Two-photon final states in peripheral heavy ion collisions

We discuss processes leading to two photon final states in peripheral heavy ion collisions at RHIC. Due to the large photon luminosity we show that the continuum subprocess $\gamma \gamma \to \gamma \gamma$ can be observed with a large number of events. We study this reaction when it is intermediated by a resonance made of quarks or gluons and discuss its interplay with the continuum process, verifying that in several cases the resonant process ovewhelms the continuum one. It is also investigated the possibility of observing a scalar resonance (the $\sigma$ meson) in this process. Assuming for the $\sigma$ the mass and total decay width values recently reported by the E791 Collaboration we show that RHIC may detect this particle in its two photon decay mode if its partial photonic decay width is of the order of the ones discussed in the literature.