Unitarity and Higher-Order Corrections in Neutralino Dark Matter Annihilation into Two Photons

The neutralino pair annihilation into two photons in our galactic halo gives a robust dark matter signal, since it would give a quasi-monotonic gamma ray. This process is radiatively-induced, and the full-one loop calculation was done previously. However, for the heavy wino-like or Higgsino-like neutralino, the one-loop cross section violates unitarity, therefore the higher-order corrections may be important. We construct a non-relativistic theory for chargino and neutralino two-body states, and estimate all-order QED corrections and two-loop corrections by $Z$ and/or $W$ exchange. We find that the critical mass, above that the two-loop contribution is larger than one-loop one, is about 8 TeV (O(10) TeV) in the limit where neutralino is wino (Higgsino)-like, respectively. Around and above the critical mass, the all-order Z and/or W exchange must be included to estimate the cross section. On the other hand, the QED corrections depend on the mass difference between the neutralino and chargino. In the wino-like limit where neutralino is highly degenerate with chargino in mass, we find that QED corrections enhance the pair annihilation cross section by 1.5-2.