Phase diagram of the 4D U(1) model at finite temperature

We explore the phase diagram of the four-dimensional compact U(1) gauge theory at finite temperature as a function of the gauge coupling and of the compactified Euclidean time dimension Lt. We show that the strong-to-weak coupling transition, which is first order at T=0 (Lt=∞), becomes second order for high temperatures, i.e. for small values of Lt, with a tricritical temporal size L⎯⎯t located between 5 and 6. The critical behavior around the tricritical point explains and reconciles previous contradictory evidences found in the literature.