What exactly are anti-neutrinos?

[Yashbhatt]

According to the the definition of anti-particles, they are particles with same mass but opposite charge. Neutrinos by definition have no charge. So, how can it have an anti-particle?

 

[jedishrfu]

Wikipedia has a brief description:

http://en.wikipedia.org/wiki/Anti-neutrino#Antineutrinos

They come out of beta particle decay. So if the beta particles is an electron then you see anti-neutrinos and if the beta particle is a positron then you see neutrinos.

http://en.wikipedia.org/wiki/Beta_decay

Its possible though not proven that they are in fact the same particle (aka Majorana particle)

 

[Bill_K]

According to the the definition of anti-particles, they are particles with same mass but opposite charge.

This is true, but not a complete definition. There are several different things called "charge", and the antiparticle has the opposite value for each.

Gluons, for example, although they have zero electric charge, they also carry color charge, and so a gluon and its antiparticle have opposite color charge.

Likewise there are neutral K-mesons K⁰ and K⁰-bar which carry opposite strangeness.

 

[Yashbhatt]

So, can we have something like anti-neutron?

 

[jedishrfu]

So, can we have something like anti-neutron?

Yes.

http://en.wikipedia.org/wiki/Anti-neutron

 

[Yashbhatt]

Thanks. I did not google anti-neutrino before posting.

 

[craigi]

Yes.

http://en.wikipedia.org/wiki/Anti-neutron

Thanks. I did not google anti-neutrino before posting.

There's some confusion here. A neutrino and a neutron aren't the same thing.

 

[Yashbhatt]

Sorry, I meant anti-neutron.

 

[ChrisVer]

Apart from all, there are studies which propose that the neutrino and antineutrino are the same particle... Those studies consider the neutrino as a Majorana particle (and that's something only neutrally charged particles can be)... That search is based in observing neutrinoless double beta decay... Unfortunately, we haven't been able to verify that nature so far...

 

[Yashbhatt]

But I think there are arguments against it being a Majorana partcile. See http://physics.stackexchange.com/questions/111358/what-exactly-is-an-anti-neutrino

 

[Bill_K]

But I think there are arguments against it being a Majorana partcile. See http://physics.stackexchange.com/questions/111358/what-exactly-is-an-anti-neutrino

Is he citing evidence against Majorana neutrinos, or evidence against leptogenesis? And how strong is it? If I were you, I'd ask the responder for the specific reference.

 

[Vanadium 50]

Stackexchange is not a valid source.

 

[Yashbhatt]

Why?

 

[jim mcnamara]

On stackexchange, like here at PF, anyone can post. Stackexchange depends on a different method of controlling posts by people who are expounding personal research or pet theories. They hope someone in the community will correct the off-the-mark post. PF depends on physicists, who try to remove crackpot posts. And there is active correction. Bill K is being polite. "rob" is expounding what appears to be a pet theory. Note that he has about 640 "status points". The point system is supposed to let you in on the believable-ness of the poster in general.

 

[Yashbhatt]

Ok. I get what you say. I just mentioned the link because I found a more detailed answer there.

 

[phinds]

Ok. I get what you say. I just mentioned the link because I found a more detailed answer there.

"detailed" does not necessarily have anything to do with "correct". Check your sources. There's a list here at PF for those that are good/bad.

 

[ChrisVer]

I never said that neutrinos are Majorana particles... What I said is that there is a lot of work being done in studying that theory and confirming it... and as I pointed out, experiments have been fruitless until now (equivalently said, there's not been experimental verification of physics beyond the Standard Model, only indirect signs). If we verify the neutrinoless double beta decay, there will be a problem with the accidental symmetries of Baryon and Lepton numbers (they will be violated) which appears in the Standard Model.
The big thing is the difficulties in measuring that experimentally (as neutrinos have always been), the rarity of the events (ordinary double beta decay is very slow itself) and of course the clearness of the subject material and "technology" .

 

[Orodruin]

Neutrino oscillations in themselves are an observation of physics beyond the Standard Model since lepton flavor is conserved in the SM. (Gravity also seems to be a fairly established observation of BSM physics.) Furthermore, B and L are not separate accidental symmetries of the Standard Model but B+L may be violated through non-perturbative effects. Anyway, breaking B-L would be good news for the possibility of generating the baryon asymmetry of the Universe.

 

[mfb]

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]

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]

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]

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?

[...] Anyway, breaking B-L would be good news for the possibility of generating the baryon asymmetry of the Universe.

 

[Orodruin]

@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]

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]

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]

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]

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.