Structure of the Hadron
Electromagnetic reactions on light nuclei, such as Compton scattering on proton, deuteron, and 3He targets, provide a unique oppertunity to perform stringent tests of the predictions of Chiral effective field theories (xEFT). These theories provide a link between a low-energy description of hadrons and Quantum Chromodynamics (QCD). An important aspect of this description is the electromagnetic properties of the nucleon. In addition to the electric and magnetic polarizabilities that parameterize the stiffness of the internal electric charge and current distributions in nucleons, spin-polarizabilities are a measure of the spin-stiffness of the nucleons. These quantities can be measured via direct scattering of photons from nucleons, but precision measurements are required to test the models. The HIGS facility is identified in the 2007 Long Range Plan (LRP) for Nuclear Sciences to be ideally suited for the next generation of experiments designed to measure the electromagnetic and spin polarizabilities.
The experiments at HIGS will test the GDH Sum Rule for the Deuteron and 3He, measure the EM-Static Polarizabilities and the Spin Polarizabilities, and once beams of energy 158 MeV are available embark on a program to carry out measurements associated with photopion production.
With the availability of both linearly and circularly polarized beams between 2 and 100 MeV, the Blowfish neutron detector array, and the HIGS NaI Detector Array (HINDA), the GDH sum rule measurements on the deuteron will start in the Fall of 2012 followed by the measurement of proton and neutron electromagnetic polarizabilities by means of Compton scattering. These measurements will be followed by the proton spin polarizability measurements in year 2013 and beyond via Compton scattering from a scintillating polarized target, followed by neutron spin polarizability measurements. The program to investigate the structure of the hadrons using Compton scattering at HIGS, as well as the GDH sum rule measurements on the deuteron, is well-aligned with the vision of the nuclear physics community as outlined in the 2007 LRP.