Orbital angular momentum small-x evolution: exact results in the large-Nc limit

Abstract

We construct an exact solution to the revised small-x orbital angular momentum (OAM) evolution equations derived in [1], based on an earlier work [2]. These equations are derived in the double logarithmic approximation (summing powers of αs ln2(1/x) with αs the strong coupling constant and x the Bjorken x variable) and the large-Nc limit, with Nc the number of quark colors. From our solution, we extract the small-x, large-Nc expressions of the quark and gluon OAM distributions. Additionally, we determine the large-Nc small-x asymptotics of the OAM distributions to beLq+q¯xQ2∼LGxQ2∼ΔΣxQ2∼ΔGxQ2∼1xαh,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {L}_{q+\\overline{q}}\\left(x,{Q}^2\\right)\\sim {L}_G\\left(x,{Q}^2\\right)\\sim \\Delta \\Sigma \\left(x,{Q}^2\\right)\\sim \\Delta G\\left(x,{Q}^2\\right)\\sim {\\left(\\frac{1}{x}\\right)}^{\\alpha_h}, $$\\end{document}with the intercept αh the same as obtained in the small-x helicity evolution [3], which can be approximated as αh ≈ 3.66074αsNc2π\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ 3.66074\\sqrt{\\frac{\\alpha_s{N}_c}{2\\pi }} $$\\end{document}. This result is in complete agreement with [4]. Additionally, we calculate the ratio of the quark and gluon OAM distributions to the flavor-singlet quark and gluon helicity parton distribution functions respectively in the small-x region.