Studies of Quark-Gluon Physics

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The research program supported by this NSF award will focus on the study of the inner structure of the proton and neutron, or more generically, the nucleon. These basic particles are the building blocks for all the visible matter around us, making up the atomic nucleus. While it is well known that the proton and neutron are composed of very light quarks held in a bound state by the very strong color force, there is still little information available about the detailed structure of this bound state. In particular, we know very little of the distribution of the quark momentum in the three space dimensions we live in. Our present knowledge is essentially limited to one dimension, similar to a shadow profile picture of a person. This group will lead two experiments at Jefferson Laboratory in which beams of high energy electrons or photons are scattered from hydrogen and deuterium targets. In one experiment photons will interact with the quarks inside the nucleus and produce a map of where the quarks are relative to each other and their momentum. The other experiment will map out the scattering over a wide range of angles and energies that produce silhouette-like images of the nucleon. Beyond the scientific goals, the proposed experiment setup, data acquisition and analysis activities will continue to offer education and research experience to undergraduates, graduate students, and postdoctoral research associates. Working in the environment of an international collaboration based at a premier accelerator facility will broaden the perspective and interests of the group members and help to train the future STEM workforce of the country.This three-year research program will study the 3D momentum distribution of the quarks by examining the momentum of the sub-atomic particles they produce when ejected from the nucleon by a high-energy a photon, produced by the high intensity accelerator at the Thomas Jefferson National Accelerator Facility. For the first time, the nuclear physics community will study this quark ejection process using photons with distinct polarization, allowing for the study of effects related to the quark spin of ½. Single spin asymmetry measurements from semi-inclusive deep-inelastic-scattering (SIDIS) will be used to study the transverse momentum dependent parton distribution (TMD) and fragmentation functions of the nucleon. SIDIS measurements are becoming a flagship program both at the 12-GeV CEBAF at Jefferson Lab, and also the newly announced electron-ion-collider to be built at Brook Haven National Lab in the coming decade. The research activities will center around JLab experiments using SIDIS to measure the cross section of charged pions and kaons from hydrogen and deuterium targets. This work is partially motivated as a test of the factorization framework used for SIDIS processes where the TMD extraction from such measurements is based upon. Another experiment focuses on performing L-T separations of SIDIS charged pion production from hydrogen and deuterium targets to test whether R from SIDIS is the same as R from inclusive DIS, where R is the longitudinal to transverse cross section. The projected measurements will significantly improve the existing scarce data from prior experiments and advance our knowledge about the underlying production mechanism.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.