Our understanding of the ultimate structure of matter has advanced greatly in the last few years. It was nearly twenty years ago that Murray Gell‐Mann and George Zweig gave us the key to much of this understanding with their revolutionary proposals that protons,neutrons and all other strongly interacting particles—the hadrons—are made of quarks, a theretofore unobserved kind of particle. Over the last ten years, this proposal has become firmly established even though we have not observed free quarks directly. More recent research has found that forces between quarks are extremely simple and universal—the same for all types of quarks—and that these forces explain why free quarks cannot be seen. In this article we will look at hadron spectroscopy, which has been one of the main venues for this great progress.
The numerous mesons and baryons are remarkably well explained as states of simple quark “atoms” that obey the same principles as ordinary atoms.