It has been suggested that pair annihilation of heavy Majorana fermions in the galactic halo into quarkonium plus a monochromatic photon could occur at an observable rate. Here we show that a calculation of Srednicki, Theisen, and Silk seriously overestimates the rate for this process, by an order of magnitude or more, as a result of neglecting the bound-state structure of quarkonium. It may nevertheless still be possible to resolve the correspondingly smaller gamma-ray line flux over the diffuse cosmic background by use of the currently planned space-borne superconducting-magnet spectrometer facility with an energy resolution of 1% or better.

• Received 31 January 1986

DOI:https://doi.org/10.1103/PhysRevLett.56.2128