Majorana Neutrinos: Different Physics than Oscillation

In summary: What does Majorano mean by 'its own antiparticle'?In summary, Majorana postulated that particles like neutrinos could have a property called "its own antiparticle" which would make their wavefunction transform into itself after the CPT transformation. However, this idea is not currently included in the Standard Model, and is only possible for particles that are electrically neutral.
  • #1
Vanadium 50
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[Moderator's note: Spin-off from previous thread due to topic change.]

A Majorana mass term is completely different physics from a neutrino oscillating to an antineutrino.
 
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  • #2
I'm not talking about oscillations, I'm talking about what Fermilab was talking about: a neutrino and an antineutrino being exactly the same particle. That would make it a Majorana particle, as I'm saying.
 
  • #3
How the neutrino (or anything else) could be its own antiparticle is something I don't understand. Wouldn't it annihilate itself? :-)>.
 
  • #4
Hornbein said:
How the neutrino (or anything else) could be its own antiparticle is something I don't understand. Wouldn't it annihilate itself? :-)>.
No, nothing can annihilate itself. It would be possible for two neutrinos to interact and create a Z-boson for instance (if they are energetic enough).
 
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  • #5
Hornbein said:
How the neutrino (or anything else) could be its own antiparticle is something I don't understand. Wouldn't it annihilate itself? :-)>.
Also, bear in mind that photons are their own antiparticle, so we have seen this before.
 
  • #6
Z boson too
 
  • #7
Is there a reason Majorano postulated that fermions could do that too, and/or is there some reason why it might only be bosons that do this?
 
  • #8
Ken G said:
But what if it is discovered that over astrophysical scales, neutrinos can oscillate into antineutrinos?

Ken G said:
I'm not talking about oscillations

Perhaps this will help you understand why people are puzzled.

Majorana mass terms are not the same as either neutrinos oscillating into antineutrinos, nor neutrinos and antineutrinos being the same particle. If you want to discuss that, I recommend a well-posed question in the appropriate section.,
 
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  • #9
Ken G said:
Is there a reason Majorano postulated that fermions could do that too
Could do what?
 
  • #10
Be their own antiparticle. So far only bosons are known to be able to do that, so I'm wondering if Majorano had some particular reason to wonder if fermions could do it too.
 
  • #11
Ken G said:
Be their own antiparticle. So far only bosons are known to be able to do that, so I'm wondering if Majorano had some particular reason to wonder if fermions could do it too.
This idea is from 1937, he was playing around with how the Dirac equation would look like for electrically neutral spin 1/2 particles. And it turns out that it qualifies as being its own antiparticle, meaning that the wavefunction will be transformed to itself after CPT (and not into -1 times itself as is the case for electrons/positrons). You seem to revert things here, Majorana was not thinking "hmm what if neutrinos are their own antiparticle", that came out as a bonus.

Note that Majorana dissapeared in 1938. He had no idea about "modern" QFT or modern particle physics. Neutrinos was not a bread and butter topic back then. Today... it is a different story. A Majorana mass term can not easliy be put into the standard model lagrangian because there is no right-handed neutrino field there. Though we can add it via the "Weinberg operator" which can be seen as some kind of "effective field theory" term - but it invites us to look for physics beyond the standard model, which some researchers think is an attractive thing to do (like myself).

"Being able to do that" sounds like you think particles have some kind of will power :)

Also note, that a particle needs to be electrically neutral to have the possibility to be its own antiparticle. That leaves us with four candidates (so far): neutrino, Z, photon, Higgs.

Finally, particle v.s. antiparticle is just a name for some property. You have too look at the mathematical properties to make sense of the physics, otherwise it is just playing with words. Like planets, does it really matter if we call pluto a planet or not - the physics is still the same.

TL;DR fermions can be their in own antiparticle (whatever that means) in theory. They (neutrino as the candidate) can not be it in the standard model for symmetry reasons.

p.s. this is getting off-topic as h**l, I suggest we ask moderator to split this thread into a new one in the "high energy physics" subforum where this discussion belongs.
 
