Photon spin and the shape of the two-photon correlation function

We use the covariant current formalism to derive the general form of the 2-photon correlation function for fully chaotic sources. Motivated by the recent discussion in the literature we concentrate on the effects from the photon spin on the correlator. We show that for locally thermalized expanding sources, like those expected to be created in relativistic heavy ion collisions, the only change relative to 2-pion interferometry is a statistical factor 12 for the overall strength of the correlation which results from the experimental averaging over the photon spin.