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?
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Two low energy neutrinos don't have the energy to annihilate into a real Z. – CuriousOne Sep 23 '15 at 04:30
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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 04:45
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Low energy neutrino and antineutrino can annihilate into three photons, but with an extremely low cross-section. So it's not "never", it's "hardly ever" (http://www.sciencedirect.com/science/article/pii/0029558266902331)
akhmeteli
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So it can be infered that the prospect of double beta decay experiment is very dark. – Ballistics Sep 23 '15 at 06:23
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Maybe the neutrino and anti-neutrino could annihilate virtually, creating a virtual Z boson. Then the virtual Z boson could decay into particles depending on the energy of the neutrino and anti-neutrino.
MiltonTheMeme
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