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  • #12
malawi_glenn said:
p.s. this is getting off-topic as h**l, I suggest we ask moderator to split this thread into a new one in the "high energy physics" subforum where this discussion belongs.
Done.
 
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  • #14
malawi_glenn said:
he was playing around with how the Dirac equation would look like for electrically neutral spin 1/2 particles. And it turns out that it qualifies as being its own antiparticle
I don't think these two are quite the same. In the current Standard Model, neutrinos are electrically neutral, but they are Dirac fermions, not Majorana fermions. (They have nonzero weak isospin, so they do have a quantum number that is of opposite sign in the antiparticle, it's just not electric charge.)
 
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  • #15
PeterDonis said:
I don't think these two are quite the same.
Perhaps I was not clear on that. Majorana only considered electric charge. And he constructed an equation of motion for such fermion where it turns out that if it is satisfied, that fermion is its own antiparticle. But, electrically neutral fermions also satisfies Dirac equation, where they are not their own antiparticle.

Today, we have more quantum numbers to take into consideration. In the (SM) standard model (Glashow, Weinberg, Salam) there is weak isospin, hypercharge and color. Electrical charge is "just" a byproduct of the spontanous symmetry breaking of the electroweak sector. This forces the neutrinos in the SM to be Dirac fermions.

Note: Lepton and Baryon numbers are not forced upon the standard model construction, they are accidental symmetries due to the gauge structure and the field content of the SM.
 
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  • #16
Vanadium 50 said:
Perhaps this will help you understand why people are puzzled.
They are puzzled because you are juxtaposing two of my posts completely out of context. That should have been obvious from the original thread-- the conversation moved away from oscillation and into Majorana neutrinos. How could a particle that is exactly the same as its antiparticle oscillate between two identical states?
Vanadium 50 said:
Majorana mass terms are not the same as either neutrinos oscillating into antineutrinos, nor neutrinos and antineutrinos being the same particle. If you want to discuss that, I recommend a well-posed question in the appropriate section.,
Once again you have chosen to put words in my mouth. Let me state it again: according to Fermilab, there is interest in the question of whether neutrinos are their own antiparticles (which would make them Majorana neutrinos, according to: https://www.npl.washington.edu/majorana/majorana-experiment, where I quote: "If this decay occurs, the neutrino is its own antiparticle, or a Majorana particle." Now what part of that sounds like neutrinos and antineutrinos not being the same particle?

Let me also repeat the whole reason this came up. You stated that we essentially already know that the neutrinos detected from SN 1987A were actually antineutrinos and not regular neutrinos, because the sensor used was much more sensitive to antineutrinos. You stated that there is hardly any point in testing that, since so much physics would need to be wrong for that not to be true. Obviously, the Majorana Collaboration I cited does not agree with you. What I said was, we don't actually know that our sensors are more sensitive to antineutrinos, because we don't know they aren't Majorana particles. If they are, then our Cerenkov detectors will detect twice as many neutrinos as you expect. This is worth testing, but we did not quite have the statistics from SN 1987A. We need a few more, or one in our own galaxy, to be sure. That is how science works, not by juxtaposing unrelated statements from two totally different places in a thread.
 
  • #17
Ken G said:
How could a particle that is exactly the same as its antiparticle oscillate between two identical states?
Neutrino oscillations are not between neutrinos and antineutrinos. They are between neutrinos of different flavors (electron, muon, tau). Neutrino flavors have nothing to do with whether or not neutrinos are their own antiparticles. You could have multiple Majorana neutrinos of different flavors, and they could oscillate between different flavors, as long as the flavor eigenstates weren't the same as the mass eigenstates.
 
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  • #18
malawi_glenn said:
This idea is from 1937, he was playing around with how the Dirac equation would look like for electrically neutral spin 1/2 particles. And it turns out that it qualifies as being its own antiparticle, meaning that the wavefunction will be transformed to itself after CPT (and not into -1 times itself as is the case for electrons/positrons). You seem to revert things here, Majorana was not thinking "hmm what if neutrinos are their own antiparticle", that came out as a bonus.

