Quark and Gluon Helicity at Small x

We determine the small-x asymptotics of the quark and gluon helicity distributions in a proton at leading order in perturbative QCD at large Nc. To achieve this, we first evaluate the quark and gluon helicity TMDs at small x, simplifying them and relating them to the so-called polarized dipole amplitudes. We then construct and solve novel small-x large-Nc evolution equations for the polarized dipole amplitudes. Our main results are the small-x asymptotics for the quark helicity distributionΔq ∼ (1x)αhq    with    αhq = 43αs Nc2π ≈2.31 αs Nc2π      (1)and the small-x asymptotics of the gluon helicity distributionΔG ∼ (1x)αhG    with    αhG = 1343αs Nc2π ≈ 1.88αsNc2π,     (2)both in the large-Nc limit.