Nuclear Physics Seminar

At leading twist, a complete picture of the one-dimensional momentum structure of the nucleon requires knowledge of three types of parton distribution functions (PDFs): the unpolarized PDFs; the helicity PDFs; and the transversity PDF, related to the transverse polarization of quarks within a transversely polarized nucleon. Current global extractions of transversity are limited by the kinematic reach of existing semi-inclusive deep-inelastic scattering (SIDIS) experiments. Beyond the open questions of one-dimensional nucleon structure, myriad opportunities abound in exploring the multi-dimensional structure of the nucleon. A step toward this goal is to investigate the nature of the transverse momentum dependence (TMD) of nucleon parton densities and their relation to nucleon spin polarization. The STAR experiment at RHIC proposes to investigate these and other spin-related phenomena through the interaction of high-energy collisions between spin-polarized protons. In preliminary results from data collected in 2011 at $\sqrt{s}=500$ GeV and in 2012 at $\sqrt{s}=200$ GeV, STAR has observed the first non-zero spin asymmetries due to the effects of transversity in proton-proton collisions. Studying these effects through both jet+hadron and di-hadron production channels and across a range of collision energy yields the potential not only to extend understanding of transversity beyond the current kinematic reach but also to address longstanding theoretical questions concerning the universality and evolution of transversity and polarized fragmentation functions. From the 2011 dataset STAR has also released the first preliminary measurements sensitive to the Sivers TMD PDF in weak-boson production. Weak boson production provides an ideal tool for isolating the unconstrained sea-quark Sivers PDF and may provide sensitivity to the expected modified universality of the Sivers PDF when compared to SIDIS. These exploratory measurements pave the way for future higher precision inv