I Majorana Neutrinos: Different Physics than Oscillation

[Ken G]

Not necessarily. In the seesaw mechanism, as I understand it, the actual physical neutrinos (the ones we detect in detectors) are neither the pure Majorana ones nor the pure Dirac ones; they are combinations of them.

Well that brings us back to the original issue, if the neutrino beam experiments force neutrinos to be mostly Dirac-type, or if they could still be essentially pure Majorana. I think the quotes I just gave make it pretty clear that there are still a lot of neutrino experts that think neutrinos could be essentially pure Majorana (after all, that paper was talking about dipole moment matrices that have zero diagonal elements, not diagonal elements that are a tiny bit smaller than you'd expect, etc.). Yet of course they know about the beam experiments, so it seems to me there must be a way that a neutrino that is flat out "its own antiparticle" (based on ability to annihilate, CPT symmetry, etc.) can still create mostly only negative leptons if it came from negative leptons. It must therefore come from some property other than lepton number, that is the possibility we have not taken into account.

 

[Vanadium 50]

Can you be more specific about which experiment this refers to?

Lots and lots. MINOS did it, and before that CCDR did it, and it goes all the way back to the CERN Wide-Band-Beam in the 70's and early 80's, and experiments like CDHS. Van der Meer invented the horn in the 1960's.

Here is one table from the latest Review of Particle Properties:

You can get a slightly more comprehensive view by looking at the experiments who have results on the top and bottom lines. (Also note that the nu cross-section is larger than nubar, in contrast to some claims made upthread)

 

[PeterDonis]

there must be a way that a neutrino that is flat out "its own antiparticle" (based on ability to annihilate, CPT symmetry, etc.) can still create mostly only negative leptons if it came from negative leptons

I looked through the "Publications" section of the Majorana Collaboration website in the hope of finding a paper that dealt with any proposed theoretical basis for such a possibility, but couldn't find one.

 

[PeterDonis]

Lots and lots.

Are all of these experimental results possible if neutrinos are pure Majorana fermions (i.e., no Dirac masses, just Majorana masses)? It doesn't seem like they would be, since pure Majorana fermions can't have a meaningful lepton number, but these results certainly seem to indicate that neutrinos do. Unless there is some other mechanism involved, but I'm not sure what it could be, and so far I've found no discussion of any such thing in the literature I've looked at.

 

[Ken G]

Exactly my question as well. I think the answer must be "yes", given all the work being done on Majorana neutrinos, but I don't know the mechanism proposed to essentially "hide" a lepton number like that.

 

[Ken G]

You can get a slightly more comprehensive view by looking at the experiments who have results on the top and bottom lines. (Also note that the nu cross-section is larger than nubar, in contrast to some claims made upthread)

Not sure what "upthread" statement is being referenced, but is it not apparent that the cross-sections could be different (here by a factor remarkably close to 2) if it is true that the target of what is being called a neutrino is a neutron, and the target of what is being called an antineutrino is a proton? So if you are implying that a different cross section requires a different neutrino, I would point to the differing target. What seemed more telling is your point that one does not see both of those reactions happening for a given beam of neutrinos, which would seem to require that if they are Majorana neutrinos, they need internal degrees of freedom that are not lepton number. But that would seem to be all they would need.

 

[malawi_glenn]

This would be a weak interaction decay conserving lepton number, correct? So if we're getting positive pions, those would decay into positive muons (antimuons, lepton number -1) and muon neutrinos (lepton number +1), whereas if we're getting negative pions, those would decay into negative muons (lepton number +1) and muon antineutrinos (lepton number -1), correct?

In the context of the standard model (pure Dirac-neutrinos) yes.

"if neutrinos are pure Majorano fermions, will there be any value in the term antineutrino, or will the term be retired." No one has yet answered that

I thought you could read between the lines in my posts. "No" (if you ask me or basically every theoretician, what experimentalists would call things - I do not know).

 

[Ken G]

Thanks for your answer, at least we have one theorist who would say that, and a claim that most any theorist would. I'm not sure if the implied separation between theorists and experimentalists is really valid though, it would be pretty strange for experimentalists to see value in a term that theorists don't. But if we take it as likely that there is not value in the term "antineutrino" if neutrinos are pure Majorana particles, we now have the issue of whether or not neutron beam experiments have already ruled that out. It looks like the answer to that has to be "no, otherwise not so many people would still be doing research on neutrinos as pure Majorana particles." Combining those two conclusions takes us all the way back to the beginning of the thread: we do not know that neutrinos are not their own antiparticles, and it would not invalidate a long list of everything else we know if they were. But we still have to understand how all that can be consistent with @Vanadium 50 's point that neutrino beams built from antileptons tend to only produce antileptons in the detectors, there has to be a way for Majorana neutrinos to pull off that trick.

