Majorana Neutrinos: Different Physics than Oscillation

[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.

 

[PeterDonis]

Is not a paper.

I don't mean the website that was originally linked. I mean the paper that was linked in post #22.

 

[PeterDonis]

we are talking about "people" at Fermilab (https://neutrinos.fnal.gov/types/antineutrinos/ ) (my bold):

Since an actual paper has been posted (post #22), it would be much better to use that as a reference. @Vanadium 50 is perfectly correct that that web page is not a good reference for a PF discussion. It contains no math and is not a scientific treatment, it's just a basic description.

 

[Ken G]

Since an actual paper has been posted (post #22), it would be much better to use that as a reference. @Vanadium 50 is perfectly correct that that web page is not a good reference for a PF discussion. It contains no math and is not a scientific treatment, it's just a basic description.

Normally I would agree, but here we have @Vanadium 50 stating : "How can one say "the neutrino is its own antipartiocle" (and its not, no matter how many people repeat ity here)"
So at issue in that claim is not the theory of neutrinos, it is a claim about "how can one say" something. Ergo, I gave examples of how one can say it, by saying it in a public forum, as they did. What I found so erroneous, and rather impolite, was the clear implication that the words "neutrino is its own antiparticle" was somehow from from "people on here." So that falsehood had to be dismissed.

 

[Vanadium 50]

because you chose to define them that way.

My definition is perfectly consistent with the standard one.

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?

Of course they will. I could zip out to Fermilab today and tell them to switch from neutrino mode to antineutrino mode and back again (well, if there wasn't a vacuum leak at the moment,) and the detectors would see negative leptons, then positive leptons, then negative leptons again.

You simply cannot take statements intended for the general public and treat them as rigorous scientific truths. If I told you that I read in My First Book of Stars (not a real book) that stars come in all sizes, shapes and colors, so there must exist somewhere a green cubical star that fits in my pocket, what would you say?

 

[Ken G]

My definition is perfectly consisten with the standard one.

You're dodging the real question: Do you think that if the Majorana Collaboration is successful in demonstrating that neutrinos are Majorana fermions, there will not be a good reason to retire the term "antineutrino"? That's the part that is not clear from your argument, because I can't see, given the language they use, why they would not find that an appropriate approach.

Of course they will. I could zip out to Fermilab today and tell them to switch from neutrino mode to antineutrino mode and back again (well, if there wasn't a vacuum leak at the moment,) and the detectors would see negative leptons, then positive leptons, then negative leptons again.

Correction, you could tell them to switch from detecting those various types of leptons. It is only you that are calling that "neutrino mode" or "antineutrino mode", based on your earlier tautology. Again, the real question is, would they have any reason to keep to your terminology here, or just retire the phrase "antineutrino" altogether? That is the question you have not answered. I don't say that you haven't got a good reason for your opinion, merely that you have not justified that opinion yet. That would be where the real insight would come from, not tautological definitions.

 

[PeterDonis]

The definition I picked does not even distinguish between mass and flavor eigenstates.

You talked about actual leptons coming out of actual processes that are detected in the lab. Don't those processes normally involve a specific flavor of lepton? If so, it would seem like they should also involve a specific flavor of neutrino.

Perhaps a more specific description of some of the processes you had in mind would help.

 

[PeterDonis]

at issue in that claim is not the theory of neutrinos, it is a claim about "how can one say" something.

I would take "how can one say" as a figure of speech. The claim @Vanadium 50 is making, as I understand it, is a claim about actual neutrinos that are involved in actual processes measured in labs like Fermilab. He is saying that such processes always produce specific charged leptons--positive or negative. They never produce equal mixtures of both charges. And he is claiming that, according to the theory of neutrinos, if neutrinos were their own antiparticles, those processes would always produce equal mixtures of both charges of leptons, not just one charge.

 

[PeterDonis]

if the Majorana Collaboration is successful in demonstrating that neutrinos are Majorana fermions

As I read the paper I linked to in post #22, it might not be that simple. The Majorana Collaboration is looking for evidence of neutrinoless beta decay. That would imply that there is a Majorana mass term in the Lagrangian, but the paper gives at least one theoretical model in which there are both Majorana and Dirac mass terms. (This model is described as a "toy model" because it only has one generation and does not include anything except neutrinos and W particles, but I don't see anything in the paper that rules out the possibility that a more sophisticated model along the same lines might not be true of actual neutrinos.) So the observation of neutrinoless beta decay would not necessarily show that neutrinos are purely Majorana fermions; it could still be the case that there are distinct neutrinos and antineutrinos.

 

[Ken G]

I would take "how can one say" as a figure of speech. The claim @Vanadium 50 is making, as I understand it, is a claim about actual neutrinos that are involved in actual processes measured in labs like Fermilab. He is saying that such processes always produce specific charged leptons--positive or negative. They never produce equal mixtures of both charges. And he is claiming that, according to the theory of neutrinos, if neutrinos were their own antiparticles, those processes would always produce equal mixtures of both charges of leptons, not just one charge.

