Do neutrinos and antineutrinos annihilate?

[pierce15]

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

 

[phinds]

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.

 

[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}...

 

[Matterwave]

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}...

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.

 

[ChrisVer]

Well I think the mass of the Z boson does nothing but suppress the interaction - and of course it's suppressed by 1/m_Z²

 

[ChrisVer]

https://www.physicsforums.com/showthread.php?t=622173

although as stated I cannot understand post #7...
mass doesn't mean that you can have annihilation to photons if your charge (vertex coupling) is zero... Or the poster meant something else...

 

[pierce15]

Is there some kind of equation applicable to this scenario?

 

[Matterwave]

Is there some kind of equation applicable to this scenario?

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.

 

[pierce15]

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?

 

[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)

 

[Orodruin]

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.

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.

 

[dauto]

https://www.physicsforums.com/showthread.php?t=622173

although as stated I cannot understand post #7...
mass doesn't mean that you can have annihilation to photons if your charge (vertex coupling) is zero... Or the poster meant something else...

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.

 

[pierce15]

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?

 

[phinds]

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?

For practical considerations, see post #2

 

[pierce15]

For practical considerations, see post #2

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.

 

[Bill_K]

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?

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

Just because it's unlikely doesn't mean it isn't possible or unworthy of consideration.

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

 

[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.

 

[phinds]

Just because it's unlikely doesn't mean it's impossible or unworthy of consideration...

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.

 

[phinds]

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.

Yes, that was the point of post #2

 

[Bill_K]

neutrinos interact hardly with matter, just imagine expecting two of them to interact with each other hehehe.

In some circumstances it can be important. http://www.researchgate.net/publication/234421034_Neutrino_annihilation_in_Type_II_supernovae says:

ABSTRACT: Neutrino-antineutrino annihilation into electrons and positrons can
deposit more than 10 to the 51st ergs above the neutrino-sphere of a
type II supernova.

 

[Bill_K]

neutrinos interact hardly with matter, just imagine expecting two of them to interact with each other hehehe.

Granted, it's a weak interaction, but there are several points to note... first of all the cross-section of the lowest order Feynman diagram for neutrino scattering is G²s - it grows rapidly with energy. The traditional "light-year of lead" remark is, I'm afraid, popsci woo. :shy: It pertains to low energy neutrinos, and does not take the energy dependence into account.

Secondly, thanks to the universality of the weak interactions, the scattering cross-section's about the same no matter what the neutrino hits - quark, electron, what-have-you.

Thirdly, the way we succeed in detecting neutrino interactions in the lab is to have a very large detector with a very large number of target particles. Neutrinos, of course,can't be focused. Their interactions become important in natural situations where the neutrino density is high.

 

[Orodruin]

To underline the fact of energy dependence of the cross section, the Earth is completely opaque to ultra-high energy neutrinos (say 1 PeV) such as the ones recently detected by IceCube.

When trying to detect dark matter annihilations in the Sun, the effect (absorbtion and tau regeneration) becomes relevant at even lower energies. Still, solar neutrinos from the usual solar fusion have far too low energy for scattering to matter although coherent forward scattering plays a key role in the flavor transitions of such neutrinos.

 

[TEFLing]

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}...

Neutrinos have no EM charge... And so they cannot generate photons? Having Weak charge only... They can only generate Weak bosons?

 

[ChrisVer]

Neutrinos have no EM charge... And so they cannot generate photons? Having Weak charge only... They can only generate Weak bosons?

Yes. Of course you can build up weird propagators that will give you as a final result photons, but that will be an extreme example.

 

[mfb]

This could work, not sure, but the alternatives don't look better. Now we just need Exawatt neutrino beams hitting each other?

 

[ChrisVer]

Is that before getting 2 Z0 and a box of fermion-propagator that results in the 2 gammas?

 

[mfb]

You can make a W or top triangle with a single Z.
Two Z with an additional box would be two loops?

 

[ChrisVer]

I think the Z--->gamma gamma by a triangle fermion loop doesn't work...
http://arxiv.org/pdf/1402.1203v1.pdf
That's why, although I thought about it at first, I didn't mention it. Or I am misunderstanding the first phrase after all in the 5.conclusions section.

 

[mfb]

Assuming this is also true for our off-shell Z: okay.

Anyway, all those diagrams have extremely small (or zero) probabilities. Way too small to look for them.

 

[ChrisVer]

Assuming this is also true for our off-shell Z.

That's why I left an open window for misunderstanding
I am not sure if these hold true for off-shell Z's too...

 

[Vanadium 50]

I think the Z--->gamma gamma by a triangle fermion loop doesn't work...

Right. It does not. The EWK SM is the theory that has no all-neutral couplings.