OUTLOOK

Elementary particle physics is a highly developed field. Over 2,000 technically sophisticated Ph.D. scientists in the United States alone work in elementary particle physics. Many features characterize the status, activities, plans, and constraints of the field as it exists.

Scientists have an elaborate and detailed understanding of the constituents and the relationships between the strong, electromagnetic, and weak forces of nature: quarks and leptons interacting through gauge forces. This is embodied in the Standard Model of particle physics, a structure that stands as a major achievement of twentieth-century science. In addition, it is known that new phenomena beyond the Standard Model must exist, most likely including a massive boson (the Higgs particle) that gives rise to the different masses of the particles.

The best approach to this new physics is at the energy frontier, which means experimenting at the highest-energy colliding beam facilities that can be constructed. Experiments and accelerator technology are highly sophisticated. These technologies include very high-speed electronics, real-time processing, and data transmission at very high bandwidth, as well as accelerator beams of unprecedented intensity and stability. New accelerators are in the multibillion-dollar category and take well beyond a decade to develop. Observational cosmology has also revealed phenomena beyond the Standard Model, such as dark matter, dark energy, and inflation.

The benefits of discoveries in particle physics are influential beyond contributions to the field. Many other fields of physics use discoveries made by particle physics. Atomic physics and certain areas of astrophysics are among those fields. In addition, many by-products of work in elementary particle physics find their way into the public sector, such as medical imaging and the World Wide Web.

Many theorists in particle physics work on string theory (where particles are viewed as incredibly tiny vibrating loops), which holds the promise of incorporating gravity among the other forces. However, the fear is that string theory may be untestable because, for consistency, it requires several extra spatial dimensions that might be so small that laboratory-based studies will never be sensitive to them.

Particle physicists have traditionally attempted to isolate the fundamental constituents in nature and study their interactions. Indeed, it remains a major thrust of the field. It is most enlightening to know what particles exist, what symmetry patterns they obey, and how to accelerate them to very high energies and perform controlled experiments with them. This field has continually revealed important new phenomena and states of nature, and this knowledge provides richer and more complete picture of the world.

Over the past forty years, many of the most important advances resulted from accelerator-based experiments. During the 1960s, the violation of CP symmetry was found: for the first time, the asymmetric decay of the neutral K meson was observed, and it distinguished matter from antimatter. Immediately this led to the question of whether the violation was connected to the matter-antimatter asymmetry in the universe. This is still a mystery. Through the scattering of electrons from protons, pointlike constituents within the proton, later identified as the quarks, were discovered. Both of these discoveries were unexpected, and they had a very significant impact on the thinking and on the development of the field.

In the 1970s came the discovery of two new quarks, charm and bottom, and a new lepton, the tau. These were striking and also largely unexpected discoveries. In addition, during this same timeframe the gluon, the force carrier of the strong interaction, was isolated. In the 1980s the force carriers of the weak interaction, the W and Z, were finally discovered; they had been sought for decades, but by the 1980s theory was able to predict their masses, and experiments found them in just the right place. Much was also learned about particles containing the bottom quark, and this allowed the determination of quark couplings with even more precision, firming up the Standard Model of particle physics.

The 1990s saw the observation of the top quark (the last one in the Standard Model) with a mass much larger than that of any of the other quarks. This brings home the mystery of the vastly different masses of the fundamental constituents of matter; however, the mass was in the range allowed by consistency with all other measurements and the Standard Model. Physicists were able to copiously produce and accurately study the decays of the Z boson, pinning down the interactions of the quarks and leptons with remarkable precision. In quark decay, a second manifestation of CP violation was also clearly observed. This came from very difficult measurements, and it helped verify the Standard Model means of accommodating CP nonconservation.

At the onset of the twenty-first century, CP violation with B mesons was observed; these measurements required the construction of what are commonly called B factories to makeB 's in sufficient quantities. The results were expected in the Standard Model.

Particle Physics is at a crossroad. Tremendous progress has been made over the past few decades: the questions now being asking could not have been conceived of earlier. Since so much is known about the constituents of matter and their patterns, the focus has shifted to explaining their origin. The forces and their relative strengths are known at low energies, and the possibility exists that at very high energies, there is only a single force. However, one must be open to surprises. The major activities today in accelerator-based particle physics are the search for the Higgs boson, the search for supersymmetric particles, the study of CP violation, and the study of neutrino oscillations.

Particle physics is strongly linked to cosmology. From observations of the cosmos, the fact that our universe has been expanding and cooling since the Big Bang has been learned. At accelerators, particle interactions are studied that occurred in the universe when it was less than a microsecond old. And, the understanding of particle physics has had great impact on cosmology. For example, from the knowledge of how forces change with particle energies, scientists are boldly able to extrapolate the forces studied at scales of a few hundred giga electron volts to much earlier epochs in the Big Bang. Such an approach is valid but must be taken with caution. Particle physics thus enables cosmology, and cosmology demonstrates the promise of new particle physics.