 

[malawi_glenn]

at least we have one theorist who would say that, and a claim that most any theorist would.

I base this on the analogy that we do not speak of "antiphotons", "anti-Z" nor "anti-higgs".

 

[vanhees71]

I tried to avoid getting into the question of mass vs. flavor eigenstates. Neutrinos are leptons, anti-neutrinos are anti-leptons. Yes, we can go into mroe detail and talk about individual flavors, but I deliberately avoided doing this.

The point is that this is not yet unanimously decided for neutrinos. Whether they are Majorana or Dirac particles is subject of active research, e.g., by the search for neutrinoless double $\beta$-decay. For a pretty comprehensive review on Majorana vs. Dirac neutrinos, see

https://arxiv.org/abs/1412.3320v1

 

[Vanadium 50]

Are all of these experimental results possible if neutrinos are pure Majorana fermions

Yes,

If that were not the case, Dirac or Majorana would not be an open issue.,

It doesn't seem like they would be, since pure Majorana fermions can't have a meaningful lepton number

You're falling back on the popularization phrase "Majorana neutrinos are their own antiparticles". The question is not "is there lepton number violation" but "how much" (and maybe "do we care?") The scale will be of order m/.E, or ~10^-12 or so in most experiments, No experiment can reach that level. because no experiment will have enough data to see even one event.,

You should consider "Majorana neutrinos are their own antiparticles" in the same category as "relativistic mass" - a story we tell so that non-experts get a glimmer of understanding, but one that leads to incorrect conclusions if taken too far. And 'too far" is not very far..

 

[Ken G]

I base this on the analogy that we do not speak of "antiphotons", "anti-Z" nor "anti-higgs".

Exactly, I agree.

 

[Ken G]

The point is that this is not yet unanimously decided for neutrinos. Whether they are Majorana or Dirac particles is subject of active research, e.g., by the search for neutrinoless double $\beta$-decay. For a pretty comprehensive review on Majorana vs. Dirac neutrinos, see

https://arxiv.org/abs/1412.3320v1

That's a very helpful article. It does make the interesting point: "In the limit of vanishingly small mass the difference between Dirac and Majorana fermions disappears. Therefore the observed smallness of the neutrino mass makes it very difficult to discriminate between different types of massive neutrinos, and it is not currently known if neutrinos are Majorana or Dirac particles." So we have that point of contact between Dirac and Majorana neutrinos, the question that seems outstanding is how do Majorana neutrinos, once they do have mass, still manage to "remember" what kind of lepton made them, to explain why neutrino beams from negative leptons act differently from those from positive leptons, even though they have zero lepton number?

 

[Ken G]

You're falling back on the popularization phrase "Majorana neutrinos are their own antiparticles".

How is it "falling back on a popularization phrase" to quote exactly from language in expert articles on the topic? From the paper @vanhees71 just cited:
"What are Majorana particles? These are massive fermions that are their own antiparticles."
It's a chapter from a book, but certainly it is written for physicists, and is not "popularized." (When is the last time you saw a popularized article contain 118 equations?)

The question is not "is there lepton number violation" but "how much" (and maybe "do we care?")

Actually, the question of whether there is complete lepton violation in neutrino reactions remains open. From the article:
"Obviously, Majorana particles must be genuinely neutral, i.e. they cannot possess any conserved charge-like quantum number that would allow one to discriminate between the particle and its antiparticle."
Lepton number is not necessarily "charge-like", but it is certainly implied by the logic of the statement that they are taking ability to discriminate a particle and its antiparticle as the given impossibility for Majorana neutrinos. Nonzero lepton number would invalidate the logic of this statement.

The scale will be of order m/.E, or ~10[suo]-12[/sup] or so in most experiments, No experiment can reach that level. because no experiment will have enough data to see even one event.,

The neutrinoless double-beta decay is indeed a very unlikely event, so they need to look at a huge sample. This article describes what is needed: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.130.062501
It is true that this experiment could get a result even if neutrinos are some kind of mix between Dirac and Majorana type, but that does not deny the truth that the experimenters think they might be of pure Majorana type, and I think it's pretty clear they are hoping that is the case (to make the reaction happen more often).

