Azimuthal asymmetries at HERA: theoretical aspects
  We comment on theoretical aspects of the measurement of azimuthal asymmetries in semi-inclusive charged particle production, made recently by the ZEUS Collaboration at HERA. By taking the ratio of the two measured asymmetries, we find good agreement between the perturbative QCD prediction and the experimental data. To separate the perturbative and nonperturbative contributions to the asymmetries, we suggest that the azimuthal asymmetries of the transverse energy flow be measured as a function of a variable $q_T$ related to the pseudorapidity of the energy flow. 
P.M. Nadolsky, D.R. Stump and C.-P. Yuan