Measurements of quark momenta in three dimensions
Keith Griffioen (College of William and Mary)

With growing experimental evidence that only a small fraction of the spin of the proton comes from the spins of either quarks or gluons, the quest has begun to look for orbital angular momentum in the proton. Orbital angular momentum is a three-dimensional phenomenon, and therefore the relevant parton distributions must also depend on transverse quark momenta. I will discuss recent results in semi-inclusive deep-inelastic scattering that probe these new transverse-momentum-dependent parton distributions (TMDs) and summarize the prospects for new experiments.