Measurement of spin structure functions at moderate Q2 using CLAS

Abstract

Spin structure functions of the nucleon in the region of large ξ and small to moderate Q 2 continue to be of high current interest. Among the topics one can study in this kinematic regime are spin-dependent resonance transition amplitudes and their interference with each other and the non-resonant background, the behavior of the asymmetry A 1 at large ξ, and the presence or absence of local duality in spin structure functions. The first moment of the spin structure function g 1 goes through a rapid transition from the photon point ( Q 2 = 0), where it is constrained by the Gerasimov-Drell-Hearn sum rule, to the deep inelastic limit where it is sensitive to the nucleon spin fraction carried by quarks. This opens up the possibility to study the transition from hadronic to quark degrees of freedom over the whole range of Q 2. Recently, we concluded a large experimental program to measure these observables with polarized proton and deuteron targets at Jefferson Lab. A highly polarized electron beam, solid polarized NH 3 and ND 3 targets and the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B were used to accumulate over 23 billion events with 4 different beam energies of 1.6, 2.5, 4.2 and 5.7 GeV. We present an overview of the experiment, its kinematic coverage and its statistical power. We show final results from the first run at 2.5 GeV and 4 GeV and preliminary results from the 5.7 GeV and the 1.6 GeV data sets.