OPTIMIZATION IN THE EXTENDED VARIATIONAL APPROACH - THE SU(2)-SO(3) PHASE-STRUCTURE ON THE LATTICE

We use the linear delta expansion with a trial action based on single links to explore the phase structure of the mixed SU(2)-SO(3) lattice gauge theory. The method can be regarded as a systematic expansion going beyond mean field theory or the variational method, to which it is closely related in first order. At O(delta2) the line of first-order phase transitions is shown to terminate, and at O(delta3) the boundaries between the different phases are well reproduced.