That's what I don't understand. The Cerenkov detectors that detected SN 1987A neutrinos were of the type you describe-- able to detect leptons of either charge. But they don't produce equal mixtures, because the targets are different-- to get an electron, the target is a neutron, to get a positron, the target is a proton. In the standard model, if we hit water with equal fluxes of neutrinos and antineutrinos, we expect more positrons, and indeed we get more positrons. If neutrinos are their own antiparticle, then we also expect more positrons, which we also see.

As for the detectors in Fermilab, if what @Vanadium 50 means by "antineutrino mode" is that the detectors are set up to detect negative leptons, then yes, they will detect negative leptons-- whether the neutrinos are of Dirac or Majorana type. So I am still not seeing any contradiction from the possibility that neutrinos are their own antiparticle.

 

[Ken G]

As I read the paper I linked to in post #22, it might not be that simple. The Majorana Collaboration is looking for evidence of neutrinoless beta decay. That would imply that there is a Majorana mass term in the Lagrangian, but the paper gives at least one theoretical model in which there are both Majorana and Dirac mass terms. (This model is described as a "toy model" because it only has one generation and does not include anything except neutrinos and W particles, but I don't see anything in the paper that rules out the possibility that a more sophisticated model along the same lines might not be true of actual neutrinos.) So the observation of neutrinoless beta decay would not necessarily show that neutrinos are purely Majorana fermions; it could still be the case that there are distinct neutrinos and antineutrinos.

Yes, it would only be one step in the path toward their objective of deciding if neutrinos are their own antiparticles or not. It is rare that a single observation is a "slam dunk." So yes, that result would not yet be enough to retire the term "antineutrino", but the issue here is merely that this remains a possibility in the future-- without holding to @Vanadium 50 's claim:
"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 a signal that looks exactly like expected."

In essence, my participation in this thread has parallel purposes: to learn a lot about neutrinos, and to assist forumgoers in retaining their open minds that we might have a lot to learn about neutrinos, including that they could be their own antiparticles, without having to put the rest of physics on its ear. Perhaps the first purpose will end up completely supplanting the second, but that has as yet not occurred.

 

[Vanadium 50]

You talked about actual leptons coming out of actual processes that are detected in the lab. Don't those processes normally involve a specific flavor of lepton?

Sure, but it's irrelevant to whether its a neutrino (yields a negative lepton) or antineutrino (yields a positive one), I'm trying to minimize complications.

I'm also trying to minimize yeahbuts. If I wrote "muon antineutrino", the peanut gallery would all chime in "mass and flavor eigenstates are not the same". Which, while true, is not necessary for this discussion.

 

[malawi_glenn]

I think theoritsts (like me) and experimentalists (like Vanadium) have different definitions of neutrino and antineutrino.

To me, an antiparticle is a particle whos state is returned after application of C, P and T transformations. Nothing more, nothing less. If that resulting state is the same as the state before the application of those operators, it is "it's own antiparticle".

One must also be careful when reading papers and articles. When you read "They detected neutrinos from SN1987A" they could be referring to $\nu$ and/or $\bar \nu$. Same when you read about say Higgs discovery that one decay channel is into four muons. Experts in the field knows that this is nonsense, Higgs can not decay in to four muons, but rather two muons and two anti-muons. But it is (for good and bad) standard lingo.

but the paper gives at least one theoretical model in which there are both Majorana and Dirac mass terms. (This model is described as a "toy model" because it only has one generation and does not include anything except neutrinos and W particles, but I don't see anything in the paper that rules out the possibility that a more sophisticated model along the same lines might not be true of actual neutrinos.)

There are papers which have all three generations, and the full SM.

 

[vanhees71]

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.

Indeed, given neutrino oscillations, that's the only way you can make sense of neutrinos at all. Only the asymptotic free mass eigenstates would have a "particle interpretation", but these cannot be detected, because all you can observe is the outcome of some interaction of the neutrinos with the detector material, and these detect flavor eigenstates due to the nature of the weak interaction.

 

[vanhees71]

I think theoritsts (like me) and experimentalists (like Vanadium) have different definitions of neutrino and antineutrino.

To me, an antiparticle is a particle whos state is returned after application of C, P and T transformations. Nothing more, nothing less. If that resulting state is the same as the state before the application of those operators, it is "it's own antiparticle".

"Particles" are asymptotic free mass eigenstates by definition. The mass eigenstates of neutrinos can, however, not be detected, because you can detect neutrinos only by interactions with some detector material and measuring the produced particles, among them the charged leptons or anti-leptons, i.e., the flavor eigenstates. In this sense we never detect neutrinos as "particles". That's also what's behind neutrino-oscillation measurements: You measure at one place, which neutrino flavor was produced and at the other place, which neutrino flavor is detected.

