Multiquark States

Why do we see certain types of strongly interacting elementary particles and not others? This question was posed more than 50 years ago in the context of the quark model. M. Gell-Mann and G. Zweig proposed that the known mesons were [Formula: see text] and baryons qqq, with the quarks known at the time, u (up), d (down), and s (strange), having charges of 2/3, -1/3, and -1/3, respectively. Mesons and baryons would then have integral charges. Mesons such as [Formula: see text] and baryons such as [Formula: see text] would also have integral charges. Why weren't they seen? They have now been seen, but only with additional heavy quarks and under conditions that tell us a lot about the strong interactions and how they manifest themselves. This review describes recent progress in our understanding of such exotic mesons and baryons.