Session PB: Invited Session: The Still Puzzling World of Up and Down Quarks

Partonic transverse distributions of up and down quarks

The quark-gluon dynamics manifests itself in a set of non-perturbative functions describing all possible spin-spin and spin-orbit correlations. The Transverse Momentum Dependent parton distributions (TMDs) and Generalized Parton Distributions (GPDs) carry information not only on the longitudinal but also on the transverse momentum and position of partons, providing rich and direct information on the orbital motion of quarks. Single and Dihadron semi-inclusive and hard exclusive production, both in current and target fragmentation regions, provide a variety of spin and azimuthal angle dependent observables, sensitive to the dynamics of quark-gluon interactions. Studies of the 3D PDFs are currently driving the upgrades of several existing facilities (JLab, COMPASS and RHIC), and the design and construction of new facilities worldwide (EIC, FAIR, and JPARC). In this talk, we present an overview of the current status and some future measurements of the orbital structure of nucleons at Jefferson Lab.   

Extracting the flavor dependence of the polarized sea quarks

The measurement of single spin asymmetry $A_L$ of W bosons in longitudinally polarized pp collisions at RHIC provides an unique probe for the flavor separation of the nucleon spin structure, especially the polarization of sea quarks. The recent $A_L$ results of W bosons from RHIC via leptonic decay provided significant new constraints on the helicity distributions of light sea quarks in addition to constraints from the semi-inclusive deep inelastic scattering data, which also indicated a symmetry breaking between anti-u and anti-d quark polarization in the nucleon. In 2013 the RHIC/STAR experiment collected a proton-proton collision data sample about 3 times larger than the previous data sample. The newest results of $A_L$ analysis from RHIC W program and the impact on sea quark polarization will be discussed.   

The (Sea)Quest to map anti-up and anti-down quarks

The SeaQuest experiment at Fermilab continues a series of Drell-Yan measurements to explore the antiquark content of the nucleon and to study the modifications to nucleon structure when the nucleon is embedded into a nuclei. To extend existing measurements to larger values of Bjorken-x, a 120 GeV proton beam extracted from Fermilab's Main Injector is used, resulting in 50 times more luminosity than previous experiments and enabling access to values of x up to 0.9. One of the key physics goals of the SeaQuest collaboration is the exploration of the origin of the intrinsic nucleon sea. The scale dependence observed in high-energy scattering experiments reflects the quantum fluctuations predicted by QCD. A sea of virtual gluons arises in the nucleon; these gluons radiate other gluons or pairs of quarks and antiquarks. Gluon splitting, e.g. into an u anti-u quark pair or a d anti-d quark pair, is a perturbative QCD process and flavor symmetric. However, there is clear experimental evidence that the nucleon sea is not flavor symmetric. The distributions for anti-d quarks and anti-u quarks differ by up to 50{\%} and suggest a substantial role of non-perturbative QCD processes in the creation of the nucleon sea. Measuring the ratio of the anti-d quark and anti-u quark distributions with high accuracy and within a large x-range is the key measurement of the SeaQuest experiment. The SeaQuest results will help to identify effective theories that can describe the intrinsic nucleon sea and help to explore its origin.