I Majorana Neutrinos: Different Physics than Oscillation

[Vanadium 50]

[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.

 

[Ken G]

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.

 

[Hornbein]

How the neutrino (or anything else) could be its own antiparticle is something I don't understand. Wouldn't it annihilate itself? :-)>.

 

[malawi_glenn]

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).

 

[Ken G]

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.

 

[malawi_glenn]

Z boson too

 

[Ken G]

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?

 

[Vanadium 50]

But what if it is discovered that over astrophysical scales, neutrinos can oscillate into antineutrinos?

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.,

 

[malawi_glenn]

Is there a reason Majorano postulated that fermions could do that too

Could do what?

 

[Ken G]

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.

 

[malawi_glenn]

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.

 

[PeterDonis]

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.

 

[PeterDonis]

I'm talking about what Fermilab was talking about

Do you mean this Fermilab page?

https://www.fnal.gov/pub/science/particle-physics/experiments/neutrinos.html

It's helpful to give specific references so we can see exactly what the reference says.

 

[PeterDonis]

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.)

 

[malawi_glenn]

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.

 

[Ken G]

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?

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.

 

[PeterDonis]

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.

 

[Ken G]

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.

"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?

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?

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.

 

[Ken G]

Do you mean this Fermilab page?

https://www.fnal.gov/pub/science/particle-physics/experiments/neutrinos.html

Yes, that's the one I cited it in the original thread.

It's helpful to give specific references so we can see exactly what the reference says.

That's exactly why I cited it.

 

[PeterDonis]

that's the one I cited it in the original thread

Ah, sorry, I didn't see the previous citation.

 

[malawi_glenn]

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.

Are you making a point that is not obvious?

Keep in mind that there are others reading this, with various education and knowledge.

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?

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.

 

[PeterDonis]

This paper looks like a more detailed treatment of the Majorana neutrino question by a Fermilab physicist:

https://arxiv.org/pdf/0903.0899.pdf

 

[Ken G]

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.

Neutrino flavors have nothing to do with whether or not neutrinos are their own antiparticles.

Obviously.

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.

 

[malawi_glenn]

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

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.

 

[Ken G]

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.

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).

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.

 

[Ken G]

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.

 

[malawi_glenn]

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.

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".

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"...

 

[Ken G]

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!

 

[Ken G]

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.

 

[Ken G]

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.

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.

 

[malawi_glenn]

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.

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".

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 :)

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.

 

[Ken G]

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.

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?

 

[malawi_glenn]

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.

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?)

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).

 

[Ken G]

"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.

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!

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?

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.

 

[malawi_glenn]

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.

 

[vanhees71]

Quantum field theory was born in 1926 by Pascual Jordan.

 

[Vanadium 50]

Let's take a strep back. After some thought, I think the best terminology is to define a neutrino as one that produces a (negative) lepton after a interaction (technically, a charged current interaction) and an anti-neutrino as one that produces a (positive) anti-lepton after a interaction.

So for this discussion, the particle is defined by what you detect, not what you (think you) made.

So, what does it mean for the neutrino and antineutrino to be the same particle? It means if I have any source of either neutrinos or antineutrinos, what I measure will be 50% neutrinos and 50% antineutrinos. Do we see that? No. That goes all the way back to the Reines-Cowan experiment. So the neutrino and antineutrino are different.

Now, a detour through theory. There are four fields: left-handed neutrino, right-handed neutrino, left-handed antineutrino and right-handed antineutrino. Left and right handed refers to the particle's chirality, which is hard to explain at I-level but is similar to helicity (spin dottend into momentum), the difference being that helicity is frame-dependent and chirality is defined to make it invariant. These are not physical particles; sometimes you will hear the term "Weyl fields".

The important thing is that in no theory can you "run past" a neutrino and make it an antineutrino.

The Higgs mechanism produces an interaction between the left and right handed neutrino feilds (which I will call "coupling: as shorthand in the future) to make a physical neutrino. Similarly, it can couple left-handed and right handed antineutrinos to make a physical antineutrino. These physical states have mass, and this is what we mean by the statement, "the Higgs mechanism gives fermions mass". This is called a "Dirac mass", and its what happens to electrons, quarks, etc.

