Leading and higher twists in the proton polarized structure functiong1pat large Bjorkenx

Abstract

A phenomenological parametrization of the proton polarized structure function ${g}_{1}^{p}{(x,Q}^{2})$ is developed for $x\ensuremath{\gtrsim}0.02$ using deep inelastic data up to $\ensuremath{\sim}50 (\mathrm{GeV}{/c)}^{2}$ 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 ${Q}^{2}=0.16\ifmmode\pm\else\textpm\fi{}0.04 (\mathrm{GeV}{/c)}^{2}.$ Then, low-order polarized Nachtmann moments are estimated and their ${Q}^{2}$ behavior is investigated in terms of leading and higher twists for ${Q}^{2}\ensuremath{\gtrsim}1 (\mathrm{GeV}{/c)}^{2}.$ 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 ${Q}^{2}\ensuremath{\gtrsim}1 (\mathrm{GeV}{/c)}^{2},$ and the singlet axial charge has been determined to be ${a}_{0}[10 (\mathrm{GeV}{/c)}^{2}]=0.16\ifmmode\pm\else\textpm\fi{}0.09.$ 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 ${Q}^{2}\ensuremath{\sim}$ few $(\mathrm{GeV}{/c)}^{2}$ 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 $\ensuremath{\Delta}(1232)$ resonance.