Nucleon spin structure (1503.02250v2)

Abstract: This paper contains three parts relating to the nucleon spin structure in a simple picture of the nucleon: (i) The polarized gluon distribution in the proton is dynamically predicted starting from a low scale by using a nonlinear QCD evolution equation-the DGLAP equation with the parton recombination corrections, where the nucleon is almost only consisted of valence quarks. We find that the contribution of the gluon polarization to the nucleon spin structure is much larger than the predictions of most other theories. This result suggests a significant orbital angular momentum of the gluons is required to balance the gluon spin momentum; (ii) The spin structure function $g_1^p$ of the proton is studied, where the perturbative evolution of parton distributions and nonperturbative Vector Meson Dominance (VMD) model are used. We predict $g_1^p$ asymptotic behavior at small $x$ from lower $Q^2$ to higher $Q^2$. The results are compatible with the data including the HERA early estimations and COMPASS new results; (iii) The generalized Gerasimov-Drell-Hearn (GDH) sum rule is understood based on the polarized parton distributions of the proton with the higher twist contributions. A simple parameterized formula is proposed to clearly present the contributions of different components in the proton to $\Gamma_1^p(Q^2)$. The results suggest a possible extended objects with size $0.2-0.3~ fm$ inside the proton.