# Do neutrinos and antineutrinos annihilate?

1. ### pierce15

254
Sorry if this is a trivial question; I don't know very much about particle physics.

2. ### phinds

8,339
Don't know. I believe it is not known for certain whether or not the neutrino is its own antiparticle.

What I believe to be the case however is that other than as a purely factual thing, it is irrelevant. Since a neutrino can go through something like a light year of solid lead without hitting anything, the chances of a neutrino hitting an anti neutrino seems vanishingly small even considering how MANY of the little buggers there are.

3. ### ChrisVer

Yes neutrinos+antineutrinos annihilate, as all the other particles+antiparticles... I don't think they can annihilate by photon emission but they can by $Z^{0}$...

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4. ### Matterwave

3,816
As the Z boson is massive (quite massive), if this is the only channel for which annihilation can occur, that would suggest the neutrinos and anti-neutrinos must have some minimum energy in their COM frame right? I think there must be other channels for this annihilation...but I don't know of them for sure off the top of my head.

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5. ### ChrisVer

Well I think the mass of the Z boson does nothing but suppress the interaction - and of course it's suppressed by 1/mZ2

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6. ### ChrisVer

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7. ### pierce15

254
Is there some kind of equation applicable to this scenario?

8. ### Matterwave

3,816
Yes, it would the sum of all Feynman diagrams with ##\nu_e \bar{\nu}_e\rightarrow Z^0 Z^0##

Or perhaps ##\nu_e \bar{\nu}_e\rightarrow \gamma\gamma##

Essentially...and perhaps other products...But...these diagrams are highly complicated and non-trivial...I wouldn't know how to evaluate them. I can only give some guesses.

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9. ### pierce15

254
Ok... slightly different question: assuming that they would annihilate if they were of the same flavor, would two antineutrinos annihilate if one was an antiparticle but the other was of a different flavor?

Related: how about a muon and a positron?

10. ### ChrisVer

the related question is obviously no...
the question itself : in principle no... except for if you allow neutrino oscillations...
But then you would have the case of neutrino+antineutrino, which is already highly suppressed... It wouldn't make any sense to suppress it even further...

Except for if you mean two antineutrinos and that was not a typo... Two antineutrinos as two positrons or two electrons don't annihilate .... (I'm trying to avoid thinking of neutrinos as Majorana, since this is not yet observed- otherwise the two neutrinos or two antineutrinos could annihilate with each other)

### Staff: Mentor

In principle, neutrinos could annihilate according to

##\nu_e \bar{\nu}_e\rightarrow e^+ e^-##

through exchange of a Z in s-channel or a W in t-channel. Since the weak bosons in these processes are virtual, the threshold for this to occur is based on the electron mass rather than the weak boson masses. Of course, the cross section for this process would typically go as the Fermi constant squared and be very low.

This reaction is also related by crossing symmetry to the elastic scattering of electron neutrinos on electrons, which gives rise to observable matter effects in neutrino oscillations.

12. ### dauto

The poster is pointing out that the annihilation to photons is possible even though the tree level vertex coupling is zero because there are radiative diagrams (that is loop diagrams) that mediate the process.

13. ### pierce15

254
Maybe this is pushing a little too far, but does whether a neutrino and an antineutrino annihilate have any significance in our view of the universe? i.e. if we experimentally showed either that this reaction forms photons/an electron-positron pair or that it doesn't occur at all, would this have any implications in particle physics?

14. ### phinds

8,339
For practical considerations, see post #2

15. ### pierce15

254
Just because it's unlikely doesn't mean it's impossible or unworthy of consideration. The big bang happened once, but we study it because it's an important part of our understanding of the universe. The point of physics is to develop a theory of how the universe works, and waving off thought experiments as unrealistic leaves gaps in the theory. This is essentially equivalent to saying that it is impossible to accelerate anything with appreciable mass to a significant fraction of the speed of light, so we don't need to consider special relativity.

Last edited: Jun 12, 2014
16. ### Bill_K

4,160
The Standard Model predicts it can happen, so if it doesn't happen, that would be a serious challenge to the Standard Model.

In fact, some of the most stringent tests of the Standard Model are those reactions that are extremely unlikely, and consequently extremely rare.

17. ### ChrisVer

Of course Bill... it's easier to find something with small contribution in soft noise than finding it in a very noisy region. At least that's what I think about the rare events being a test for the SM.
Of course, if the OP checks the other thread I posted, he'll find that people there said the process of neutrino-antineutrino annihilation is not yet observed, because it's very small... neutrinos interact hardly with matter, just imagine expecting two of them to interact with each other hehehe.

18. ### phinds

8,339
I was not suggesting otherwise. I think we're talking at cross purposes. I was commenting on the practical significance to everyday life which sort of addressed the first part of your question, not the second part.

19. ### phinds

8,339
Yes, that was the point of post #2

20. ### Bill_K

4,160
In some circumstances it can be important. This paper says: