The Experiment 

• Spokesmen: A. Klein (LANL) and D. Keller (UVA)
• Configuration: Transversely-Vertical Pointing Polarized Target
• Group: Fermilab SeaQuest Collaboration

Importance of Polarized Drell-Yan

The importance of studying transverse momentum dependent parton distributions (TMDs) and advancing the related theory of the nucleon spin is well summarized by the goals of the nuclear theory TMD Collaboration. The details of which can be found in the Proposalfor a topical collaboration in nuclear theory for the coordinated theoretical approach to transverse momentum dependent hadron structure in QCD. Nucleons are the fundamental building blocks of atomic nuclei and make up essentially all the visible matter in the universe. Our modern understanding of the strong interaction is based on Quantum Chromodynamics (QCD), in which the nucleon arises as a strongly interacting, relativistic bound state of quarks and gluons (referred to as partons). The nucleon is not static, but has complex internal structure, full of features that emerge from QCD dynamics and that are only now beginning to be revealed in modern experiments. Explaining the origin, the evolution, and the structure of the visible world is a central goal of nuclear physics. In order to do this, it is vital to understand the internal structure of the nucleon in terms of its partonic constituents. Over the last 50 years, since the first deep inelastic scattering experiments, there have been many advances in our understanding of the partonic structure of the nucleon, including its momentum and spin structure. The most significant progress has been in understanding the (one-dimensional) longitudinal momentum distributions of quarks and gluons encoded in the standard unpolarized collinear parton distribution functions (PDFs) in terms of the longitudinal momentum fraction of the nucleon carried by the parton and the resolution momentum scale of the external probe. However, there are still unknown and important aspects of the nucleon structure to be further explored, especially the ones related to the transverse momentum distribution of partons and its full 3-dimensional landscape. Several recent experiments have helped to uncover the first layers of transverse partonic structure of the proton but there is still essential helicity dependent information missing that requires polarized nucleon reaction so that the full picture of QCD dynamics and nucleon structure. Transverse momentum dependence in the parton distributions of the nucleon allows for the appearance of unsuppressed single spin azimuthal asymmetries, such as Sivers and Collins asymmetries. The measured spin azimuthal asymmetries will enable us to extract the TMD parton distributions through a global analysis of the experimental data, from which we can obtain an image of the nucleon in transverse, as well as in longitudinal momentum space.