The Generalized GDH Sum Rule: Measuring the Spin Structure of $^3$He and the Neutron using Nearly Real Photons

The generalized Gerasimov-Drell-Hearn (GDH) sum rule is a versatile and important tool for studying nucleon spin structure over the entire $Q^2$ range. Originally derived for real photon absorption ($Q^2$=0), the GDH sum rule has been extended to non-zero $Q^2$. The goals of Jefferson Lab experiment E97-110 are to measure the $Q^2$ dependence of the generalized GDH integral between 0.02 and 0.3 (GeV/c)$^2$, to study its slope near $Q^2 = 0$, and to extrapolate to the real photon point for the neutron and $^3$He. In this low $Q^2$ domain, such measurements serve to test Chiral Perturbation Theory and probe its limits of applicability. Our measurement will also provide new constraints on understanding the neutron and $^3$He spin structure and a better understanding of the effect of nucleon resonances. The experiment utilized the highly polarized electron beam and polarized $^3$He target at Jefferson Lab. Preliminary results will be presented.