Exotic Particles - 57 Years and Counting

Abstract: For nearly 40 years, the only known baryons could be classified as states of three quarks, while mesons could all be explained as quark-antiquark systems. There was no reason why one could add one or more quark-antiquark pairs, leading to (for example) tetraquark mesons or pentaquark baryons. That changed with the discovery in 2003 of a particle known as the X(3872), composed of a charmed quark, a charmed antiquark, an up quark, and an up antiquark. The key was the presence of two heavy (charmed) quarks, leading to additional stability. This talk will trace the subsequent spectroscopy of particles containing two or more heavy quarks, and how it affects our understanding of the strong interactions.