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.
And yet you then jumped in and put some words in my mouth., Please don;t do that. hat's disrepsectful, unhelpful, unbcollegial and beneath you.
I put no words in your mouth, it was exactly what you said. You suggested defining antineutrinos by any neutrino that makes an antilepton, is that not just what you said? And does it not immediately follow that by this definition, a neutrino is not its own antiparticle, even if it is a pure Majorano fermion? What is clear from the thread is terms like "is its own antiparticle", and "the particle and antiparticle are the same thing" do not have a consensus interpretation. This is precisely the reason I reframed the question some time ago, yet without any answer from anyone so far:
I am trying to get us away from that woolly language.
Can you be more specific about which experiment this refers to? The only specific experiment you've referred to so far is the Cowan Reines experiment, which used a nuclear reactor as the neutrino source and so could not choose what kind of neutrinos were being detected--it was whatever kind were being emitted by the nuclear reactions in the reactor. What you're describing here seems like a neutrino source where we can choose what kind of neutrinos it emits at will.
I meant "that is why particle physicsists in general write that pure majorana fermions are their own antiparticle". You should be able to read this in any quantum field theory textbook.
What I meant is that we do not make a "fuzz" about the photon, the higgs, or the Z boson being their own antiparticles (whatever that means). Is there any value to you, the fact that those particles are their own antiparticle? To me, its not at all important. What is important to me is that if neutrinos are not pure Dirac, we have a strong case for beyond the standard model physics - there are more things to be discovered after this :)
I can write down equations for you if you prefer that, after all - the language of physics is math. It would be better I think if we just said "the quantum field is transformed back to itself after successive CPT-transformations" ;) Or just, "pure Dirac fermion", "pure Majorana fermion" or "pseudo-Dirac fermion". Or, just stick with the Weyl spinors...
Ok now I understand better what you meant by that.
Perhaps this https://lbnf-dune.fnal.gov/how-it-works/neutrino-beam/
Yes, this is very helpful, thanks!
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?
I never asked "is it a fuss if neutrinos are their own antiparticle," I asked, "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, even the person who said that there is no possible way that neutrinos are their own antiparticle.
Yes of course, that is the exciting physics here. But to understand the significance to the lexicon, we need the answer to the question I have posed so many times now.
Actually, the language of physics is the language we find in physics books, and that is not all math. Words matter, that's why we are literally using them right now. Why do I have to justify the importance of words just to get a fairly straightforward question answered?
Does the Majorana Collaboration have anything to say about this? At any rate they would seem to be the ones in the best position to give an argument for the "just retire the term" position.
I'm not sure if this is even a possibility. It seems to be established that the Standard Model could be expanded to accommodate a Majorana mass for neutrinos, but I'm not sure whether it could accommodate neutrinos only having a Majorana mass. (If nothing else, one would have to explain why the standard Higgs mechanism doesn't give neutrinos Dirac masses, even though left-handed neutrinos, at least, are weak isospin doublets with the charged leptons.)
There's an item in this paper that I'm not sure I understand. Fig. 1 in the paper shows the Feynman diagram that is expected to dominate neutrinoless beta decay. The accompanying text notes that each vertex has to conserve lepton number; that means the neutrino coming out of the left vertex has to be an antineutrino (since it's created along with an electron), but the neutrino going into the right vertex has to be a neutrino (since it gets converted to an electron). Hence, according to the text, the diagram cannot happen unless the neutrino is its own antiparticle, so the same particle can participate in both vertices.
Yes I agree. Their paper (https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.130.062501 ) does not speak directly to the issue of neutrinos being their own antiparticle, instead they focus on violation of lepton number conservation, but the violation can be a rare thing. Their interest is in the creation of a matter universe, because they say: "Neutrinoless double-beta decay (0νββ) is a hypothetical nuclear process involving the unbalanced creation of two new matter particles but no antimatter [1–5]. This lepton number-violating process is predicted generically by many grand-unification theories, as well as by leptogenesis [6], a leading explanation of why the Universe is matter dominated."
Yes I see your point, reflected in @Vanadium 50 's argument as well, but you see the issue with the first quote I gave above from the Majorana Collaboration. If they want to explain the tiny neutrino mass, and they say that Dirac fermions have no natural explanation there, they must be relying pretty heavily on the Majorana characteristics. How does a tiny Majorana mass term account for a tiny total mass?
In at least one version of the seesaw mechanism, as I understand it, the Majorana mass is tiny and the Dirac mass is very large, so there are two sets of neutrinos, a set of very light ones (which are the ones we have detected) and a set of very heavy ones (which we haven't detected yet). I don't know how much of the potential parameter space of such theories the LHC results have ruled out.
Interesting, but that would still imply that the neutrinos we are detecting are pure Majorana neutrinos, so would need to have some way of knowing what type of lepton they came from, while still showing the CPT symmetry of a particle that is its own antiparticle. I'm wondering if the pro-Majorana folks have a trick up their sleeve that we are overlooking.
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.
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.
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.
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.
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.
In the context of the standard model (pure Dirac-neutrinos) yes.
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).
I base this on the analogy that we do not speak of "antiphotons", "anti-Z" nor "anti-higgs".
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
Yes,
Exactly, I agree.
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?
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:
Actually, the question of whether there is complete lepton violation in neutrino reactions remains open. From the article:
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
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."
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).