Compton Scattering and Nucleon Polarizabilities at HIγS

Abstract

The electric and magnetic polarizabilities ($\alpha_E$ and $\beta_M$) are fundamental quantities encoding the internal structure of the nucleon. They characterize the response of the nucleon to an external electromagnetic field and can be probed via the Compton scattering process. In recent decades, nucleon polarizabilities have been extracted from low-energy Compton scattering data using dispersion relations and chiral effective field theories ($\chi$EFTs). However, due to the difficulties in experimental measurements, $\alpha_E$ and $\beta_M$ of the nucleon, particularly of the neutron, are still not precisely determined. Furthermore, lattice QCD has also been promising in making predictions on nucleon polarizabilities. Nowadays, high precision data are badly needed to benchmark theoretical calculations and to enable accurate extractions of nucleon polarizabilities. In aim of providing the most precise determination of $\alpha_E$ and $\beta_M$ of the nucleon, Compton scattering experiments have been performed at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility located at Duke University. In the experiments, intense, quasi-monoenergetic and nearly 100\% circularly or linearly polarized $\gamma$-ray beams were incident on a liquid $^4$He or a liquid hydrogen target. The scattered photons were detected by eight NaI(Tl) detectors placed at various scattering angles. Compton scattering cross sections of the proton have been measured with a circularly polarized photon beam at about 81\,MeV and a linearly polarized photon beam at about 83\,MeV to extract $\alpha_E$ and $\beta_M$ of the proton using the $\chi$EFT frameworks. In addition, differential cross sections of elastic Compton scattering from $^4$He at about 81\,MeV have been extracted with high precision to provide a complementary approach to determine the neutron polarizabilities. The $^4$He results strongly motivate the development of theoretical calculations of Compton scattering from $^4$He.