Leading-twist proton parton distribution amplitude from scalar and axial-vector diquark correlations

Abstract

We compute the leading-twist proton parton distribution amplitude (PDA) using algebraic representations of the nucleon Faddeev wave function inspired by perturbation-theory integral representations. The PDA is obtained from a light-front projection of the covariant quark--diquark Faddeev amplitude and analysed in terms of its scalar- and axial-vector-diquark components. We derive analytic expressions for the corresponding diquark distribution amplitudes and their contributions to the proton PDA. The resulting distribution is broad and concave, with its maximum $(x_1,x_2,x_3)\approx (0.55,0.20,0.25)$ displaced from the symmetric configuration $(1/3,1/3,1/3)$, which corresponds to the maximum of the perturbative-QCD asymptotic form. Scalar-diquark correlations generate a strong asymmetry in the longitudinal momentum distribution, whereas axial-vector correlations redistribute momentum among the valence quarks and significantly reduce this asymmetry. We calculate low-order Mellin moments and obtain a compact parametrisation of the proton PDA at the hadronic scale, ${\mu }_0=0.51\mathrm{GeV}$. We then perform the leading-order QCD evolution of its normalized shape to $\mu =2\mathrm{GeV}$, finding that evolution reduces the momentum asymmetry while preserving a clear imprint of the non-perturbative quark--diquark correlations. The results provide insight into the role of such correlations in shaping the proton light-front structure and furnish non-perturbative input for studies of hard exclusive baryonic processes.

arXiv comment24 pages, 9 figures, 3 tables