You should consider "Majorana neutrinos are their own antiparticles" in the same category as "relativistic mass" - a story we tell so that non-experts get a glimmer of understanding, but one that leads to incorrect conclusions if taken too far. And 'too far" is not very far..

At this point, this is sounding like a personal opinion masquerading as authoritative, and worse, being used as a kind of club to dismiss discussion of this very interesting topic, among "non-experts" who can hope for no more than a "glimmer of understanding."

 

[PeterDonis]

Yes,

Is there a mathematical model that explains how? I've been unable to find one in any of the papers I've read (including the ones that have been linked in this thread, in post #22 and another linked by @vanhees71 in post #100).

 

[vanhees71]

What do you look for? In the above quoted paper there's a review about different models of Dirac and Majorana neutrinos.

The appealing feature of Majorana neutrinos, i.e., Weyl spinors vs. Dirac spinors, is that although Majorana neutrinos have a mass (and at least 2 of the 3 known neutrino flavors should have mass due to the observed mixing), there are only left-handed (chirality -1) neutrinos and no right-handed ones as in the Dirac case.

The notion that Majorana neutrinos with mass were "their own antiparticles" indeed doesn't make much sense, because there's no conserved lepton number any more which could distinguish between particles and antiparticles. Nevertheless the consequence of this is that there's the possibility of "neutrino-less double $\beta$ decay", if the neutrinos are Majorana particles, and that's why this is the key observable to empirically demonstrate this. So far, there's neither evidence for nor a clear exclusion of neutrino-less double-$\beta$ decay. So the question whether neutrinos are Majorana or Dirac fermions is undecided.

 

[PeterDonis]

What do you look for?

When a neutrino hits a neutron, it produces a proton and an electron. When an antineutrino hits a proton, it produces a positron and a neutron. Individually, there is no problem with pure Majorana neutrinos participating in these reactions, as long as we don't mind violating lepton number.

However, in the experiments @Vanadium 50 describes, a neutrino source produces a beam that is fired into a target containing both protons and neutrons; and as he describes it, the source can be made to produce a beam that only produces electrons when fired into the target (indicating that only the neutron reaction above is happening) or a beam that only produces positrons when fired into the same target (indicating that only the proton reaction above is happening).

The question is, if neutrinos are pure Majorana fermions, how is it that possible? If neutrinos are pure Majorana fermions, it should be impossible to make a source that can produce a beam that only produces electrons, or only produces positrons, when fired into a target that contains both protons and neutrons; in any such target, both reactions described above should happen with any neutrino beam, because the neutrinos have no way of distinguishing the neutron reaction from the proton reaction.

 

[vanhees71]

I'm not sure, whether the sensitivity of neutrino experiments of this kind is high enough to exclude that both reactions happen. Though, if this were the case I'd say that then the question were settled in favor of Dirac fermions, and then nobody would ask anymore, whether the neutrinos could be Majorana fermions.

Note that at the level you can neglect the neutrino masses there's no difference between Majorana and Dirac fermions within the standard model, i.e., there are only left-handed neutrinos and right-handed antineutrinos.

Now you have three possibilities to introduce masses:

-pure Dirac mass terms and conserved lepton number; then you have "sterile" right-handed neutrinos since the mass terms mix in the right-handed parts of the Dirac fields, which however don't participate in the weak interaction. Then the neutrino mass eigenstates carry lepton number +1 and the anti-neutrinos lepton number -1 and are thus distinguishable

-pure Majorana mass terms; then lepton number is not conserved and there are only left-handed fields in the game (left-handed Weyl fermions), i.e., there are no "sterile" right-handed neutrinos

-both Majorana and Dirac mass terms; then lepton number is not conserved and you have sterile right-handed neutrinos.

 

[PeterDonis]

I'm not sure, whether the sensitivity of neutrino experiments of this kind is high enough to exclude that both reactions happen.

It should be easy: the charges of the reaction products are opposite, so they bend in opposite directions in a magnetic field and can easily be distinguished. As I understand it, that's how the detectors in experiments like MINOS work.

 

[vanhees71]

But if it were so easy, why are then the searches for neutrinoless double-$\beta$ decay are still pursued? I guess @Vanadium 50 can answer this much better than I.

 

[Ken G]

Or put differently, how does a Majorana neutrino know that if it was made by antileptons, it should hit protons and make antileptons, and if it was made by leptons, it should hit neutrons and make leptons? That seems to be what happens to the neutrinos in the experiments @Vanadium 50 was describing.

