EiR: Constraining the Symmetry Energy in Nuclear Equation of State for Neutron Rich Systems: A Study of Isovector Giant Quadrupole Resonance by Compton Scattering

The researchers propose a program in nuclear Compton scattering to carry out a series of measurements that will perform stringent tests of the predictions of Chiral Effective Field Theories (EFT), which provide a link between a low-energy description of hadrons and the underlying theory of strong interactions.

In the first of these proposed studies, the researchers will measure the quasi-free Compton scattering cross-section on deuterium to determine the electromagnetic (EM) polarizabilities (aE and aM), which describe the EM structure of the nucleon and are calculated in the framework of EFT. The polarizabilities play an important role in determining the EM contribution to the self-energy of the nucleons and are needed to precisely determine the overall neutron-proton mass difference. The proposed measurement will aim to reduce to one-half the present uncertainties in polarizabilities for the case of the neutron.

The second set of proposed Compton scattering measurements are at longer wavelengths, and lower-ray energies on nuclei will allow us to study the collective motion of protons and neutrons, such as the iso-vector giant quadrupole resonance (IVGQR). The IVGQR is a collective mode of the nucleus characterized by the out-of-phase oscillation of protons against neutrons. The restoring force is due to the symmetry energy term, which appears in the nuclear equation of state and is a key parameter for describing neutron-rich astrophysical systems such as neutron stars. This resonance can be observed via the interference with the tale of the giant dipole resonance (GDR) due to an interference term that gives rise to a fore/aft asymmetry in linearly polarized Compton scattering, from which the IVGQR parameters can be obtained.