Glueball masses in 4d U(1) lattice gauge theory using the multi-level algorithm

We take a new look at plaquette-plaquette correlators in 4d compact U(1) lattice gauge theory which are separated in time, both in the confined and the deconfined phases. From the behaviour of these correlators we extract glueball masses in the scalar as well as the axial-vector channels. Also in the deconfined phase, the non-zero momentum axial-vector correlator gives us information about the photon which appears as a massless particle in the spectrum. Using the Luescher - Weisz multi-level algorithm, we are able to go to large time separations which were not possible previously.