Rumsfeld hadrons

A missing link in the Standard Model is understanding hadrons, particles that respond to the strong interactions. In this article I summarise our knowledge in three classes, which are reminiscent of Donald Rumsfeld's (in)famous: 'things that we know we know; things that we know we don't know; and things that we don't know that we don't know'. Recent discoveries in particle physics concerning strongly interacting hadrons fall into those categories. It is of course the third category that is the most tantalising, but lessons from the first two may help resolve the third.