Full one-loop calculation of neutralino annihilation into two photons

For the first time, a full one-loop calculation of the process χχ -> 2γ is performed, where χ is the lightest neutralino in the minimal supersymmetric extension of the Standard Model. This process is of interest for dark matter detection, since it would give a sharp γ ray line with E_γ = m_χ. We improve upon and correct published formulas, and give cross sections for supersymmetric models with χ masses between 30 GeV and several TeV. We find a new contribution, previously neglected, which enhances the 2γ rate for TeV higgsinos by up to an order of magnitude. As a byproduct, we obtain a new expression for the related process χχ -> 2 gluons, which on the other hand can be smaller than previously calculated. There has been a recent claim that evidence for a 3.5 TeV higgsino annihilating into a γ line may already exist from balloon emulsion and Air Cherenkov Telescope data. We comment on attractive features and problems with this interpretation.