As for whether it makes sense to describe Majorana neutrinos as "their own antiparticle", I believe the generally taken meaning is "it is impossible to regard the particle as either matter or antimatter because there is no way to distinguish those possibilities." (Or as you put it, "there's no conserved lepton number any more which could distinguish between particles and antiparticles"). It does not follow that if a particle has no matter or antimatter properties, then it does not make sense to describe it as its own antiparticle, that is simply what the term is taken to mean. By your reasoning, the phrase "is its own antiparticle" would be itself fundamentally meaningless, because what particle that is its own antiparticle could possibly have attributes that could distinguish matter from antimatter? So there is no point in defining a phrase such that it becomes meaningless, just assume the usage that does mean something (can annihilate with identical versions of itself).

 

[Vanadium 50]

At this point, this is sounding like a personal opinion masquerading as authoritative, and worse, being used as a kind of club to dismiss discussion of this very interesting topic, among "non-experts" who can hope for no more than a "glimmer of understanding."

Again, disrespectful, unhelpful, uncollegial and beneath you. Thar's third time,

I point out that only one of us has actually done these kinds of experiments.

 

[Ken G]

Again, disrespectful, unhelpful, uncollegial and beneath you. Thar's third time,

Well it's certainly the third time you've said that, but that's the only thing I can agree with.

I point out that only one of us has actually done these kinds of experiments.

And I point out that, despite that fact, you have still not answered the important question: How can Majorana neutrinos made by antileptons be restricted to generating antileptons in a detector? That is the question we need the answer to, so if you don't know, then just say that.

ETA: Let me be very clear that I do respect your extensive knowledge of particle physics, and I do appreciate your willingness to share it on a forum. I'm sure you have other things to do, yet you choose to take time to explain things here, which is fundamentally a generous act. That's not what I'm talking about, I'm talking about the way you have tried to steer this thread away from useful inquiry into the issue of "under what circumstances can neutrinos be their own antiparticles, within the constraints of what has already been observed about them, and what is the generally taken meaning of that expression." But it really doesn't matter who said what when in this thread, because it has all come down to the question I just stated, also posed above by @PeterDonis. That is where the inquiry has led us, and the only real payoff we can get at this point is the answer to that question.

 

[ohwilleke]

I had thought, maybe inaccurately connecting dots that shouldn't be connected, that a Majorana neutrino can come in a left-parity neutrino and a right-parity anti-neutrino form, and that the reason that we say that it is its own antiparticle is that with all other Standard Model fermions you have four possibilities:

Left parity particle, right parity particle, left parity antiparticle, and right parity antiparticle,

while in the case of a neutrino, you collapse those four combinations into two:

left parity particle and right parity antiparticle - which is indistinguishable from having a left parity particle and a right parity particle, and no antiparticles.

I also had understood, perhaps wrongly, that a left parity particle could transition to a right parity particle in some kind of interaction related to the Higgs mechanism (by which quarks, antiquarks, charged leptons, charged antileptons, W+ bosons, W- bosons, Z bosons, and Higgs bosons get their rest masses in the Standard Model) in some fashion.

I thought that transitions of the same particle from left parity to right parity and back were what drove the Higgs mechanism (seemingly implied, e.g., here, although I've seen more explicit illustrations to that effect)

Hence, in such a transition, a left parity neutrino could convert to a right parity neutrino (perhaps improperly called an antineutrino), which is why Majorana neutrinos don't conserve lepton number.

In contrast, in a Dirac neutrino scenario you have a left parity neutrino and a right parity antineutrino and one can't transition into the other, which is great for explaining that neutrinos and antineutrinos are different in their interactions with protons/neutrons due to lepton number conservation.

But, in the Dirac neutrino case, I had thought that this screws up the Higgs mechanism that requires transitions between a left parity and right parity fermion, and transitions between a left parity and right parity antifermion - which would necessitate sterile neutrinos because W and Z bosons don't interact with right handed particles or left handed antiparticles, neutrinos don't have electromagnetic charge, and neutrinos don't have strong force color charge (so they would interact only via gravity and Higgs mechanism related parity transitions).

Also, for some reason I've never really understood, the usual proposal is that the sterile counterparts of weak force interacting Dirac neutrinos (a.k.a. active neutrinos), unlike the parity counterparty of every other SM fermion which has the same mass regardless of its parity or matter/antimatter status, should have a different mass than the active neutrinos of opposite parity in a see saw mechanism.

Along the same lines it also isn't entirely obvious why Dirac and Majorana mass generation mechanism are the only options for neutrino mass.

