phenomenon. Uncovering those missing ingredients and deciphering their role was a major focus of the Superconducting Super Collider (SSC), and remains one of the most important goals of high-energy physics today. E. MATTER-ANTIMATTER ASYMMETRY Another outstanding problem in elementary particle physics is the very small asymmetry between the properties of matter and antimatter (particles and antiparticles), related to CP violation. When first observed in a 1964 Brookhaven National Laboratory (BNL) experiment, this asymmetry came as a complete surprise. Since then we have learned that CP violation is a necessary ingredient for explaining the dominance of matter over antimatter in our universe. The origin of CP violation remains mysterious to this day. Within the framework of the Standard Model, CP violation can be accommodated through quark mixing effects. Such mixings give testable predictions that are being studied in K meson decays and will be further scrutinized in B decays. The Standard Model, however, does not really explain the underlying reason for CP violation. Furthermore, it appears that an additional source of CP violation from some as-yet-undiscovered new physics may be necessary to explain the matter-antimatter asymmetry of our universe. Testing the Standard Model's description of this phenomenon and searching for non-standard CP violation are major goals of high-energy physics. Following that path may lead us to an understanding of the origin of mass and our universe. F. Beyond the Standard Model Many of the elements of the simple Higgs mechanism for electroweak symmetry breakdown can be retained if an additional symmetry between bosons and fermions, called supersymmetry, were to exist. This elegant symmetry alleviates quantum instabilities in the theory, at the expense of introducing a host of new elementary particles at masses near 1 TeV. In supersymmetric theories, essentially every particle in Table A has a supersymmetric boson or fermion partner. Currently, supersymmetry has no direct experimental support; however, supersymmetric grand unified theories correctly predict low-energy coupling strengths. Additional strong motivation for supersymmetry is provided by superstring theories, which unify the Standard Model and gravity by replacing point particles with tiny strings. Many supersymmetric theories, furthermore, predict the existence of heavy, stable, neutral particles that have the potential to explain the missing mass of the universe. Astronomical observations indicate that visible objects might comprise less than 10% of the total mass of the universe. With its plethora of new particles, supersymmetric theories can solve this problem. If true, this would have profound implications for our place in the universe: we would not be made of the material that comprises the bulk of the universe! Alternative to an elementary Higgs particle is dynamical symmetry breaking via fermion-antifermion interactions. This is analogous to the Bardeen-Cooper-Schrieffer theory of superconductivity in which electron- electron Cooper pairs replace the scalar order parameter of the Landau- Ginsberg phenomenological theory. Scenarios for electroweak symmetry breaking along these lines range from minimal top-antitop interactions to more ambitious schemes modeled on QCD. These models often predict many new heavy particles below the TeV scale. Although the basic premise of these speculations is very appealing, no complete dynamical theory currently exists. We do, however, expect that new particles or interactions should appear, at a mass scale below a few TeV. To make headway in unfolding dynamical symmetry breaking will require accelerators of the highest possible energy to discover new heavy fermions and bosons or some complete surprise. Such discoveries would provide the clues necessary to help guide our imaginations about the underlying dynamics. In addition to supersymmetry and dynamical symmetry breaking, there have been many other possible suggestions for new physics. They include: extended symmetries with additional heavy gauge bosons W', Z', neutrino masses and associated oscillations among the three different species, new sources of CP violation, grand unification of strong and electroweak interactions, etc. The menu of possibilities is rich. Full exploration will require a diverse and broad-based experimental program that utilizes accelerator and non-accelerator facilities. Theorists may speculate, but data rules supreme in the study of nature. G. SEARCHING FOR NEW PHYSICS Testing the Standard Model and probing for new phenomena at accelerators can be roughly categorized by three approaches: high energy, high precision, and high intensity. The most direct way to find new physics is to go to higher energy and explore completely uncharted territory. The Fermilab Tevatron currently has the highest center-of- mass energy of any accelerator in the world. It is the only existing facility where top quarks can be produced and where there still remains the possibility that other new high-mass phenomena might be discovered. The Main Injector upgrade will increase the Tevatron's intensity and allow a better look at the top quark's properties. Pushing the high- energy frontier ever forward is the lifeblood of elementary particle physics. Beyond the Tevatron, one