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What exactly are anti-neutrinos?

by Yashbhatt
Tags: antineutrinos
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mfb
#19
May6-14, 04:31 PM
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I think you mixed + and - here. Many models propose B and L non-conservation, but a conserved B-L (which implies a non-conserved B+L).
ChrisVer
#20
May6-14, 04:36 PM
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I am not sure whether B and L can be violated in the Standard Model... in which case?
As for the neutrino oscillations, that's not something the SM could not deal with. I guess it should be better called extension of the standard model rather than physics beyond it... PMNS is fine with doing that job.
Bill_K
#21
May6-14, 04:48 PM
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Quote Quote by mfb View Post
I think you mixed + and - here. Many models propose B and L non-conservation, but a conserved B-L (which implies a non-conserved B+L).
He's got it right, hasn't he? Sphalerons conserve B - L and violate B + L, which is what he said.
mfb
#22
May6-14, 04:49 PM
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Quote Quote by ChrisVer View Post
I am not sure whether B and L can be violated in the Standard Model... in which case?
That depends on what exactly you call SM. Example

As for the neutrino oscillations, that's not something the SM could not deal with. I guess it should be better called extension of the standard model rather than physics beyond it... PMNS is fine with doing that job.
Some physicists even see it as part of the SM now.


@Bill_K: Why would you need B-L violation for baryogenesis?
Quote Quote by Orodruin View Post
[...] Anyway, breaking B-L would be good news for the possibility of generating the baryon asymmetry of the Universe.
Orodruin
#23
May6-14, 05:22 PM
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Quote Quote by mfb View Post
@Bill_K: Why would you need B-L violation for baryogenesis?
Since B+L is already broken by sphalerons which will tend to wipe out any asymmetry you create unless you also break B-L. You can do this by introducing processes breaking either B or L (which is what Majorana neutrinos do). If you break L you can generate a B-L asymmetry which you can partially convert into an asymmetry in B using sphalerons. This is baryogenesis via leptogenesis, which is quite popular to try to achieve in seesaw models of neutrino mass. Standard Sakharov conditions obviously still apply.

Edit: The problem with the PMNS is that it is introduced either through Majorana or Dirac neutrinos and the oscillation phenomenology does not care which. I still would not consider it part of the Standard Model since it is simply a phenomenological construct that happens to appear for both possibilities without actually needing to refer to the underlying lagrangian. That being said, it is obviously an important ingredient in the extension of the SM.
Yashbhatt
#24
May7-14, 04:15 AM
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Rob at stackexchange said the same thing that although they neutrinos and anti-neutrinos are different, there are experiments going on to find out if they are the same.
andrien
#25
May7-14, 05:43 AM
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Quote Quote by Orodruin View Post
The problem with the PMNS is that it is introduced either through Majorana or Dirac neutrinos and the oscillation phenomenology does not care which.
This is wrong. The oscillation phenomenology do care for neutrino nature, a Majorana neutrino contributes phases to PMNS matrix, but it does not affect oscillation mechanism.

EDIT-
Since B+L is already broken by sphalerons which will tend to wipe out any asymmetry you create unless you also break B-L.
I am actually bothered with this statement. Why would you necessarily need B-L non-conservation if you are generating baryon asymmetry using lepton number non-conservation? It is possible in principle that B-L may be conserved like it having some conserved charge for an extended gauge group.
Orodruin
#26
May7-14, 06:09 AM
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Quote Quote by andrien View Post
This is wrong. The oscillation phenomenology do care for neutrino nature, a Majorana neutrino contributes phases to PMNS matrix, but it does not affect oscillation mechanism.
This is exactly what I said ... How do you propose there is a difference between oscillation phenomenology not caring about Majorana phases and them not affecting oscillations?
Orodruin
#27
May7-14, 06:24 PM
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Quote Quote by andrien View Post
I am actually bothered with this statement. Why would you necessarily need B-L non-conservation if you are generating baryon asymmetry using lepton number non-conservation? It is possible in principle that B-L may be conserved like it having some conserved charge for an extended gauge group.
If you do not break it sphalerons will strive to erase any B+L you manage to create as long as they are active. I believe there are some scenarios of leptogenesis that work without B-L violation, but those would typically include things like the RH neutrinos decaying more or less at the same time as sphalerons are turning off or hiding a non-zero lepton number in right-handed leptons, which the sphalerons do not see. Thus, it is not an absolute necessity and people seem to find ways around it that are more or less contrived.

In the example of vanilla leptogenesis sphalerons violate B+L (while conserving B-L) and the Majorana nature violates L (while conserving B). Since you can change L without changing B, this means you have violation of B-L. In fact, you cannot gauge B-L if you have Majorana neutrinos - unless you break it, which seems quite popular as breaking it by small amounts may lead to inverse seesaws and similar low-energy seesaw scenarios.


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