The Physics of the Next Decade
Previous chapters have shown that the great successes of the past quarter of a century in elementary-particle physics are embodied in the Standard Model, which contains both the electroweak interaction and the strong force. The former, a unification of weak and electromagnetic forces, has been successfully tested at the 0. 1% level, and all carriers of the interaction have been observed and studied. The strong force is described by a fundamental theory whose predictions have been checked at both high-energy hadron and electron accelerators. Also, what are believed to be all of the elementary constituents of matter—three pairs of quarks and three pairs of leptons—have been observed.
However, it has been shown as well that the Standard Model is very likely an incomplete description of nature. Many particle properties are not predicted by the theory, including the masses of quarks and leptons and the way flavors are mixed by weak interactions. Physicists do not know whether the strong and electroweak interactions are different manifestations of a single force, and the origins of the breaking of electroweak symmetry, either by a Higgs particle or by some other mechanism, have not been discovered.
It is the need to answer these and other essential questions that drives the program of the next decade described in this chapter. The facilities in which this program will be carried out are distributed throughout the world as shown in Figure 5.1 and listed in Tables 6.1 and 6.2. They include an upgrade to the LEP electron-positron collider to run at energies up to 190 GeV; an upgrade to the Tevatron at Fermi National Accelerator Lab (FNAL) to produce higher rates of
proton-antiproton collisions at an energy of 2,000 GeV; three electron-positron machines—the Cornell Electron-positron Storage Ring (CESR) at Cornell University, the Positron-Electron Project II (PEP-II) at the Stanford Linear Accelerator Center (SLAC), and the High-Energy Accelerator Research Organization (KEK) B factory in Japan, operating at 10.6 GeV to study B mesons; and the Large Hadron Collider (LHC), which will come on-line in 2005 and collide protons at energies of 14,000 GeV. The new data will investigate the very underpinnings of the theory. Many questions should be answered by the experimental program of the next 10 years, and there will almost certainly be surprises that alter physicists' view of the world. In Chapter 7, the remaining questions that will have to be addressed at facilities not yet planned are described. The issues addressed here represent only the major thrusts of particle physics today. The list of topics in this chapter is not inclusive, and many other problems are being attacked that are important to a full understanding of the elementary constituents and forces in nature.
WHAT IS THE ORIGIN OF MASS?
As discussed in detail in Chapter 3, the origin of mass is the least understood part of the picture of elementary particles and forces. In the very tightly constructed theories of the strong, electromagnetic, and weak forces, the most natural state would be for all elementary particles discussed to be massless. Indeed, if nature consists of only the presently known particles, then none of the fundamental particles should have mass.
As discussed earlier, one mechanism to describe how particles can have mass requires that at least one more particle exist in nature, the Higgs boson. There could be only one such particle, or there could be several. This might be an elementary particle, or it might be a composite particle in the same way that a proton is made of quarks or an atom is made of electrons and a nucleus. Physicists may find, however, that there is no Higgs particle at all; in this case, some new physics must take its place and give particles mass.
In the simplest case, the Higgs boson is a single elementary particle. The Standard Model does not determine the mass of the Higgs boson, but various lines of reasoning based on empirical data suggest loose upper and lower bounds on its mass. Therefore, experiments now planned or running must incorporate strategies for detecting the Higgs boson within a wide range of possible mass values.
If the Higgs boson has a mass of less than about 95 GeV, it is most easily found in electron-positron annihilation experiments at the LEP collider at CERN (the European Laboratory for Particle Physics). The Tevatron proton-antiproton collider at Fermilab may also be sensitive to Higgs bosons in this range in the next decade. If the Higgs has a higher mass, its detection must await the 14 TeV (1012 electron volts) center-of-mass proton-proton collisions in the Large Had-
ron Collider (LHC) at CERN. Below about 800 GeV, it should be detected directly by ATLAS and CMS, the two large detectors being constructed at LHC. Above this value, it can still be detected indirectly because it will affect the rate for producing pairs of vector bosons. It is a tremendous challenge to design and build an experiment that will discover the Higgs if its mass is anywhere from 95 to more than 1,000 GeV. As the mass changes, the way in which the Higgs will reveal itself changes. Detailed studies have been done to ensure that the LHC experiments are sensitive to the entire range of Higgs mass. The LHC will either observe Higgs particles directly or rule out their existence and provide the first clues to the new physics that is responsible for mass.
WHY ARE THERE ENERGY SCALES THAT ARE SO VASTLY DIFFERENT?