All this has little to do with the question, whether there are Majorana mass terms or not in the correct description of neutrinos. That's why experiments look for, e.g., neutrino-less double $\beta$ decay, which would indicate that there are such Majorana mass terms. So far no such events have been found though. So it's still an open question.

 

[malawi_glenn]

"Particles" are asymptotic free mass eigenstates by definition

Yeah that is also an aspect of QFT that complicate things when we try to discuss these kinds of matters ;)

 

[Ken G]

I think theoritsts (like me) and experimentalists (like Vanadium) have different definitions of neutrino and antineutrino.

Yet you don't go around disparaging "people here" for repeating language used by those at Fermilab, correct? (Besides, the issue here is not your personal definitions, it is whether or not the general canon will, or will not, include the term "antineutrino" if neutrinos are found to be Majorana fermions. Are you saying theorists would retire the term but experimentalists would continue using it? I really don't think we'd have such a Tower of Babel.)

To me, an antiparticle is a particle whos state is returned after application of C, P and T transformations. Nothing more, nothing less. If that resulting state is the same as the state before the application of those operators, it is "it's own antiparticle".

So does a Majorana fermion return to its state after CPT transformation?

 

[Ken G]

Indeed, given neutrino oscillations, that's the only way you can make sense of neutrinos at all. Only the asymptotic free mass eigenstates would have a "particle interpretation", but these cannot be detected, because all you can observe is the outcome of some interaction of the neutrinos with the detector material, and these detect flavor eigenstates due to the nature of the weak interaction.

So the same question shall be put to you: if neutrinos are demonstrated by experiment to be Majorano fermions, will the community retire the term "antineutrino" on grounds that it contains zero value, or will they retain the term any time an antilepton is produced? That I believe is the question that carries the insight here, whichever is the answer.

 

[Ken G]

Indeed, given neutrino oscillations, that's the only way you can make sense of neutrinos at all.

You make a nice point that what we call "neutrinos" (the flavor eigenstates) don't formally fall under the definition of particle, but this is not directly related to the common fact that we need to wait for the outcome of an experiment to be able to talk about its constituent stages. All that simply stems from Bell's theorem and the problems with local realism. If a photon does not even "carry with it" its own polarization in all situations, then we should not be surprised that any particles does not "carry with it" its own identity at all stages of an experiment. But that's not the issue here, the issue is whether or not there is any value in distinguishing neutrinos and antineutrinos at all, not how we would do it.

the asymptotic free mass eigenstates would have a "particle interpretation", but these cannot be detected, because all you can observe is the outcome of some interaction of the neutrinos with the detector material, and these detect flavor eigenstates due to the nature of the weak interaction.

Yet even in situations that are not challenges to local realism, we do conceptualize hypothetical states all the time. In Faraday rotation, a linearly polarized photon passing through a plasma with a line-of-sight magnetic field will have its polarization rotated. The photon is never in a state of circular polarization, yet a nice way to conceptualize what is happening is to imagine the photon as being in a superposition of circular polarizations with different propagation speeds. This would be true even if all we had were ways of measuring linear polarization. Note that in such a situation, we would still be able to imagine photons that were circularly polarized, as a useful conceptual device, even without the means for a circular polarization to be an outcome of an experiment. So what my question is driving at is, if neutrinos were Majorana fermions, would there be any value in the "antineutrino" conceptualization, independent of any issues of how to define it or measure it?

 

[malawi_glenn]

Are you saying theorists would retire the term but experimentalists would continue using it?

We do not go around and say "Z boson is its own antiparticle" all day long either.

After all, it is the physics that matters, not what we call things - just as with Pluto and planets.

if neutrinos are demonstrated by experiment to be Majorano fermions, will the community retire the term "antineutrino" on grounds that it contains zero value, or will they retain the term any time an antilepton is produced?

I think you mean "should" here, not will - because we have no time machines.

So does a Majorana fermion return to its state after CPT transformation?

A pure majorana fermion would yes, that is why we call it "its own antiparticle".

 

[Ken G]

We do not go around and say "Z boson is its own antiparticle" all day long either.

Hang on, we do say the "photon is its own antiparticle." Maybe not "all day long", but we do say it. Are you saying the situation is different with the Z boson? I mean, can it annihilate with another one or not?

After all, it is the physics that matters, not what we call things - just as with Pluto and planets.

Not at all like Pluto and the planets, the logic does not hold. Your syllogism is, if words don't really matter in regard to Pluto and the planets, then words never matter. I know you don't think that. After all, we're on a forum right now-- and what are we using to communicate? Words! So they don't matter, this is the point of a forum?

I think you mean "should" here, not will - because we have no time machines.

The device here is "if and when it is discovered that neutrinos are Majorana fermions, what will we do." It's asking for a prediction.

A pure majorana fermion would yes, that is why we call it "its own antiparticle".

Careful now, who is "we"? I think you'd better let @Vanadium 50 speak for themself, given that they suggested we define antineutrinos by anything that produces a negative lepton-- and no particle that is defined that way could ever be its own antiparticle.