Parton helicities at arbitrary x and $$Q^2$$ in double-logarithmic approximation

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 $$Q^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>Q</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:math> , we generalize the results in order to obtain formulae valid at arbitrary x and $$Q^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>Q</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:math> . 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