Why can't we theorize that neutrinos acquire mass in some third way, call it "Wilma mass" or "Zappa mass" the conserves lepton number in some manner that does not require that there be a sterile neutrino - e.g. through W boson or Z boson interactions?

This would seem to be a rather modest theoretical tweak compared to creating a whole new set of three sterile neutrinos with three new mass parameters of their own and some sort of mixing parameters.

If you are making up new physics with new rules of physics solely to explain neutrino mass anyway, why not do it in some other fashion than the Dirac mass and Majorana mass options that we know and love (or know and hate)?

What do I have right? Where have I gone astray?

 

[PeterDonis]

I had thought

Please read post #37. It is a good summary of the basics of neutrinos and addresses pretty much all of the issues you raise.

 

[Ken G]

All except for the conclusion of that post, which was:
"So we're left with two possibilities:
1. We see antineutrinos, more or less as expected.
2. Our understanding 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 a signal that looks exactly like expected."

That does not seem to be borne out by the rest of this thread. Instead, we appear to have a third possibility, which is what the thread has been about from the start:
3. We think we were seeing antineutrinos, but actually there is no meaningful difference between neutrinos and antineutrinos, so we were actually seeing the versions of this Majorana particle that stem from creation by left- and right-handed leptons, which can only (for reasons we don't yet seem clear on in the thread) go on to create either left- or right-handed leptons in the detector, rarely crossing over.

The fact that reasonable people are searching for evidence of this situation clearly establishes that we might not be seeing "antineutrinos" when we thought we were, and this requires no significant modification to stellar collapse theory, QM, nor SR, nor any special conspiracies to have fooled us all this time. Instead, our mistake would have been to get carried away with conservation of lepton number, so we decided to associate the handedness of the lepton that created it with handedness of the neutrino via the usual particle/antiparticle dichotomy that we are used to, which ends up not being meaningful for it. We may then also conclude that we had no easy way to spot our mistake, until (if) we detect neutrinoless double beta decay at a rate associated with a Majorana particle expectation.

This possibility exhibits the crucial feature that distinguishes science from pretty much everything else we see around us: openmindedness.

 

[PeterDonis]

our mistake would have been to get carried away with conservation of lepton number

The open question in this thread, regarding the neutrino experiments like MINOS and Majorana neutrinos, has nothing to do with getting carried away with conservation of lepton number. As I said in post #107, individually, if we accept violation of lepton number conservation, there is no problem with the reactions, and indeed that seems to be the common belief among physicists. Nobody is arguing that Majorana neutrino models are impossible because they violate lepton number conservation; everybody seems to accept that yes, that's a consequence and that's just how it is.

The problem is that our ability to tune the neutrino source so that leptons of only one charge get produced in the detector (electrons or positrons, but not both) does not seem to be consistent with a Majorana neutrino model--on such a model, electrons and positrons should be produced in the detector in roughly equal numbers no matter what we do to the neutrino source. The alternatives here seem to me to be these:

(1) My description above of the experimental results is wrong; electrons and positrons are in fact produced in roughly equal numbers in these experiments, no matter what is done with the neutrino source. However, some other factor involved, which has not been explained, prevents us from detecting this.

(2) A Majorana neutrino model can in fact explain the results as I have described them above. If that is in fact the case, I would expect to find at least a rough theoretical model showing how this can be done somewhere in the literature, but so far I have not found one, nor even any hint that anyone thinks there should be one.

(3) There is a big disconnect between two communities in neutrino physics: those who take it as a routine fact that we can tune sources to produce either only neutrinos, or only antineutrinos, on demand, and think of the two as distinct particles; and those who take it as a routine fact that Majorana neutrinos are a real theoretical possibility and look for experiments (different from the ones the first group gets its routine facts from) to explore it.

 

[PeterDonis]

I point out that only one of us has actually done these kinds of experiments.

That's true, and it's why I have asked you if you know of a mathematical model that explains how the experimental results you described can be consistent with a Majorana neutrino model. In terms of the alternatives I just described in post #117, it seems like you think alternative (2) is obvious, but it's not obvious to me, and if it is obvious to you, it would be helpful to get at least a brief explanation of why.

 

[Ken G]

The open question in this thread, regarding the neutrino experiments like MINOS and Majorana neutrinos, has nothing to do with getting carried away with conservation of lepton number. As I said in post #107, individually, if we accept violation of lepton number conservation, there is no problem with the reactions, and indeed that seems to be the common belief among physicists. Nobody is arguing that Majorana neutrino models are impossible because they violate lepton number conservation; everybody seems to accept that yes, that's a consequence and that's just how it is.

