Leading and higher twists in the proton polarized structure function g1p at large Bjorken x

A phenomenological parametrization of the proton polarized structure function g1p(x,Q2) is developed for x≳0.02 using deep inelastic data up to ∼50 (GeV/c)2 as well as available experimental results on both photo- and electroproduction of proton resonances. According to the new parametrization the generalized Drell-Hearn-Gerasimov sum rule is predicted to have a zero-crossing point at Q2=0.16±0.04 (GeV/c)2. Then, low-order polarized Nachtmann moments are estimated and their Q2 behavior is investigated in terms of leading and higher twists for Q2≳1 (GeV/c)2. The leading twist is treated at NLO in the strong coupling constant and the effects of higher orders of the perturbative series are estimated using soft-gluon resummation techniques. In the case of the first moment, higher-twist effects are found to be quite small for Q2≳1 (GeV/c)2, and the singlet axial charge has been determined to be a0[10 (GeV/c)2]=0.16±0.09. In the case of higher order moments, which are sensitive to the large-x region, higher-twist effects are significantly reduced by the introduction of soft gluon contributions, but they are still relevant at Q2∼ few (GeV/c)2 at variance with the case of the unpolarized transverse structure function of the proton. Our finding suggests that spin-dependent correlations among partons may have more impact than spin-independent ones. As a by-product, it is also shown that the Bloom-Gilman local duality is strongly violated in the region of polarized electroproduction of the Δ(1232) resonance.