However, because neutrinos are uncharged, they can also be connected the other way: left handed neutrino to right handed antineutrino and vice versa, (And more than one theory describes how this might happen) This is called a Majorana mass. But such a mass does not change the mixing: we covered that in the first three paragraphs: it predicts something different from what we actually observe.

Finally, there was a proposal that neutrinos can oscillate to antineutrinos. Theoretically, this is troublesome: it violates a number of conservation laws that we don't see violated anywhere else. Experimentally this is a problem, because we did not observe a"neutronization pulse" at the start of thge SN. Furthermore QM tells us for this oscillation to occur neutrinos and antineutrinos need different masses. However,m something called the CPT theorem says that Lorentz invariance prevents this.

So we're left with two possibilities:
1. We see antineutrinos, more or less as expected.
2. Our underdstanding of stellar collapse is grossly wrong (no neutronization), and QM is wrong, and SR is wrong, and all three are wrong in just the right way to conspire to give asignal that looks exactly like expected.

 

[PeterDonis]

I think the best terminology is to define a neutrino as one that produces a (negative) lepton after a interaction (technically, a charged current interaction) and an anti-neutrino as one that produces a (positive) anti-lepton after a interaction.

This appears to define neutrino and antineutrino in terms of flavor eigenstates. But the Fermilab paper that was linked to earlier defines them in terms of mass eigenstates, which makes the question of whether the neutrino and antineutrino are the same the question of whether the mass eigenstates have Majorana or Dirac masses. Is it possible that both definitions can be valid? I.e., that in terms of flavor eigenstates, neutrinos and antineutrinos are different, but that they could still have Majorana instead of Dirac masses?

 

[malawi_glenn]

@PeterDonis are you meaning this?
If Neutrinos got their mass solely by a Majorana mass-term, what would the flavor eigenstates (weakly interactive states) be?

 

[PeterDonis]

@PeterDonis are you meaning this?
If Neutrinos got their mass solely by a Majorana mass-term, what would the flavor eigenstates (weakly interactive states) be?

The answer to that question is relevant to what I asked. But what I asked is more general. The experimental results referred to by @Vanadium 50 would seem to indicate that a model in which neutrinos are their own antiparticles can't be right; but we also have a paper by a Fermilab physicist that says that physicists think it probable that neutrinos are Majorana fermions, which would mean they are their own antiparticles. I'm asking if it's possible that both claims could be correct--if there is some model in which, in some contexts, neutrinos are their own antiparticles, but in other contexts, they're not; or in which some neutrinos are their own antiparticles and others are not (for example, perhaps left-handed neutrinos aren't but right-handed neutrinos are).

 

[malawi_glenn]

Well there are models with Pseudo-Dirac neutrinos, e.g. where neutrinos get their mass from both a Dirac mass-term and from a Majorana mass-term.

This is perfect topic for @Orodruin long time since I saw him here :/

 

[Ken G]

Remarkable how quick that all happened, Schroedinger equation in 1925, QFT in 1926, and Dirac equation in 1928. What a crazy era that must have been. Fast forward to the current situation, where it's looking like we might need astroparticles, perhaps including a supernova in our galaxy, to take things to the next level-- our accelerators may be reaching their limits. (I recall it being said about 10 years ago that if all CERN is able to do is verify the Higgs, it will be a kind of disaster for laboratory particle physics because they will have discovered nothing beyond what was widely expected in the first place. It is starting to look like that is just what happened, though of course there is always hope for the upgrade.)

 

[PeterDonis]

recall it being said about 10 years ago that if all CERN is able to do is verify the Higgs, it will be a kind of disaster for laboratory particle physics because they will have discovered nothing beyond what was widely expected in the first place.

Actually, we did get at least one other valuable piece of information from the LHC, although in this case it was the opposite of what was widely expected: namely, the failure to find any evidence of supersymmetry. That was a blow to many physicists, but it's clearly a valuable piece of information since it rules out a whole swatch of proposed models.

 

[malawi_glenn]

Do not neglect the hadron physics findings too, for instance penta quarks.

And not finding evidence of SUSY, which was kinda expeted. Sure, not finding anything is not as near as exciting to finding things for sure.

 

[Vanadium 50]

This appears to define neutrino and antineutrino in terms of flavor eigenstates.

