The Mystery of Nucleon Spin; July 1999; Scientific American Magazine; by Rith, Schäfer; 6 Page(s)

Protons and neutrons were among the first subatomic particles discovered this century. They reside in the nuclei of atoms and are hence known as nucleons; they make up more than 99.9 percent of the matter in the everyday world around us, including this page and you yourself. (The other 0. 1 percent is electrons.) Eighty years of experimental study and theoretical analysis have taught us much about the nucleons, yet certain of their fundamental properties still hold puzzles and surprises. For the past decade, physicists have labored to resolve a particular quandary known as the spin crisis.

This crisis emerged from the highly successful quark model of subatomic particles. Theorists developed this model as a neat, compact description of the myriad of new particles detected during the 1950s and 1960s, as well as of old familiars such as the proton and neutron. The properties and interactions of the particle zoo fell into patterns that could be explained by their being made of just three species of quark, called up, down and strange.