Paper
Helicity Evolution at Small $x$: Flavor Singlet and Non-Singlet Observables
Authors
Abstract
We extend our earlier results for the quark helicity evolution at small to derive the small- asymptotics of the flavor singlet and flavor non-singlet quark helicity TMDs and PDFs and of the 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- evolution equations by employing the large- limit. All evolution equations resum double-logarithmic powers of in the polarization-dependent evolution along with the single-logarithmic powers of 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- evolution equations for the flavor singlet case is presented in the accompanying Letter and is further discussed here.