It does not. I said "lepton". The definition I picked does not even distinguish between mass and flavor eigenstates.

(I picked it partially to avoid introducing more potential sources of confusion)

But the Fermilab paper

Is not a paper. It is a description for the public of the neutrino program. And if I posted it, one of the Mentors would smack me silly for using a popularization and maybe toss in a few points.

Dirac vs. Majorana is a statement of how one links the theoretical Welv fields together to make physical particles. Any other words are just words. ("Is the fabric of spacetime polyester?"), The fact that we do not know which combination is right means that the properties of the two cases are very similar - one cannot use this as a rationale for the sort of gigantic effects proposed earlier.

 

[Ken G]

Let's take a strep back. After some thought, I think the best terminology is to define a neutrino as one that produces a (negative) lepton after a interaction (technically, a charged current interaction) and an anti-neutrino as one that produces a (positive) anti-lepton after a interaction.

If the neutrino is its own antiparticle, why is it important to have any distinction between neutrino and antineutrino?

So, what does it mean for the neutrino and antineutrino to be the same particle? It means if I have any source of either neutrinos or antineutrinos, what I measure will be 50% neutrinos and 50% antineutrinos. Do we see that? No. That goes all the way back to the Reines-Cowan experiment. So the neutrino and antineutrino are different.

Can you explain the reasoning there? There would seem to be additional assumptions required. For example, it sounds like you are saying that if neutrinos and antineutrinos were the same particle, then Reines-Cowan would have detected twice as many inverse beta decays as they did. That is a similar argument to what I said in the other thread about SN 1987A, that we should see twice as many as we thought. But I am now questioning that, because I believe there is an impact on the calculation of the cross section, where if one assumes neutrinos and antineutrinos are different, one gets half the cross section for inverse beta decay than if they were the same.

This would seem to be required by reciprocity-- if we imagine a bath of neutrinos, antineutrinos, electrons, and positrons all encountering protons and neutrons in thermal equilibrium, all rates must equal their inverse rates. So let's say in the standard picture, electrons encountering protons have a rate R_en for creating neutrinos, and positrons encountering neutrons have a rate R_pa for creating antineutrinos. Those rates, whatever they are, will equal the rates for neutrinos and antineutrinos to create electrons and positrons, in this equilibrium. But now if we say neutrinos and antineutrinos are the same particle, R_en and R_pa won't be any different, because those are just processes that make neutrinos of whatever type they are allowed to make, it's no advantage to those processes whether there are two types, but only one helps, or just one type.

But if R_en and R_pa are the same either way, and they balance the inverse processes, then the inverse beta decays all must have half the cross sections if neutrinos are the same as antineutrinos, because when the processes are seen in that direction, there are now twice as many viable candidates for making it happen, yet the total rates for it to happen must be the same. So I don't think you can distinguish thinking you have a source of 50% neutrinos and 50% antineutrinos coupled with thinking you can only detect the antineutrinos, via a process that respects the principle of reciprocity, from thinking you have a source with 100% undifferentiated neutrino/antineutrinos and you can detect them all by that same process.

The kind of process that would allow us to observe a factor 2 different from what we expect is a process that we think can make both a neutrino or an antineutrino, but that has only half the chance to occur if neutrinos and antineutrinos are the same thing. But the problem is, the whole point of distinguishing neutrinos and antineutrinos is what you are saying, we would distinguish them by what they produce, i.e., we would explicitly not let their inverse processes produce either type of particle, so we have no access to the kind of rate that would be sensitive to this distinction. In short, I don't see how the Reines-Cowan experiment tells us neutrinos are different from antineutrinos.

Now, a detour through theory. There are four fields: left-handed neutrino, right-handed neutrino, left-handed antineutrino and right-handed antineutrino. Left and right handed refers to the particle's chirality, which is hard to explain at I-level but is similar to helicity (spin dottend into momentum), the difference being that helicity is frame-dependent and chirality is defined to make it invariant. These are not physical particles; sometimes you will hear the term "Weyl fields".

The important thing is that in no theory can you "run past" a neutrino and make it an antineutrino.

The Higgs mechanism produces an interaction between the left and right handed neutrino feilds (which I will call "coupling: as shorthand in the future) to make a physical neutrino. Similarly, it can couple left-handed and right handed antineutrinos to make a physical antineutrino. These physical states have mass, and this is what we mean by the statement, "the Higgs mechanism gives fermions mass". This is called a "Dirac mass", and its what happens to electrons, quarks, etc.

