Compton scattering from 4He at the TUNL HIγS facility

Prof. Gao's research focuses on understanding the structure of the nucleon in terms of quark and gluon degrees of freedom of Quantum Chromodynamics (QCD), search for QCD exotics, and fundamental symmetry studies at low energy to search for new physics beyond the Standard Model of electroweak interactions. Most recently, her group's studies of the structure of the nucleon have been focusing on a precision measurement of the proton (see her group's 2019 Nature paper on this topic) and deuteron charge radii to elucidate on the proton and the deuteron charge radius puzzles, and on imaging the three-dimensional structure of the nucleon in momentum space through the extraction of transverse momentum dependent parton distribution functions (TMDs), employing polarized semi-inclusive deep inelastic scattering processes. The nucleon tomography provided by TMDs will uncover the rich QCD dynamics, and provide quantitative information about the quark orbital angular momentum contribution to the proton spin. TMDs will also provide information on fundamental quantities such as the tensor charge of the nucleon, a quantity not only important for testing lattice QCD predictions, but also important for searches of new physics beyond the Standard Model together with the next generation of nucleon electric dipole moment experiments. Her group is playing leading roles in the Solenoidal Large Intensity Device (SoLID) project at Jefferson Lab, a high profile program which will make major impact on TMD physics, proton mass puzzle through precision measurement of J/psi production near threshold, and search for new physics beyond the Standard Model using parity-violating deep inelastic scattering. Most of her work utilizes the novel experimental technique of scattering polarized electrons or photons from polarized gas targets. Her group has built a number of state-of-the-art polarized gas targets including H/D internal gas target and a high-pressure polarized ³He target for photon experiments using the High Intensity Gamma Source (HIGS) facility at the Duke Free Electron Laser Laboratory (DFELL). Her research is being carried out mostly at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, and the HIGS facility at DFELL.

Professor Howell’s research is in the area of experimental nuclear physics with emphasis on the quantum chromodynamics (QCD) description of low-energy nuclear phenomena, including structure properties of nucleons and nuclei and reaction dynamics in few-nucleon systems.   The macroscopic properties of nucleon structure and the residual strong nuclear force between neutrons and protons in nuclei emerge from QCD at distances where the color interactions between quarks and gluons are strong.  However, the details of the mechanisms that generate the strong nuclear force are not well understood.   Effective field theories (EFT) and Lattice QCD calculations provide theoretical frames that connect low-energy nuclear phenomena to QCD.  Professor Howell and collaborators are conducting experiments on few-nucleon systems that test predictions of ab-initio theory calculations for the purpose of providing insight about the QCD descriptions of low-energy nucleon interactions and structure.  His current projects include measurements of the electromagnetic and spin-dependent structure properties of nucleons via Compton scattering on the proton and few-nucleon systems and studies of two- and three-nucleon interactions using few-nucleon reactions induced by photons and neutrons.  In the coming years, a focus will be on investigating the neutron-neutron interaction in reactions and inside nuclei.  In addition, his work includes applications of nuclear physics to national nuclear security, medical isotope production, and plant biology. Most of his research is carried out at the High Intensity Gamma-ray Source and the tandem laboratory at TUNL.