Saying "carried away by Lepton conservation" is by no means a claim that Majorana neutrino models are impossible, so I don't understand the connection you are drawing there. Indeed, I am saying the opposite-- "getting carried away by Lepton conservation" is another way of saying "rejecting Majorana neutrino models without experimental justification." This is because, any time we demand that neutrinos be either particles or antiparticles in a distinguishable way, we are making it so we can attach a lepton number to them based on the handedness of leptons they produce (indeed, we saw @Vanadium 50 make precisely that suggestion above). That's the "carried away" part, equal to rejecting the Majorana possibility without cause.

The alternatives here seem to me to be these:

(1) My description above of the experimental results is wrong; electrons and positrons are in fact produced in roughly equal numbers in these experiments, no matter what is done with the neutrino source. However, some other factor involved, which has not been explained, prevents us from detecting this.

That possibility seems ruled out by what @Vanadium 50 has said.

(2) A Majorana neutrino model can in fact explain the results as I have described them above. If that is in fact the case, I would expect to find at least a rough theoretical model showing how this can be done somewhere in the literature, but so far I have not found one, nor even any hint that anyone thinks there should be one.

I agree, this must be the situation and it is odd that it is so difficult to understand how it would work. I don't know enough about the neutrino constraints, but one idea would be to say Majorana neutrinos preserve the handedness of the lepton that made them when they go to make leptons, but not doing it by being either particles or antiparticles, they do it in some other way.

Adding to this sense is that I got the impression from the seesaw mechanism that the low-mass neutrinos are the ones that couple to the weak force while the high-mass ones are sterile, so that would be kind of the trick right there: Majorana neutrino fields could involve some kind of simultaneous superposition of left/right and particle/antiparticle such that the low-mass particles they generate can couple to the weak force without being either a particle or an antiparticle, and the high-mass particles are sterile. So if the weak force is looking for credentials like "|left>|particle>" or "|right>|antiparticle>", the Majorana neutrinos pass the test by being "|left>|particle>+|right>|antiparticle>", and the transformation left-->right and particle-->antiparticle leaves them invariant (which is what is meant by "is its own antiparticle"). If so, then the way they "remember" the handedness of what made them would need to have something to do with the difference between that wavefunction and a mixture of "|left>|particle>" and "|right>|antiparticle>". In short, it would have to matter whether there was a + in the superposition or a -. We know the regular Dirac "|left>|particle>" states interact with neutrons to make leptons, and the "|right>|antiparticle>" interact with protons to make antileptons, so we just need a way for the + or - in the Majorana superposition to do the same thing. One way to do that would be to set it up so the sign controlled whether the neutron or proton interactions suffer destructive interference. I don't know how that could be done, but it's just one possible suggestion for a way that a particle that was its own antiparticle could still "remember" whether it was made by a lepton or an antilepton.

(And I note in this scheme, the transformation left-->right and particle-->antiparticle would induce a change in the sign of the wavefunction, so that would have to be an ignorable difference somehow, maybe it doesn't work but it has the spirit of how a particle could do it. We note that if we look at the neutrino beams we have today, and imagine doing left-->right and particle-->antiparticle for the entire apparatus, we produce no contradiction in the observed behavior if Majorana neutrinos transform into the identical particle.)

(3) There is a big disconnect between two communities in neutrino physics: those who take it as a routine fact that we can tune sources to produce either only neutrinos, or only antineutrinos, on demand, and think of the two as distinct particles; and those who take it as a routine fact that Majorana neutrinos are a real theoretical possibility and look for experiments (different from the ones the first group gets its routine facts from) to explore it.

We have certainly seen that people manipulating accelerator beams like to think of them as being beams of neutrinos or antineutrinos, because this works for them to do so. That's a very different issue as to whether or not it is actually correct or required. So I would say the answer looks very much like "both (2) and (3)," but it's not necessarily a disconnect, so much as one group deciding to commit to a certain essentially philosophical stance, and the other group saying "not so fast-- you don't really know that." Or maybe: "you are making a distinction that seems to work to make, but we might end up showing it does not actually exist." An analogy could be the aether in physics in the late 1800s.

 

[PeterDonis]

"getting carried away by Lepton conservation" is another way of saying "rejecting Majorana neutrino models without experimental justification."

The point I was making is that I don't see anyone doing that. @Vanadium 50 doesn't seem to be; he explicitly said the experimental results he describes are consistent with a Majorana neutrino model (though he hasn't explained how that can be the case).