That is a better explanation of the Higgs mechanism than I have ever seen (and no "Mexican hat" either)!

However, because neutrinos are uncharged, they can also be connected the other way: left handed neutrino to right handed antineutrino and vice versa, (And more than one theory describes how this might happen) This is called a Majorana mass. But such a mass does not change the mixing: we covered that in the first three paragraphs: it predicts something different from what we actually observe.

Here you must be cautious, because you are on the brink of calling the Majorana Collaboration (https://phys.org/news/2023-03-results-majorana-collaboration-neutrinoless-double-beta.html) a bunch of fools!

So we're left with two possibilities:
1. We see antineutrinos, more or less as expected.
2. Our underdstanding of stellar collapse is grossly wrong (no neutronization), and QM is wrong, and SR is wrong, and all three are wrong in just the right way to conspire to give asignal that looks exactly like expected.

No, those are quite clearly not the only possibilities, though I cannot state the case that a member of the Majorana Collaboration would provide here.

 

[Vanadium 50]

If the neutrino is its own antiparticle, why is it important to have any distinction between neutrino and antineutrino?

I gave a definition. We define things so we can talk about them sensibly. This thread has suffered from people treating wooly, non-precise (and certainly non-mathematical) statements as the gospel truth.

How can one say "the neutrino is its own antipartiocle" (and its not, no matter how many people repeat ity here) and expect it to have meaning without stating what a neutrino and and an antineutrino actually is?

 

[Vanadium 50]

Here you must be cautious, because you are on the brink of calling the Majorana Collaborationa bunch of fools!

That's incorrect, disrepsectful, unhelpful, unbcollegial and beneath you.

 

[Ken G]

I gave a definition. We define things so we can talk about them sensibly. This thread has suffered from people treating wooly, non-precise (and certainly non-mathematical) statements as the gospel truth.

Well, if your definition is "every neutrino that produces a positron was an antineutrino", then it is tautological that when SN 1987A produces a bunch of neutrinos that caused us to detect positrons, then those were antineutrinos. I can say that without knowing anything about neutrinos, because you chose to define them that way. But the real issue is, if the Majorano Collaboration detects neutrinoless double beta decay, will anyone even bother to use the term "antineutrino" ever again? Perhaps you see some reason they will, but I must say it seems to me the term will be retired immediately. That may be what I am missing-- perhaps you see reasons why, unlike for photons, it will make sense to keep thinking about neutrinos and antineutrinos even though they are Majorana particles.

How can one say "the neutrino is its own antipartiocle" (and its not, no matter how many people repeat ity here) and expect it to have meaning without stating what a neutrino and and an antineutrino actually is?

Again, making a tautological definition might not necessarily be a step forward. And I confess I do not have your deep understanding of these issues (I wish I did!), but that does not make you infallible. When you say "people" are repeating "the neutrino could be its own antiparticle", we should be clear we are not talking about "people here", as you imply. No, we are talking about "people" at Fermilab (https://neutrinos.fnal.gov/types/antineutrinos/ ) (my bold):

"Does that mean neutrinos and antineutrinos are the same thing, only differing in the particles (positrons or electrons) produced along with them? Scientists aren’t sure. There are many experiments under way or proposed to discover whether that’s the case."

and in the Majorana Collaboration
(https://enapphysics.web.unc.edu/res...particle, and therefore a “Majorana fermion.”):

"The terms “Dirac particles” and “Majorana particles” are used today to denote whether a particle and its antiparticle are distinct (Dirac) or the same (Majorana)."

 

[Ken G]

That's incorrect, disrepsectful, unhelpful, unbcollegial and beneath you.

Not at all, as the quotes I just gave so clearly demonstrate. You are pretending that the only people who are talking about neutrinos and antineutrinos being the same particle, i.e., neutrinos being their own antiparticle, are people "on here," and that is what is uncollegial here. I'm sure we all appreciate your insights into neutrinos, I for one simply don't understand why you claim we know things about them that many other experts have quite clearly stated we do not in fact know. Science is about discovery, it is about being honest about what could yet surprise us.