114 Nuclear Physics B (Proc. Suppl.) 9 (1989) 114-118 North-Holland, Amsterdam PHASE STRUCTURE OF COMPACT LATTICE QED WITH DYNAMICAL FERMIONS H.C. Hege and A. Nakamura FB Physlk, Freie Universit~t Berlin, D-]O00 Berlin 33, FRG The phase structure of compact lattice QED with dynamical Wilson fermions is studied by Monte Carlo simulation. The fermion contribution is calculated using an exact algorithm, capable of large Markov steps in configuration space. A first order phase transition line in the (~,~) plane, ending at the quenched point (~I.0,<=0) has been found. The phases are explored by measuring gauge depen- dent quantities, photon and fermion propagators, besides the more conventional gauge invariant observables. For this we applied a non-iterative gauge fixing algorithm. From the phenomenological point of view weak coupling QED is the most successful physical theory due to the excellent agreement of expe- rimental results and those obtained within the perturbative approach. However, to understand QED as a complete field theory it is necessary to get insight into the theory at strong coup- ling, too. Recently it has been suggested, based upon Schwinger-Dyson calculations in the quenched ladder approximation, that in the strong coupling regime a nontrivial zero of the beta function exists ] . If the fixed point really exists and survives in the presence of dynamical fermions, it may have phenomenologi- cal consequences. It has been pointed out, that such an unexpected behaviour of strong coupling QED may revive certain technicolor models by suppression of undesired flavour-changing neutral currents 2. The speculation that the sharp lines observed in positron and electron spectra in heavy ion collisions may be caused by a new phase of QED 3 (but see i.e. Ref.4), raised the interest in non-perturbative inves- tigations of QED, too. The lattice formulation of QED allows non-perturbative calculations which may be more reliable than the ladder approximation. The existence of a second order transition in strong coupling lattice QED, which might allow a non-trivial continuum limit to exist, would be of great interest. For the pure gauge part of lattice QED there are two formulations, the compact (peri- odic) and the non-compact one. Whi~e the non-compact version models conventional weak coupling continuum theory directly, the com- pact theory contains additional topological excitations (monopoles, whose density vanishes exponentially in the weak coupling region), but fits into the general lattice gauge theory scheme. Since gauge invariant lattice Dirac operators are customarily defined in terms of compact gauge field variables Un, =exp(i@n,u) , the compact lattice version looks more natural from a conceptual point of view. Up to now, only staggered fermions have been employed in Monte Carlo simulations of lattice QED. Recently Dagotto and Kogut found phase transitions of first order in compact QED 5 and of second order in noncompact QED 6 with staggered fermions coupled to compact gauge field variables. Here we report the first results of a compact lattice QED Monte Carlo simulation with dynamical Wilson fermions, that is with action S=B~Re Un,~Un+~,vU~+v,uU~,v+~n(1-<Q)nn.~n. n,u, (]) Qnn'=[( ( 1-~U)Un,u~n+~,n "+ ( 1+Y~)U~.,~n_u,n. ] 0920-5632/89/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)