Optimized delta-Expansion in QCD; a challenge
  We split the Yang-Mills Lagrangian into a free and an interaction part in such a way, that the free part is non-local and contains an arbitrary form factor.   Manifest gauge invariance is guaranteed by connecting the field-strength tensors at different space-time points by a string. As a result the gluon propagator, which, due to the presence of the string, now contains many different contributions, comes out strictly transversal in one-loop order.   We also calculate the ghost self-energy and the ghost-gluon vertex in one-loop order. Subsequently we discuss how one can determine the "optimal" form of the form factor. To apply well known principles like "fastest apparent convergence" or "principle of minimal sensitivity" one has first to solve some problems connected with divergences and renormalization. Here we concentrate on the calculation of the anomalous dimensions and the beta-function. This is technically simpler because only the divergent contributions of the integrals have to be determined. The beta-function becomes independent of the gauge parameter as it should. A puzzle with respect to the principle of minimal sensitivity shows up. Really interesting non-perturbative results are expected when applying the above principles directly to the propagator. For the beta-function a two loop calculation would be required to obtain non-trivial results. 
Dieter Gromes (Universitaet Heidelberg)