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?

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