Multiquark States

Why do we see certain types of strongly interacting elementary particles and not others? [his question was posed more than 50 years ago in the context of the quark model. Al. Cell-Mann and C. Zweig proposed that the known mesons were 0 and baryons qqq, with the quarks known at the time, u (up), d (down), and s (strange), having charges or 2/3, 1 /3, and 1 /3, respectively. Mesons and baryons would then have integral charges. Mesons such as gqiH and baryon such as 0061 would also have integral charges. Why weren't they seen? They have now beets 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.