FIGURE 2.1

Schematic illustration of the substructure of a proton or neutron (left) and of a meson (right), according to the theory of quantum chromodynamics (QCD). Among the constituents confined within the nucleon are three point-like valence quarks, shown here as heavy colored dots, which interact by exchanging gluons shown as spring-like lines. Instead of three quarks, the meson has one quark and one antiquark (dot with white center) as valence constituents. The strong interactions induce additional gluons and a "sea" of virtual quark-antiquark pairs, shown as smaller, fainter dots. Quarks are labeled "q" and antiquarks "¯q". The colors of the constituents represent their intrinsic strong charges, the source of their participation in QCD interactions. Note that quarks appear only in groups of three (with different colors) or in quark-antiquark pairs. The nature of the strong interactions inside a nucleon and the relative contributions of various types of valence and sea quarks, as well as gluons, to the nucleon's overall properties have become major topics of research in nuclear physics.