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 quasimonotonic gamma ray. This process is radiatively induced, and the full one-loop calculation was done previously. However, for the heavy W-ino-like or Higgsino-like neutralino, the one-loop cross section violates unitarity; therefore, the higher-order corrections may be important. We construct a nonrelativistic 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 which the two-loop contribution is larger than the one-loop one is about 8 TeV [O(10) TeV] in the limit where the neutralino is W-ino- [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 W-ino-like limit where the neutralino is highly degenerate with the chargino in mass, we find that QED corrections enhance the pair annihilation cross section by 1.5-2.