Exotica Robert Jaffe, MIT

For years, high energy physicists have struggled with the problem of exotics in quantum chromodynamics (QCD), the theory of the strong interactions. Before 2003 all hadrons could be classified as qq* (mesons) or qqq (baryons), except of course for nuclei, which are loosely bound collections of baryons. Suddenly experimenters are reporting evidence for several long-lived exotic baryons, made of qqqqq*. The experiments are controversial, but if they are right, they give us insight into the inner workings of QCD, and reveal the importance of quark-quark correlations in the spectrum and in low energy interactions.