Speaker
Description
The fundamental properties of the nucleon reflect the internal dynamics of a composite system. In particular, the application of external electromagnetic fields induces a response in the constituent charge and current distributions in the nucleon, which is reflected in the electromagnetic polarizabilities. Nuclear Compton scattering is an ideal reaction for exploring these effects. While the first-order response to the incident photon is determined by the charge and magnetic moment, the second-order response is given by the static electric and magnetic polarizabilities $\alpha_N$ and $\beta_N$ and of the nucleon, which are structure-dependent. The next highest order effect is parameterized by the spin polarizabilities $\gamma$, which encode the stiffness of the nucleon spin.
High-precision measurements of these observables provide necessary benchmarks of theories describing hadron structure such as Chiral effective field theories and lattice QCD. Knowledge of the polarizabilities helps address the proton radius puzzle through their contributions to the Lamb shift in muonic hydrogen. In addition, the proton-neutron mass difference can be related to the difference in the respective magnetic polarizabilities $\beta_p-\beta_n$.
The High Intensity Gamma-Ray Source (HI$\gamma$S), housed at the Free Electron Laser Laboratory at Duke University, is able to produce intense, quasi-monoenergetic photon beams with nearly 100% circular or linear polarization. These beam characteristics, along with a cryogenic cooling system capable of creating liquid targets of hydrogen, deuterium, and helium, are essential components in performing such measurements. Using these tools, the Compton@HIGS collaboration aims to collect high-statistics Compton scattering data on a series of light nuclei at incident beam energies below pion threshold in order to provide stringent constraints on the scalar polarizabilities of the proton and neutron. To date, data has been collected on $^1$H, $^2$H, and $^4$He, and preparations are underway for a planned measurement on He. The status of these data, along with implications for the polarizabilities, will be discussed.
This research is supported in part by the U.S. Department of Energy under grant no. DE-SC0016581.