must take large enough steps to ensure a significant new discovery potential. In that regard, the SSC energy of 40 TeV represented a factor of twenty increase over the Tevatron, and was chosen to allow thorough exploration of electroweak symmetry breaking, including discovery of the Higgs over its entire mass range. The European Laboratory for Particle Physics' (CERN) Large Hadron Collider (LHC), with an energy of 14 TeV, represents a significant step beyond the Tevatron on the energy frontier. Although the LHC is not as energetic as the SSC, it has considerable discovery potential. A TeV- scale electron-positron collider would also extend our discovery potential and would be well-suited for thorough investigations of new phenomena. Complementary to high-energy searches are high precision studies of the Standard Model. In this approach, one tests the consistency of standard-model predictions through precision experiments. Such studies allow us to refine our understanding of the Standard Model. In addition, any deviation from expectations would indirectly signal the presence of new physics. Examples of precision measurements include the W and Z masses, the electroweak mixing angle, as well as the quark mixing angles. Of particular importance are plans to measure the W mass to an accuracy of about 50 MeV (better than 0.1%) both at the Tevatron with the Main Injector upgrade, and at LEP II, along with the ongoing effort at SLAC to measure the electroweak mixing angle with similar accuracy using polarized electrons. The third means of testing the Standard Model and hunting for new physics involves studies of very rare, or even forbidden processes, including CP violation. At accelerators, such experiments require high intensity. Traditionally, the ¾ and K mesons have been used because of their relatively long lifetimes and copious production rates. Indeed, K decays presently provide our only evidence for CP violation. They also indirectly probe for new physics at the 200 TeV scale, a domain well beyond the reach of our highest energy accelerators. Ongoing experiments at BNL and Fermilab continue to push the search for rare K decays to unprecedented levels and probe for the origin of CP violation. Rare decays of the bottom and charm quarks as well as the tau lepton are starting to reach significant limits. For example, the CLEO collaboration at CESR recently found the first evidence for rare radiative b quark decays. Studies of B mesons (that contain b quarks) are particularly exciting because they open a new window to CP violation. Indeed, the standard model of CP violation predicts relatively large effects in B decays. Studies of these predictions will be possible at high-luminosity electron-positron B factories as well as at high-energy hadron colliders. Other examples of exotic phenomena that require high rates or massive detectors include neutrino oscillations from one type to another, non- standard CP violation searches and proton decay. Proton decay experiments are particularly impressive because they are our most direct window to physics at the grand unification mass scale. Indeed, present bounds on the proton lifetime already test physics at 1015 GeV. A joint Japan-U.S. experiment presently under construction at the Kamioka mine in Japan should push the proton lifetime search more than a factor of ten. Discovery of any reaction forbidden by the Standard Model would revolutionize physics and open up many new avenues of investigation. A well-balanced experimental program must include this three-pronged approach of high energy, high precision, and high intensity experiments, along with a variety of complementary non-accelerator initiatives. Only in that way can we hope to broaden our frontiers and increase our chances for discovery. What then are the most compelling questions and issues which currently drive our experimental program in high-energy physics and how can they be best addressed? As representative of the many exciting questions still to be answered by particle physics we propose the following list and briefly indicate with what facilities these questions may be answered. H. COMPELLING QUESTIONS Top Quark Physics: What is the precise value of the top quark mass? Why is it so heavy? What are its properties? The Fermilab Tevatron is currently the only accelerator in the world capable of directly exploring top quark physics. The LHC, when commissioned about a decade from now, will produce many millions of top pairs per year, making it a veritable top factory. An electron-positron collider with energy just beyond twice the top mass would provide a clean environment for measuring top quark properties. Electroweak Symmetry Breaking: Is there an elementary Higgs boson? Is it part of a supersymmetry scenario? How do we uncover the Higgs boson and explore its properties? Alternatively, is the electroweak symmetry broken dynamically? The LHC offers the opportunity to search for an elementary Higgs boson over the broad range of masses between 80 and 800 GeV. It can also explore extended Higgs models as suggested by supersymmetry. To understand all possible Higgs particles in this case, however, it would be important to also have access to a high-energy, high-luminosity electron-positron collider. There are scenarios in which the LHC discovery potential is limited