DEPARTMENT COLLOQUIUM | In Search of L: Orbital Motion in the Land of Quarks

Speaker: Naomi Makins (University of Illinois, Urbana-Champaign)

The proton is a puzzling creation made of quarks and gluons. The HERMES experiment in Hamburg, Germany and others in recent years have focused on a particular quandary: ``the Spin Puzzle''. It is now well known that the spins of the quarks account for very little of the proton's total angular momentum ... so where is the rest? The most mysterious and least accessible source is L: orbital angular momentum. L is a familiar friend in the study of bound systems such as the atom and the nucleus, but not so for the proton. Are the quarks in the proton in an s-shell like the electrons of the hydrogen ground state ... or is that even a valid question in the strange world of relativistic quantum mechanics? New data from HERMES will be presented that show glimpses of quarks in orbital motion, both within the proton and in the process of hadron formation in high-energy collisions. A summary will be presented of the ongoing theoretical efforts to understand these data and to determine how best to measure -- and think about -- L in the land of quarks.