Nature of the phase transition in compact QED

The four-dimensional O(2) gauge theory is studied by computer simulations and its phase structure is investigated by three methods: (i) by accurately measuring the action per plaquette on a ${12}^4$ lattice, (ii) through a scaling analysis of a certain function of Wilson loops, and (iii) via its string tension. All these methods consistently indicate that the theory has a line of fixed points. The end point of this line exhibits a power-law divergence of the correlation length with index $\nu =0.35\mathrm{}\pm 0.05$.