Exotica

The sudden appearance of narrow, prominent exotic baryons has re-invigorated light hadron spectroscopy. At present the experimental situation is confused; so is theory. The recent discoveries are striking. So too is the complete absence of exotic mesons, and, except for the recent discoveries, of exotic baryons as well. Whether or not the new states are confirmed, the way we look at complicated states of confined quarks and gluons has changed. Perhaps the most lasting result, and the one emphasized in these notes, is a new appreciation for the role of diquark correlations in QCD.