Measurements of the Q2-dependence of the proton and deuteron spin structure functions g1 and g1

The ratio g1/F1 has been measured over the range 0.03 < x < 0.6 and 0.3 < Q < 10 (GeV/c) using deep-inelastic scattering of polarized electrons from polarized protons and deuterons. We find g1/F1 to be consistent with no Q-dependence at fixed x in the deep-inelastic region Q > 1 (GeV/c). A trend is observed for g1/F1 to decrease at lower Q . Fits to world data with and without a possible Q-dependence in g1/F1 are in agreement with the Bjorken sum rule, but ∆q is substantially less than the quark-parton model expectation. The longitudinal spin-dependent structure function g1(x,Q ) for deep-inelastic leptonnucleon scattering has become increasingly important in unraveling the quark and gluon spin structure of the proton and neutron. The g1 structure function depends both on x, the fractional momentum carried by the struck parton, and on Q, the fourmomentum transfer squared of the virtual photons used as a probe of nucleon structure. Of particular interest are the fixed-Q integrals Γp1(Q ) = ∫ 1 0 g p 1(x,Q )dx for the proton and Γ1 (Q ) = ∫ 1 0 g n 1 (x,Q )dx for the neutron. These integrals are directly related to the net quark helicity ∆q in the nucleon. Measurements of Γp1 [1–5], Γ d 1 [6–7], and Γ n 1 [8] have found ∆q ≈ 0.3, significantly less than a prediction [9] that ∆q = 0.58 assuming zero net strange quark helicity and SU(3) flavor symmetry in the baryon octet. A fundamental sum rule originally derived from current algebra by Bjorken [10] predicts the difference Γp1(Q 2)− Γ1 (Q). Recent measurements are in agreement with this sum rule prediction when perturbative QCD (pQCD) corrections [11] are included. There are two main reasons for measuring g1 over a wide range of x and Q . The first is that experiments make measurements at fixed beam energies rather than at fixed Q. To evaluate first moment integrals of g1(x,Q ) at constant Q [typically between 2 and 10 (GeV/c)], extrapolations are needed. Data at low x are at lower Q than desired