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

A phenomenological parametrization of the proton polarized structure function $g_1^p{(x,Q}^2)$ is developed for $x\gtrsim 0.02$ using deep inelastic data up to $\sim 50\hspace{0.5em}(\mathrm{GeV}{/c)}^2$ 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 $Q^2=0.16\mathrm{}\pm 0.04\hspace{0.5em}(\mathrm{GeV}{/c)}^2.$ Then, low-order polarized Nachtmann moments are estimated and their $Q^2$ behavior is investigated in terms of leading and higher twists for $Q^2\gtrsim 1\hspace{0.5em}(\mathrm{GeV}{/c)}^2.$ 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 $Q^2\gtrsim 1\hspace{0.5em}(\mathrm{GeV}{/c)}^2,$ and the singlet axial charge has been determined to be $a_0\left[10\hspace{0.5em}\right(\mathrm{GeV}{/c)}^2]=0.16\mathrm{}\pm \mathrm{0.09.}$ 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 $Q^2\sim$ 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 $\Delta \mathrm{}\left(1232\right)\mathrm{}$ resonance.