and higher energy is required. Dynamical symmetry breaking would be such a case where the LHC's success would depend on the physics. In this case one might need a higher-energy hadron collider, with a broad-band discovery potential at least as great as that of the SSC. Fermion Masses, Mixings, and CP Violation: What is the underlying physics of fermion mass generation? Can we test standard-model predictions for quark mixing and CP violation? Whatever generates fermion masses apparently couples most strongly to heavy quarks, so it is very important to study the properties of the top and bottom quarks. K and B decays offer the best means of measuring the quark mixing parameters and refining our understanding of standard-model CP violation. Searches for very rare or even forbidden decays are a sensitive probe of the underlying physics of mass generation. With the BNL and Fermilab fixed-target programs, the Tevatron Collider, CESR at Cornell and the SLAC B-factory, the U.S. is well-positioned to study the physics of quark masses and the origin of CP violation. Neutrino Masses and Mixings: Do neutrinos have nonzero masses? Are they part of dark matter? Do neutrinos oscillate from one type to another? Neutrino masses and oscillations can be studied using accelerator, reactor, solar, or atmospheric neutrino sources. Exploring the full panoply of neutrino masses and mixings probably will require both long and short baseline neutrino oscillation experiments, as well as beta decay studies, necessitating both accelerator and underground facilities. QCD Dynamics: What is the structure of the proton? Can we better understand quark confinement? Are there exotic bound states? What is the precise value of the strong coupling constant? Full exploration of QCD and its properties requires studies of nucleon structure, high-energy scattering, and searches for new forms of matter. Monte Carlo computer simulations provide a powerful means of investigating QCD properties. The study of QCD dynamics overlaps strongly with the future nuclear physics programs at the Continuous Electron Beam Accelerator Facility (CEBAF) and the Relativistic Heavy Ion Collider (RHIC), while important studies of QCD structure functions are underway at the Hadron-Elektron-Ring-Anlage ( HERA) accelerator in Hamburg, Germany, as well as at SLAC and Fermilab. Electroweak Parameters and Quantum Corrections: What are the precise values of electroweak masses and couplings? Can we observe quantum loop effects? Present precision electroweak experiments range from low energy studies such as atomic parity violation and anomalous magnetic moments to Z studies at SLAC and CERN and W mass measurements at Fermilab. A high-energy, high-luminosity electron-positron collider can make precision measurements of the gauge-boson interactions and open a window to physics well beyond the energy of the machine. Supersymmetry: Is supersymmetry manifest at or below 1 TeV? If so, can we uncover the supersymmetric spectroscopy? Do supersymmetric particles contribute to the missing mass of the universe? The LHC is capable of finding signals for supersymmetry up to mass scales of about 1.5 TeV. Full exploration of the supersymmetric spectrum can be accomplished by an electron-positron collider with sufficient energy to pair-produce the supersymmetric particles. Underground searches for dark matter could also uncover such particles. Additional Gauge Bosons: Are there W' and Z' bosons? How can we find them? Direct production of W' or Z' bosons requires high-energy colliders. The LHC, for example, can search up to about 3 to 4 TeV, while a TeV electron-positron collider can indirectly probe similar scales and would provide constraints on the gauge symmetry of the new interaction. Low- energy experiments such as those on atomic parity violation and polarized electron scattering can also indirectly provide evidence for Z' bosons via deviations from Standard Model predictions. Non-Standard CP Violation: Is there CP violation beyond the Standard Model? Is it related to the matter-antimatter asymmetry of the universe? Searches for electric dipole moments and CP violating asymmetries such as the transverse muon polarization in K+ -> (pi^0)(mu^+)(nu) decay are examples of experiments that can be sensitive to CP violation beyond the Standard Model. A full program of CP violation studies in B and K decays will probe not only standard-model predictions, but could uncover, through precision studies, a new source of CP violation. Grand Unification: Can we confirm a grand unification of strong and electroweak interactions? Can we observe proton decay? Magnetic monopoles? Can we test supersymmetric unification? String theory? Super-Kamiokande offers an opportunity to push searches for proton decay more than an order of magnitude beyond current bounds, to within the range predicted by some supersymmetric theories. It is also capable of studying solar and atmospheric neutrinos and searching for magnetic monopoles from grand unification. Although the Standard Model provides an apparently complete description of particle physics at present energies, and answers many questions, it gives rise to many more. With a vigorous, broad-based program on the energy, intensity and precision frontiers, we can look forward to great progress during the coming years.