Flavor Singlet Contribution to the Structure Function g1 at Small-x

The singlet contribution to the g1(x;Q) structure function is calculated in the double-logarithmic approximation of perturbative QCD in the region x 1. Double logarithmic contributions of the type ( s ln (1=x)) which are not included in the GLAP evolution equations are shown to give a power-like rise at small-x which is much stronger than the extrapolation of the GLAP expressions. The dominant contribution is due to the gluons which, in contrast to the unpolarized case, mix with the fermions also in the region x 1. The two main reasons why the small-x behavior of the double logarithmic approximation is so much stronger than the usual GLAP evolution are: the larger kinematical region of integration (in particular, no ordering in transverse momentum) and the contributions from non-ladder diagrams. Supported by Bundesministerium f ur Forschung und Technologie, Bonn, Germany under Contract 056HH93P(5) and EEC Program "Human Capital and Mobility" through Network "Physics at High Energy Colliders" under Contract CHRX-CT93-0357 (DG12 COMA) The research described in this paper has been made possible in part by the Grant R2G 300 from the International Science Foundation and the Russian Government. This work has also been supported by the Volkswagen-Stiftung. Work supported by the Grant INTAS-93-0079 and by the Volkswagen-Stiftung