Note that Majorana dissapeared in 1938. He had no idea about "modern" QFT or modern particle physics. Neutrinos was not a bread and butter topic back then. Today... it is a different story. A Majorana mass term can not easliy be put into the standard model lagrangian because there is no right-handed neutrino field there. Though we can add it via the "Weinberg operator" which can be seen as some kind of "effective field theory" term - but it invites us to look for physics beyond the standard model, which some researchers think is an attractive thing to do (like myself).
Thank you, that is interesting information.
malawi_glenn said:
"Being able to do that" sounds like you think particles have some kind of will power :)
So you dislike colorful language? Isn't elementary particles the field with "strangeness" and "charm"? Seriously now?
malawi_glenn said:
Also note, that a particle needs to be electrically neutral to have the possibility to be its own antiparticle. That leaves us with four candidates (so far): neutrino, Z, photon, Higgs.

Finally, particle v.s. antiparticle is just a name for some property. You have too look at the mathematical properties to make sense of the physics, otherwise it is just playing with words. Like planets, does it really matter if we call pluto a planet or not - the physics is still the same.
That is correct, words have meaning, and mathematical words have mathematical meaning. Are you making a point that is not obvious?
malawi_glenn said:
TL;DR fermions can be their in own antiparticle (whatever that means) in theory. They (neutrino as the candidate) can not be it in the standard model for symmetry reasons.
Yes I know that, that's why I didn't ask that. What I did ask, has not been answered. If you don't know, that's fine, and I appreciate the information you do know, but there's no point in stating the obvious.
 
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  • #20
Ken G said:
that's the one I cited it in the original thread
Ah, sorry, I didn't see the previous citation.
 
  • #21
Ken G said:
So you dislike colorful language? Isn't elementary particles the field with "strangeness" and "charm"? Seriously now?
This is a science forum. Being able to "do" something could suggest that there is some underlying mechanism.
Ken G said:
Are you making a point that is not obvious?
Keep in mind that there are others reading this, with various education and knowledge.
Ken G said:
What I did ask, has not been answered.
You have asked so many things that it is hard to keep track of everything.
What question(s) of your have you not found a satisfactory answer to?
Ken G said:
but there's no point in stating the obvious.
It is, because (1) I have no idea what you know and don't know. Obviously you are not a particle physcists (like me) so why is it so bad that I provide some extra information for clarity? (2) there are other people reading this, with various background. This is public information, not a private conversation.
 
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  • #23
PeterDonis said:
Neutrino oscillations are not between neutrinos and antineutrinos. They are between neutrinos of different flavors (electron, muon, tau).
Yes, I know that, the question was rhetorical. It was to point out the absurdity of @Vanadium 50 claiming the topic of neutrinos oscillating between antiparticles and particles is the same issue as them being Majorana particles. Nobody is talking about neutrino oscillations at this point, other than Vanadium 50-- the issue is if they are Majorana particles.
PeterDonis said:
Neutrino flavors have nothing to do with whether or not neutrinos are their own antiparticles.
Obviously.
PeterDonis said:
You could have multiple Majorana neutrinos of different flavors, and they could oscillate between different flavors, as long as the flavor eigenstates weren't the same as the mass eigenstates.
Yes, I know all that. I realize you can't know what I do and do not know, so you are just giving helpful information. Had I not already known all that, it would have been quite helpful, and I realize there might also be other people reading the thread who might not know that.
 
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  • #24
PeterDonis said:
This paper looks like a more detailed treatment of the Majorana neutrino question by a Fermilab physicist:

https://arxiv.org/pdf/0903.0899.pdf
It is always good to link to the papers page on arxiv for additional information (like journal publication etc) instead of just the pdf
https://arxiv.org/abs/0903.0899
Ken G said:
Nobody is talking about neutrino oscillations at this point
To be fair, that original thread derailed into to several other discussions pretty quick. It is hard to keep track of things when it does so. That is why this thread was created for instance. Try to stay on topic, if you have a new question - start a new thread instead.
 
