Measurement of the Spin Structure Function G1 of the Deuteron and Its Moments at Low

MEASUREMENT OF THE SPIN STRUCTURE FUNCTION G1 OF THE DEUTERON AND ITS MOMENTS AT LOW Q2 Krishna P. Adhikari Old Dominion University, 2014 Director: Dr. Sebastian E. Kuhn Double polarization cross section differences (∆σ‖) for proton and deuteron targets have been measured in the EG4 experiment using the CLAS detector at Jefferson Lab. Longitudinally polarized electron beams at relatively low energies of 1.056, 1.337, 1.989, 2.256 and 3.0 GeV from the CEBAF accelerator were scattered off longitudinally polarized NH3 and ND3 targets. Scattered electrons were recorded at very low scattering angles (down to θ = 6o) with the help of a new dedicated Cherenkov counter and a special magnetic field setting of the CLAS detector in order to measure the cross section differences in the resonance region (1.08 GeV<W < 2.0 GeV) at very low momentum transfers (Q2 for the deuteron was as low as 0.02 GeV2). These measurements on the deuteron were used to extract the deuteron’s spin structure function g1 as well as the product A1F1 of the virtual photon asymmetry A1 and the unpolarized structure function F1. These extracted quantities, in turn, were used to evaluate three important integrals for the deuteron the first moment (Γ1) of g1, the extended Gerasimov-Drell-Hearn (GDH) integral (ĪT T ), and the generalized forward spin polarizability (γ0). These measurements extend and improve the world deuteron data on g1 to the previously unmeasured low Q2 region. The data, in combination with the corresponding proton data from the same experiment, will be valuable to extract g1 on the neutron in the same kinematics. They will shed more light on the nucleon spin structure in the region of quark-confinement as well in the transition region between hadronic and partonic degrees of freedom. In addition, the three integrals evaluated from the measured data are compared to predictions from different Chiral Perturbation Theory (χPT) calculations and phenomenological models. Extrapolations of the integrals (especially the GDH sum and the polarizability) to the real photon point (Q2=0) enable us to test the validity of the predictions for their real photon counterparts. The new results have extended and improved the very low Q2 data on g1 and the corresponding results on moments compare very well with the latest χPT and phenomenological calculations (especially near the photon point).