Why can't we detect neutrino-antineutrino annihilation?

In summary: It seems to me that this also isn't true, as two Z bosons ( which are there own antiparticles) can self-annihilate through a higgs boson.
  • #1
bcrowell
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Various astrophysical processes produce antineutrinos, which then fly off into outer space. I assume there are pretty accurate estimates of the production rates. I can imagine three possible fates for such an antineutrino: (1) annihilating with a neutrino, (2) interacting with baryonic matter, (3) ending up as the only particle inside its own cosmological horizon. It seems like we ought to have pretty good estimates of the rate of the neutrino-antineutrino annihilation process. Each such annihilation produces two back-to-back photons. If the neutrino masses are on the order of 0.1 eV, then these are infrared photons with wavelengths on the order of 10^4 nm. Why can't we detect these photons and thereby determine the neutrino mass spectrum? Are the peaks too weak? Too spread out by Doppler broadening?
 
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  • #2
bcrowell said:
Too spread out by Doppler broadening?

Surely the Doppler broadening is so great that there would be nothing you could call a "peak" in the spectrum? Essentially all neutrinos are relativistic; the energy of the annihilation photons would be almost entirely determined by the neutrinos' kinetic energies and not their masses.
 
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  • #3
The_Duck said:
Surely the Doppler broadening is so great that there would be nothing you could call a "peak" in the spectrum? Essentially all neutrinos are relativistic; the energy of the annihilation photons would be almost entirely determined by the neutrinos' kinetic energies and not their masses.

Makes sense!
 
  • #4
The vv → γγ cross-section is truly microsocpic. (zeptoscopic?)

It's got to be photons, because everything else is kinematically blocked. It's got to go through the Z-pole, at 90 GeV when the neutrinos are at 1/40,000 of an eV. It also has to go through a loop which gives additional kinematic reduction, higher powers of the couplings, and a 1/16π2, which in light of the other factors, is almost too small to worry about.
 
  • #5
Vanadium 50 said:
The vv → γγ cross-section is truly microsocpic. (zeptoscopic?)

It's got to be photons, because everything else is kinematically blocked. It's got to go through the Z-pole, at 90 GeV when the neutrinos are at 1/40,000 of an eV. It also has to go through a loop which gives additional kinematic reduction, higher powers of the couplings, and a 1/16π2, which in light of the other factors, is almost too small to worry about.

The cross-section is small ... compared to what? Is the probability of this fate small compared to both of the other possibilities listed in #1? Negligibly small?
 
  • #6
I would say "so small that I don't trust the estimation to be done correctly".

If you want a ballpark, I'd estimate that the vv cross-section divided by the vp cross-section is alpha x m(v)/m(p) x [m(v)/m(e)]^4 /16 pi^2.
 
  • #9
Thanks Bill. I had acquired the notion that Majorana particles do not self annihilate. Upon further review, I find the consensus is that is only true for bosons, not leptons.
 
  • #10
? If neutrinos are majorana they can annihilate - we are supposed to find the Majorana nature by neutrinoless double beta decay.
 
  • #11
Don't we know already that neutrinos can self-annihilate through a Z boson, because we observe that the Z can decay to neutrinos? ( simply the reverse process)
 
  • #12
Thanks Bill. I had acquired the notion that Majorana particles do not self annihilate. Upon further review, I find the consensus is that is only true for bosons, not leptons.

It seems to me that this also isn't true, as two Z bosons ( which are there own antiparticles) can self-annihilate through a higgs boson.
 

FAQ: Why can't we detect neutrino-antineutrino annihilation?

1. Why is it difficult to detect neutrino-antineutrino annihilation?

The main reason it is difficult to detect neutrino-antineutrino annihilation is because neutrinos and antineutrinos have very low interaction cross-sections, meaning they rarely interact with other particles. This makes it challenging to detect their annihilation events, which produce very little detectable energy.

2. What methods have been used to try to detect neutrino-antineutrino annihilation?

Scientists have used various methods to try to detect neutrino-antineutrino annihilation, including large underground detectors such as the Super-Kamiokande and IceCube experiments, as well as detectors placed on satellites and in space, such as the Fermi Gamma-ray Space Telescope.

3. Are there any proposed future experiments that could detect neutrino-antineutrino annihilation?

Yes, there are proposed future experiments such as the PTOLEMY experiment, which aims to use a large-scale detector to search for the signals of neutrino-antineutrino annihilation in our galaxy.

4. Can neutrino-antineutrino annihilation events be detected indirectly?

Yes, while directly detecting neutrino-antineutrino annihilation events is difficult, scientists can indirectly observe their effects, such as through the production of high-energy particles or the emission of gamma rays. This indirect detection method is often used in astrophysics to study cosmic sources of neutrinos.

5. What are the potential implications of detecting neutrino-antineutrino annihilation?

If neutrino-antineutrino annihilation events are detected, it could provide evidence for new physics beyond the Standard Model and help us better understand the properties of neutrinos and antineutrinos. It could also have implications for our understanding of the early universe and the processes that occur within it.

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