Parton helicities at arbitrary x and Q2 in double-logarithmic approximation

TL;DR

This paper derives explicit formulas for parton helicities across all x and Q^2 in the double-logarithmic approximation, advocating for KT factorization over collinear approaches when considering orbital angular momentum.

Contribution

It provides generalized helicity formulas valid at arbitrary x and Q^2 in the DLA, and argues for the use of KT factorization over collinear factorization in spin-dependent processes.

Findings

KT factorization is preferable when including orbital angular momentum.

Helicity asymptotics at small x are less singular in DGLAP than in Regge.

Explicit formulas for helicities at arbitrary x and Q^2 are derived.

Abstract

Description of spin-dependent hadronic processes at high energies in terms of parton helicities is a both effective and technically convenient means. In the present paper, we obtain explicit expressions for the parton helicities when either Collinear or KT forms of QCD Factorization are used. Starting our studies with calculation of the helicities in the double-logarithmic approximation (DLA) in the region of small x and large Q^2, we generalize the results in order to obtain formulae valid at arbitrary x and Q^2. We argue against using Collinear Factorization, when the parton orbital angular momenta are accounted for, and prove that KT Factorization should be used instead. We also consider in detail the small-x asymptotics of the parton helicities, compare them with the DGLAP-asymptotics in LO,NLO, etc and prove that the DGLAP asymptotics are less singular at small x than the Regge asymptotics