Full One-loop Calculation of Neutralino Annihilation into Two Photons

For the first time, a full one-loop calculation of the process $\chi\chi\to 2\gamma$ is performed, where $\chi$ 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 $\gamma$ ray line with $E_\gamma=m_\chi$. We improve upon and correct published formulas, and give cross sections for supersymmetric models with $\chi$ masses between 30 GeV and several TeV. We find a new contribution, previously neglected, which enhances the $2\gamma$ rate for TeV higgsinos by up to an order of magnitude. As a byproduct, we obtain a new expression for the related process $\chi\chi\to 2 gluons$, which on the other hand is generally smaller than previously calculated. There has been a recent claim that evidence for a 3.5 TeV higgsino annihilating into a $\gamma$ line may already exist from balloon emulsion and Air Cherenkov Telescope data. We comment on attractive features and problems with this interpretation.