Finite size renormalization group study of U(1) gauge theory

We apply the finite size renormalization group method to the analysis of the phase transition of U(1) lattice gauge theory in four dimensions. We show that it is possible to construct gauge invariant quantities ('gauge-spin') to compute renormalized variables corresponding to large blocks. We calculate the thermal critical exponent and obtain a result near the theoretical value expexted in a first order phase transition for lattice sizes much smaller than the ones required with standard methods based on finite size scaling analysis of the specific heat exponent. (orig.)