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

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 Q2, we generalize the results in order to obtain formulae valid at arbitrary x and Q2. 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 and prove a property of the asymptot1ics often used in the literature though without a proof. Namely, the asymptotics of all parton helicities and the asymptotics of the spin structure function g1 are the same despite the parent formulae for these objects are quite different.