U(1) lattice gauge theory with a topological action

We investigate the phase diagram of the compact U(1) lattice gauge theory in four dimensions using a non-standard action which is invariant under continuous de-formations of the plaquette angles. Just as for the Wilson action, we find a weakly first order transition, separating a confining phase where magnetic monopoles condense, and a Coulomb phase where monopoles are dilute. We also find a third phase where monopoles are completely absent. However, since the monopoles do not influence the long-distance prop-erties of the Coulomb phase, the physics is smooth across the singularity in the monopole density. The topological action offers an algorithmic advantage for the computation of the free energy