There are three major puzzles in cosmology that have a direct bearing on particle physics. The first is the existence of dark matter. Dark matter was discovered by simply using Newtonian gravity to analyze the motion of stars in galaxies and galaxies in clusters of galaxies. This analysis makes it clear that there is more than meets the eye, that is, the dominant matter that holds galaxies and galaxy clusters together is not luminous. It is also believed that this nonluminous matter is not the stuff that humans are made of or that has been studied in the laboratory. This belief is supported by the successful theory of how the light elements were created during the first few minutes of the universe. The prediction for the relative fractions of H, He, Li, and D holds together only if the density of such ordinary matter is much smaller than the density of all the matter, which can be determined by a number of independent techniques.

Particle physics provides good candidates for particles that could comprise this dark matter, particles that were created very early in the history of the universe and have survived to the present day. The extension of the Standard Model known as supersymmetry posits a new set of particles that mirrors known particles but with different quantum numbers. The lightest of these is likely stable, that is, it has a lifetime exceeding that of the universe, and if it has a mass of approximately 100 GeV, it could comprise the dark matter and be detectible by a number of means. The search for such particles is a major objective of the field.

Another puzzle from cosmology that directly impacts particle physics is that of dark energy, a dominant component of the universe whose constitution is completely unknown and which is apparently causing the universe's expansion to accelerate. This could result from some residual energy in the vacuum, or it might be some new dynamical field. There are new observations of the cosmos that may help unravel this phenomenon, but how to address this puzzle within the context of particle physics is not yet known. The third deep puzzle beyond the Standard Model is the apparent acausal nature of the universe. If one looks at the sky with detectors sensitive to microwave radiation, one sees the radiation left over from the birth of the universe, the so-called cosmic microwave background radiation. This radiation split off from the hot soup of photons, electrons, and protons less than a million years after the birth of the universe and has been traveling unperturbed, as it cools, to the present day. The surprise is that everywhere one looks one sees, to very high precision, the same temperature for this radiation, about 3° absolute. The puzzle is that regions far from each other (just a few degrees on the sky) have never been in causal contact with each other since the birth of the universe. Accelerated expansion very early in the history of the universe has been postulated to explain why the universe is so extremely smooth: this theory says that everything was initially in causal contact, but by expansion faster than the speed of light, these regions that had come into thermal equilibrium with each other disappeared from each other's causally connected regions. Further tests of cosmic microwave background lend more support to this notion, called inflation, and definitive tests are in the planning stage.

There is another connection: it is now known, as a result of some very incisive experiments studying neutrinos emitted from the Sun and as by-products of the interactions of cosmic rays in the atmosphere, that neutrinos oscillate. They have a rich structure not unlike what has been known for a long time about the weak interactions of the quarks. Experiments under construction at accelerators will further characterize this new system.

To get to every smaller distance scale requires, through a basic relation in quantum mechanics, ever higher energies. This means larger, more costly devices and inevitably international machines. The Large Hadron Collider (LHC) in Geneva, Switzerland, will be the next major instrument in the field. It has an energy about seven times greater than the Tevatron currently operating at Fermilab in Batavia, Illinois, and can make collisions more than ten times more frequently. A wealth of new phenomena will very likely be discovered and explored at that facility, which is scheduled to begin operations around the year 2007. The Higgs boson and several of the supersymmetric partners are prime candidates for discovery.

Beyond the LHC, a worldwide consensus is developing for a facility that collides electrons and positrons at an energy of about 1,000 billion electron volts. This machine could perform precision studies of the Higgs boson and of some supersymmetric states within its energy range. It is technically very challenging and will require the intellectual and financial support of the United States, Europe, and Japan. It is being planned from the start as an international facility.

There is another class of experiments that deals with focused in-depth studies. These experiments tend to be smaller and more limited in scope than those searches discussed above. These include studies of particle decays in beams. Here one can create a well-defined beam of a known particle, such as a K meson, and build a detector to study a particular decay that is expected to occur rarely. These experiments could not be performed in the collider environment: to reach needed sensitivity, it requires 10,000 or more times the number of protons that can be obatined in collider experiments. Another example of a special purpose experiment is building a storage ring to capture muons and make a precison measurement of the structure of the muon. Recently such an experiment was performed at the Brookhaven National Laboratory (BNL) with intriguing results. Dedicated experiments are mounted to study and search for the violation of certain fundamental symmetries: How do matter and antimatter behave differently? How does nature distinguish left from right?

As the scale of experiments grew ever larger and as there were fewer focused experiments, the field experienced some changes. The physics that the largest facilities reveal cannot be studied any other way: it simply takes large accelerators and large collaborations to explore that kind of science. However, as the domain of particle physics is expanding, there are other opportunities. For example, smaller efforts in observational cosmology address fundamental science, as do experiments performed deep under-ground to look for proton decay or to detect dark matter particles. There are many new initiatives to explore this science, and the prospects for new discovery are bright.