Helicity Evolution at Small $x$: Flavor Singlet and Non-Singlet Observables

TL;DR

This paper derives small-$x$ asymptotics for quark helicity distributions and the $g_1$ structure function, extending previous results and constructing new evolution equations, with solutions confirming earlier intercept calculations.

Contribution

It extends helicity evolution equations to small $x$, deriving new flavor non-singlet equations and providing analytical and numerical solutions for both flavor singlet and non-singlet cases.

Findings

Derived small-$x$ asymptotics for helicity PDFs and $g_1$.

Constructed new evolution equations in the large-$N_c$ limit.

Confirmed the intercept matches previous theoretical calculations.

Abstract

We extend our earlier results for the quark helicity evolution at small $x$ to derive the small-$x$ asymptotics of the flavor singlet and flavor non-singlet quark helicity TMDs and PDFs and of the $g_1$ structure function. In the flavor singlet case we re-derive the evolution equations obtained in our previous paper on the subject, performing additional cross-checks of our results. In the flavor non-singlet case we construct new small-$x$ evolution equations by employing the large-$N_c$ limit. All evolution equations resum double-logarithmic powers of $\alpha_s \, \ln^2 (1/x)$ in the polarization-dependent evolution along with the single-logarithmic powers of $\alpha_s \, \ln (1/x)$ in the unpolarized evolution which includes saturation effects. We solve the linearized flavor non-singlet equation analytically, obtaining an intercept which agrees with the one calculated earlier by Bartels, Ermolaev and Ryskin using the infra-red evolution equations. Our numerical solution of the linearized large-$N_c$ evolution equations for the flavor singlet case is presented in the accompanying Letter and is further discussed here.