Effects of Jet Azimuthal Angular Distributions on Dijet Production Cross Sections in DIS

A Monte Carlo study of the azimuthal angular distribution around the virtual boson-proton beam axis for dijet events in DIS at HERA is presented. In the presence of typical acceptance cuts on the jets in the laboratory frame, the azimuthal distribution is dominated by kinematic effects rather than the typical $\cos\phi$ and $\cos 2\phi$ dependence predicted by the QCD matrix elements. This implies that the $\phi$ dependent part of the QCD matrix elements contributes even to the dijet production cross section. Neglecting this $\phi$ dependence leads to an error of about 5-8\% in the production cross section for typical acceptance cuts in the laboratory frame. We also present first NLO results on the $\phi$-decorrelation of the jets through NLO effects.