If there is a fundamental theory that explains all the forces that have been observed, the natural energy for describing such a theory is at or near the scale of gravity (1016 to 1019 GeV). Chapter 3 has already noted that it is extremely difficult to produce a theory that relates physics at the scale of gravity with physics at the 100-GeV scale of the electroweak interaction. This is often called the naturalness or fine-tuning problem because, to achieve a viable theory, it is necessary for a parameter in the theory to have a value that must be specified to 34 significant digits!
Chapter 3 discusses two possible solutions to this problem that have been considered: supersymmetry and technicolor. The more thoroughly explored explanation is supersymmetry (SUSY), which postulates an as-yet unobserved ''partner" particle for each elementary particle. If supersymmetry is to solve the naturalness problem, the masses of supersymmetric particles must be less than approximately 1,000 GeV. This puts them in the range of existing or planned accelerators.
Because SUSY contains many new particles, each with a short lifetime and complex decay scheme, there is not a unique way to search for it. On the other hand, many supersymmetric processes produce very distinctive signatures. For example, if a pair of supersymmetric relatives of the carriers of the weak force is produced, three electrons or muons could be seen in a detector after decay and nothing else. The observation of such processes would be strong evidence for supersymmetry since no other known process would create such unusual events.
The search for supersymmetry is being carried out at the Fermilab Tevatron. CERN's LEP collider, and the Stanford Linear Collider (SLC) accelerator at SLAC. Thus far there is little compelling evidence for any new processes, including SUSY. This is not surprising since only a small fraction of the possible SUSY mass range is accessible at these accelerators. Between now and 1999, the energy of the LEP collider will increase so that more of the SUSY range will be explored. The increase in beam intensity at Fermilab's Tevatron.
beginning in 1999, will further extend the search for supersymmetric quarks and gluons, as well as partners to electrons, muons, and W and Z particles.
When operation begins at CERN's LHC collider in 2005, the available energy in accelerator collisions will increase by a factor of seven, and a huge range of SUSY masses will be accessible to experiment. If supersymmetry is discovered, a great deal will be learned at the LHC. Different types of SUSY particles will be seen, and many of their properties studied. However, it will not be possible to observe all of the SUSY particles and their decays.
If the LHC does not find supersymmetry, it is very unlikely that SUSY is the explanation of the naturalness puzzle. An alternative model to SUSY is a heretofore unobserved very strong force associated with a number of new, high-mass particles; this is the scheme that physicists call technicolor. This model can be distinguished from supersymmetry both by the types of new particles seen and by the way the new particles decay. The LHC and future accelerators will allow a thorough search for a number of types of such particles.
If neither supersymmetry nor a new strong force is observed, then the new high-energy accelerators will reveal the onset of unexpected phenomena. These will serve as signposts to the new physics that produces the enormous difference between the energy scale at which the electromagnetic and weak forces become unified and that above which all forces become unified.
WHAT IS THE ORIGIN OF MATTER-ANTIMATTER ASYMMETRY?
At the moment, studies of K decays offer the only clue to matter-antimatter asymmetry: K mesons are the only experimentally accessible system that has manifested such asymmetry. Physicists believe that if they further study CP violation in the K meson system and in the B meson system, which might exhibit a similar matter-antimatter asymmetry, they can learn something about this cosmological mystery of the universe.
Chapter 3 shows that the Standard Model with three generations might explain why CP violation appears in the K meson system; if this model is correct, similar phenomena should be evident in the B meson system. In fact, the B system should display a rich variety of CP violating asymmetries in many different decay modes, and some of them are predicted to be quite large. For example, the rates for a B meson and its antiparticle to decay to particular final states of interest are very low but may differ by 20% or even more. Furthermore, asymmetries that might be measured in the B system can be related directly to parameters describing quark flavor-changing transitions in the Standard Model, so with enough data, the B system can provide a definitive test of the theory. K and B experiments are in progress or planned at a variety of facilities—Cornell, SLAC, KEK, FNAL, DESY, Brookhaven, Frascati, and CERN—to see if this model is correct.
The experiments that will pursue this study have several common features. The effects are small, so large numbers of particles, either K mesons or B mesons, are necessary. The technology frontier of accelerators is being pushed, particularly in the case of B mesons, to create enough data for these studies. In addition, detectors that study CP violation have to be very sophisticated to reveal evidence for a small matter-antimatter asymmetry in a convincing fashion, and new state-of-the-art detectors are either coming on-line or being built expressly for this purpose. What is the hoped-for outcome of these studies? These experiments should definitively confirm or rule out the current understanding of this phenomenon. Also, if physicists can learn more about the asymmetry between matter and antimatter already evidenced in the K system, and perhaps show that B asymmetry fits into a similar pattern, they may be able to determine the mechanism for CP violation.
In addition to answering the question of where the observed CP violation comes from, physicists are struggling to understand why more CP violation is not observed. This is because there is a natural mechanism to produce CP violation in the strong interactions at a much larger level than that observed.