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  • #25
malawi_glenn said:
This is a science forum. Being able to "do" something could suggest that there is some underlying mechanism.
I withdraw the expression if it offends.
malawi_glenn said:
You have asked so many things that it is hard to keep track of everything.
What question(s) of your have you not found a satisfactory answer to?
Why Majorana was inspired to speculate that neutrinos might be their own antiparticle. I believe it has to do with their tiny mass. In supersymmetry, particles with tiny mass might be paired with particles of large mass, the latter being hard to create and therefore hard to discover. But I don't know if supersymmetry was around in Majorana's day (sorry to hear he disappeared).
malawi_glenn said:
It is, because (1) I have no idea what you know and don't know. Obviously you are not a particle physcists (like me) so why is it so bad that I provide some extra information for clarity? (2) there are other people reading this, with various background. This is public information, not a private conversation.
Fair enough, I reacted with frustration about being told a bunch of things I regard as obvious. I should not have gotten defensive, you cannot know this.
 
  • #26
malawi_glenn said:
It is always good to link to the papers page on arxiv for additional information (like journal publication etc) instead of just the pdf
https://arxiv.org/abs/0903.0899

To be fair, that original thread derailed into to several other discussions pretty quick. It is hard to keep track of things when it does so. That is why this thread was created for instance. Try to stay on topic, if you have a new question - start a new thread instead.
The topic was if what we detected from supernovae were antineutrinos. @Vanadium 50 claimed there was no point in wondering this, as it was already well known they were antineutrinos. So it was completely on topic to discuss whether or not we really do know that.
 
  • #27
Ken G said:
I withdraw the expression if it offends.
No one is offended, but as I wrote, this might suggest that you thought there is a mechanism behind the "ability" to be its own antiparticle.
Ken G said:
Why Majorana was inspired to speculate that neutrinos might be their own antiparticle. I believe it has to do with their tiny mass.
He never did. I did write that he was only interested in seeing what the equations of motion would be for an electrical neutral fermion. I am not gonna repeat what I wrote. But let me be crystal clear, to answer this question: "He never did speculate about that".
Ken G said:
So it was completely on topic to discuss whether or not we really do know that.
Yes that was a good question and it was on topic. But then it went quickly from "what about neutrinos and anti-neutrinos could oscillate over galactic scales" "why did Majorana speculate about neutrinos being their own anti-particle"...
 
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  • #28
malawi_glenn said:
It is always good to link to the papers page on arxiv for additional information (like journal publication etc) instead of just the pdf
https://arxiv.org/abs/0903.0899
That is indeed an interesting paper, thank you both for finding it. Note that it points to certain attractive aspects of imagining that neutrino mass is indeed of the Majorana type. Also note, that if this is true, then the detections of the 1987A supernova detected twice as many neutrinos as the standard model predicted they would, but this was not caught because of the uncertainties (low statistics and uncertain models). I think that is all quite relevant to the original thread, but I agree that the theory of neutrinos is probably deeper than what was originally being asked there!
 
  • #29
PeterDonis said:
Ah, sorry, I didn't see the previous citation.
It's all right, no one can be expected to search through a whole thread. Besides, you entered both the Fermilab citation, and the paper, into this thread, both of which are helpful here.
 
  • #30
malawi_glenn said:
He never did. I did write that he was only interested in seeing what the equations of motion would be for an electrical neutral fermion would be. I am not gonna repeat what I wrote. But let me be crystal clear, to answer this question: "He never did speculate about that".
Thanks for the clarification of what you wrote before. That does indeed answer the question! But gives us a new one: why do people today like the idea that neutrinos are Majorana particles! But that's what's covered in that paper. On the surface, it is surprising that anyone would like that idea, because it ruins Lepton number conservation.
malawi_glenn said:
Yes that was a good question and it was on topic. But then it went quickly from "what about neutrinos and anti-neutrinos could oscillate over galactic scales" "why did Majorana speculate about neutrinos being their own anti-particle"...
In any event, the real issue was whether or not we already know that we should regard the SN 1987A detections as antineutrinos because that's all we can detect (i.e., 1/2 the full flux). I believe the answer to that, at least, is completely clear: we do not in fact know that, in contradiction to claims made by some on that thread.
 
