Polarized Drell-Yan Measurements with the Fermilab Main Injector
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Abstract
The proton is a unique bound state, unlike any other yet confronted by physics. We know its constituents, quarks and gluons, and we have a theory, QCD, to describe the strong force that binds these constituents together, but two key features make it a baffling system that defies intuition: the confining property of the strong force, and the relativistic nature of the system. Real understanding of the proton can only be claimed when two goals are accomplished: precise calculations of its properties from first principles, and the development of a meaningful picture that well approximates the system’s dominant behavior, likely via effective degrees of freedom. The excitement and challenge of the quest for this intuitive picture is well illustrated by the ongoing research into the spin structure of the proton, and in particular, into the contribution from quark orbital angular momentum (OAM). As experiment provides new clues about the motion of the up, down, and sea quarks, theory continues to make progress in the interpretation of the data, and to confront fundamental questions concerning the very definition of L in this context. Yet crucial pieces are still missing on the experimental side. This proposal aims to fill in such a piece: the lack of any spin-dependent data on one of the most powerful probes of hadronic substructure available, the Drell-Yan process.