The symmetries of the Standard Model allow an additional interaction among gluons, which leads to the strong interactions that break CP symmetry. However, the observed breaking of CP symmetry is a tiny effect, so such a strong breaking must be absent. All interactions allowed by symmetries are expected to occur—why should this one be absent? This is known as the strong CP problem. One solution involves a new, hypothetical particle known as the axion-new symmetries dictate interactions for the axion, which lead to a cancellation of the CP violating effects in strong interactions.
Axions are predicted to have mass and to interact very weakly with matter; they are believed to have been produced in the very early universe, at the same time hadrons were produced. Searches for them are based on the fact that axions can be detected by their decay into two photons, and a team of physicists from U.S. universities and national laboratories is perfecting an experiment that will reach a significant limit in the next decade.
PATTERNS OF QUARK AND LEPTON MASSES AND TRANSITIONS
Chapter 3 listed 18 input parameters of the Standard Model, which are so far unexplained. There is currently particularly intense interest in measuring all of the parameters of quark flavor-changing interactions, which act to change one type of quark into another (e.g., an s quark to a d quark). The most active work is going on at the CESR accelerator at Cornell; at the soon-to-be-completed PEP-II at SLAC and the KEK B factory; and at the K decay experiments at Fermilab, Brookhaven, and CERN.
If neutrinos have no mass, the Standard Model weak interactions do not change lepton flavor. However, if neutrinos have small masses, then flavor-
changing processes are possible for leptons. There are two places in which lepton flavor-changing interactions might be important. First, they could result in leptons that exhibit the same kind of matter-antimatter asymmetry that quarks do. Second, they would allow for the lepton flavor oscillations discussed in Chapter 4, which could then be related to the observed deficit of neutrinos coming from the Sun. Over the next decade, a strong experimental program with several dedicated experiments using a variety of different techniques will search for evidence of neutrino mass and neutrino mixing in order to address the question of whether or not lepton flavors mix.
One technique takes advantage of the Sun as a copious source of neutrinos. with about 60 billion incident on each square centimeter of Earth every second. The effect of the matter in the Sun on the propagating neutrinos makes these searches sensitive to very small quantum mechanical mixing. The Sudbury Neutrino Observatory in Canada and the Superkamiokande experiment in Japan are sensitive to neutrinos from the Sun and will produce measurements that, in a few years, could definitively establish that neutrinos have mass and that lepton flavor-changing interactions occur.
Another approach to this problem uses the cosmic rays that are constantly raining down on Earth. When primary cosmic rays that have traveled great distances through space strike the nuclei of atoms in Earth's upper atmosphere, they produce showers of particles that eventually decay to neutrinos. One can calculate how many of each neutrino type are expected in a detector located deep underground. Two experiments, the Kamiokande and 1MB collaborations, observe that the ratio of events containing a muon to events containing an electron indicates that neutrinos change their flavor over the distances involved. The results are statistically significant, and their similarity is striking; however, they have to be confirmed. Progress will come soon from the Superkamiokande experiment, which significantly extends the sensitivity of its predecessor and definitively provides evidence for lepton flavor mixing.
The NuMI (Neutrinos at Main Injector project) experiments being built at FNAL, along with similar efforts in Japan and at CERN, have the potential to definitively resolve the issue of lepton flavor mixing. These experiments are accelerator-based experiments in which beams of neutrinos are produced in a controlled environment. Detectors hundreds of kilometers away from the source then search for direct evidence of neutrinos changing flavor, for example, from a muon neutrino into an electron neutrino or tau neutrino.
The ultimate goal is to explain the patterns of mixing of leptons and quarks, as well as their masses. In addition to precision measurements of the mixing of quarks (and perhaps leptons) in Standard Model weak interactions, it is very important to continue searching for evidence of flavor-changing interactions that do not occur or are highly suppressed in the Standard Model. A variety of dedicated experiments will come into operation over the next decade, which will look for proton decay, muon decay into an electron plus a photon, or CP viola-
tion in electrons or neutrons. A positive result from any of these experiments would be fantastically exciting because it would be evidence of physics beyond the Standard Model.
UNDERSTANDING THE STRONG FORCE
Although there is general agreement that the basic elements of the theory of the strong force, quantum chromodynamics (QCD), are correct, major unsolved problems remain. It has not yet been fully demonstrated that quarks are "confined." Already mentioned is the puzzle that QCD naturally accommodates a matter-antimatter asymmetry far beyond what has been observed. And many experimental manifestations of the strong force cannot yet be adequately predicted because of limited calculational tools.
