# What is the product of low energy neutrino annihilation?

Since neutrinos are only subjected to weak interaction(ignore gravitation), thus the interaction of neutrinos are only governed by Z bosons, and photon is unlikely produced. If two neutrinos annihilate at an energy scale sufficient to produce a Z boson, then the Z boson can decay into various particles including photons. But what if the energy scale is insufficient for Z boson production? Does that mean the neutrinos will never annihilate?

• Two low energy neutrinos don't have the energy to annihilate into a real Z. – CuriousOne Sep 23 '15 at 4:30
• Perhaps $\nu\bar\nu \to W^+W^- \to \gamma\gamma$? It'd be super-suppressed. Note that according to the PDG, $Z\to\gamma\gamma$ decays are ruled out at the part-per-million level for lone $Z$s. – rob Sep 23 '15 at 4:45