Neutralino Annihilation into Two Photons

We compute the annihilation cross-section of two neutralinos at rest into two photons, which is of importance for the indirect detection of neutralino dark matter in the galactic halo through a quasi-monochromatic gamma-ray line. We include all diagrams to one-loop level in the minimal supersymmetric extension of the standard model. We use the helicity formalism, the background-field gauge, and an efficient loop-integral reduction method. We confirm the result recently obtained by Bergstrom and Ullio in a different gauge, which disagrees with other published calculations.