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theory of quark confinement within hadrons. The next decade should bring enormous progress toward a meaningful confrontation of theory and experiment.
The new facilities at CEBAF and RHIC, together with continuing opportunities for selected experiments at high-energy laboratories, will dramatically improve empirical knowledge of nucleon structure. Ongoing investigations of the meson spectrum should confirm or disprove the discovery of hadrons whose primary constituents are gluons, thereby testing a prediction directly tied to the confinement mechanism of QCD. Measurements of pion interaction probabilities at low energies will reveal whether QCD leads to a self-consistent treatment of the observed violations of one of the theory's basic symmetries. This experimental progress will exploit a wide variety of accelerator facilities and state-of-the-art instrumentation. Still, the quest for ultimate understanding of the excess baggage carried by quarks within nucleons is likely to require upgrades to currently available facilities. For example, continuous electron beams of higher energy or a spin-polarized, electron-proton collider may be needed to probe correlated behavior among pairs of quarks, or spin contributions from the abundant gluons, which each carry less than a few percent of the nucleon's momentum.
In parallel with experimental progress, advances in computer performance and in theoretical techniques will fuel more quantitatively credible numerical solutions of QCD on a space-time lattice. Direct comparisons of experimental results to quantitative QCD predictions for aspects of meson and nucleon structure will have to form one of the primary testing grounds for the theory, just as the validity of quantum electrodynamics has been established in good part by quantitative accounts for observed details of atomic energy levels. On the other hand, it is unlikely that numerical solutions of QCD will be viable within a decade for systems containing more than a single nucleon. Further development of more phenomenological approaches, inspired by QCD but also guided by experimental results, will be needed to establish more firmly the QCD basis for the force between nucleons, or the structure of hadrons in dense nuclear matter.
Investigations of the evolution of hadron structure with the density and temperature of surrounding nuclear matter are just beginning. Resolution of some of the relevant issues, such as QCD effects on the passage of fast hadrons through nuclear matter, may well require accelerator facilities with beam energies and characteristics beyond those now available. Other questions, such as the occurrence of pion or kaon condensation or of a transition from nuclear to quark matter at very high densities, may only be settled by combining substantial extrapolations from laboratory experiments with successful models of stellar behavior, constrained by astronomical observations. In addressing these issues, nuclear physicists are attempting to build an essential bridge, linking the fundamental theory of nature's strongest force to the makeup of the densest objects in the cosmos, and spanning the microscopic structure of the atomic nuclei that constitute nearly all of the mass we observe around us.