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  • #31
Ken G said:
I think that is all quite relevant to the original thread, but I agree that the theory of neutrinos is probably deeper than what was originally being asked there!
Also detection mechanisms should have deserved a thread on its own. Also, the original thread was in the astrophysics section. Discussion of detection of elementary particles should be in this subforum.
Ken G said:
Sorry, but I still see that as an "ability"
I don't, same with electric charge. The electron does not have the "ability to have electric charge" it just "has electric charge".
Ken G said:
But gives us a new one: why do people today like the idea that neutrinos are Majorana particles! But that's what's covered in that paper. On the surface, it is surprising that anyone would like that idea, because it ruins Lepton number conservation.
Why is lepton number conservation such a precious thing? Lepton number is violated in neutrino oscillations between flavor eigenstates anyway :)
Ken G said:
Please don't repeat @Vanadium 50 's irrelevant juxtoposition.
I am not, I simply rephrased some of your questions in that thread that could be considered off-topic therein.
 
  • #32
malawi_glenn said:
Also detection mechanisms should have deserved a thread on its own. Also, the original thread was in the astrophysics section. Discussion of detection of elementary particles should be in this subforum.
The whole thread was about detection mechanisms, that was literally the question.
malawi_glenn said:
Why is lepton number conservation such a precious thing? Lepton number is violated in neutrino oscillations between flavor eigenstates anyway :)
Why don't the flavors preserve lepton number?
 
  • #33
Ken G said:
The whole thread was about detection mechanisms, that was literally the question.
"Do supernovae generate neutrinos or antineutrinos?" is a question about what is being produced. That question of that thread was answered in the first reply... namely that both are genereted/produced in supernovae according to our theories.

Ken G said:
Why don't the flavors preserve lepton number?
Lepton number as a whole is conserved yes. But not Lepton flavor number.
If neutrinos would be massless, we would have more accidental symmetries in the SM, ##L_e##, ##L_\mu## and ##L_\tau##. Now, we "just" have Lepton number. I should have been more clear.

To get back to my question then (you seem very keen on others to answer your questions but are you equally proficient in answering questions directed to you?)
Ken G said:
On the surface, it is surprising that anyone would like that idea, because it ruins Lepton number conservation.
Why is lepton number conservation such a precious thing? (after all, it is just an accidental symmetry - other accidental symmetries are broken, such as Lepton flavor number).
 
  • #34
malawi_glenn said:
"Do supernovae generate neutrinos or antineutrinos?" is a question about what is being produced. That question of that thread was answered in the first reply... namely that both are genereted/produced in supernovae according to our theories.
You are assuming the question was presaged with, "in our theories...". It could have just as easily been presaged with "According to observation..." Why would you assume the former and not the latter? If you look back at the OP, you'll see the issue was much more about the detections than the theories. But this isn't a productive channel of discussion, what is interesting is that we really don't know what kind of neutrinos we are detecting because fundamental uncertainties remain about the nature of the neutrino.
malawi_glenn said:
Lepton number as a whole is conserved yes. But not Lepton flavor number.
Well, I would have thought that lepton number, as a whole, would be a convenient thing to conserve!
malawi_glenn said:
If neutrinos would be massless, we would have more accidental symmetries in the SM, ##L_e##, ##L_\mu## and ##L_\tau##. Now, we "just" have Lepton number. I should have been more clear.
I take your meaning-- we have lost so many conserved quantities already, what's one more?
malawi_glenn said:
To get back to my question then (you seem very keen on others to answer your questions but are you equally proficient in answering questions directed to you?)

Why is lepton number conservation such a precious thing? (after all, it is just an accidental symmetry - other accidental symmetries are broken, such as Lepton flavor number).
I presumed your question was rhetorical. You really want my opinion on the value of lepton number conservation? I don't even want my opinion on the value of lepton number conservation.
 
  • #35
Ken G said:
But I don't know if supersymmetry was around in Majorana's day
I forgot to reply on this one. No it was not, quantum field theory was basically born around that era. Supersymmetry was devoloped in the early 1970's.
 
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