Since QCD is involved wherever there are quarks and gluons, almost all experiments involving these constituents are relevant. Experiments at Fermilab are probing QCD at the very highest energies, where calculations are thought to be most reliable. Experiments at electron-positron colliders provide valuable inputs on the low-energy behavior of QCD. Several experiments, however, have been or soon will be constructed with an emphasis on exploring QCD, and there already have been some surprises.
The HERA facility, at the DESY laboratory in Hamburg, Germany, is the world's first and only electron-proton collider. HERA is the next giant step in a fruitful line of experiments in which leptons (electrons, muons, neutrinos) are scattered from protons. These experiments measured how quarks and gluons are configured inside the proton, a property that cannot yet be calculated accurately from basic QCD principles. By using the technology of modern colliders, HERA has taken lepton-nucleon scattering into new regimes, and new things are being learned about how the proton is constructed.
In addition to the three component quarks and the gluons that bind them in a proton, additional quarks and gluons can exist as brief "virtual" particles, whose existence can be detected in an energetic collision. Experiments have observed a remarkable increase in the numbers of these quarks and gluons that carry only a very tiny fraction of the proton energy. This trend cannot continue to increase without violating fundamental conservation laws, but how the increase becomes limited is unclear. It is hoped that the densities of quarks and gluons inside the proton will eventually be calculable from the basic principles of QCD in the same way that the densities of electrons in atoms can be calculated from the principles of electricity and quantum mechanics.
Another puzzle involves the proton's internal rotation, or spin. How the total proton spin is built up from the spins of constituent quarks and gluons must be a consequence of QCD. Naively, this spin would be formed from the principal quark constituents of the proton; surprisingly, experiments show these quarks to contribute only about 20% of the proton spin. Experimental studies of the
spin structure of the proton are being done at many laboratories, including SLAC, CERN, DESY, and the new nuclear physics facilities at Brookhaven and the Continuous Electron Beam Accelerator Facility (CEBAF). Again, the challenge is to someday calculate the spin properties of the proton from QCD.
The new facilities at Brookhaven and CEBAF will also be able to explore new phases of matter that result from the strong force. It is not understood under what conditions the neutrons and protons in the nucleus dissolve into their constituent quarks and gluons. Experiments of the next decade will start to address the nature of quark gluon confinement and try to understand how the early universe evolved from a quark gluon plasma to the nuclear matter that makes up most of the visible mass in the universe today.
In parallel with the important new experimental information that is being obtained, new calculational tools are being developed and specialized parallel processing computers are being designed and constructed to perform QCD calculations. These techniques have already provided important information about the theory, and there is promise for much more.
The examples cited above illustrate that many aspects of QCD are still unexplored, both experimentally and theoretically. The QCD force underlies the bound microscopic states (protons and other atomic nuclei) of the universe's known matter, as well as the wealth of particles observed in high-energy experiments. A combination of imaginative experimental exploration, theoretical insight, and innovative calculational tools is providing a deeper understanding of QCD. There may also be exciting consequences such as new types of matter, and experimental searches for exotic states will continue at high-energy and nuclear physics facilities throughout the decade.
ARE THERE UNEXPECTED PHENOMENA?
For each of the currently operating accelerators, as well as those under construction, a full experimental program is planned. The motivation of each experiment is quite explicit, usually focusing on the issues addressed in this chapter. However, experimenters are always on the lookout for the unexpected. Although answers to questions posed incrementally can increase knowledge, the observation of an unanticipated phenomenon can revolutionize the way physicists and eventually society think about the universe. The original electron scattering experiment revealing for the first time that the proton had an internal structure made of quarks; the discovery that a fundamental symmetry of nature, CP, was violated; and the discovery of the charm quark, which introduced a second generation of quarks—these are three spectacular examples of experiments that profoundly influenced the field with unanticipated results. In fact, many of the most important scientific discoveries have been unexpected. In experiments under way, as well as at facilities now being designed, physicists
are keenly aware of this lesson. Experimenters will continue to study their data for signs of the next surprise that could significantly alter views of nature.
The experiments beginning during the coming decade in this country and abroad will explore very important issues in elementary-particle physics. Researchers stand to learn a tremendous amount about the fundamental differences between matter and antimatter, enough so that the current understanding of CP violation should be either confirmed or refuted. Experiments should be able to establish generation-changing interactions among the leptons, and if so, this has important conclusions for the ultimate unification of all elementary particles. Data on the strong force will give new insights into QCD and could establish an important new phase of matter. The Higgs boson and/or some of the expected states in supersymmetry should be discovered, if they exist. If not, then physicists expect compelling evidence for a new force in nature.
Even so, one wonders what questions will remain after the coming decade of experimentation: This subject is treated after the accelerators and detectors used in particle physics are described in Chapter 6.