I Do supernovae generate neutrinos or antineutrinos?

[swampwiz]

This Wikipedia particle says that neutrinos are generated:
https://en.wikipedia.org/wiki/Supernova

The Supernova Early Warning System (SNEWS) project uses a network of neutrino detectors to give early warning of a supernova in the Milky Way galaxy.[44][45] Neutrinos are particles that are produced in great quantities by a supernova, and they are not significantly absorbed by the interstellar gas and dust of the galactic disk.[46]

But this article (about SN1987A) seems to say that both neutrinos & antineutrinos were detected:
https://en.wikipedia.org/wiki/SN_1987A

Approximately two to three hours before the visible light from SN 1987A reached Earth, a burst of neutrinos was observed at three neutrino observatories. This was likely due to neutrino emission, which occurs simultaneously with core collapse, but before visible light is emitted. Visible light is transmitted only after the shock wave reaches the stellar surface.[11] At 07:35 UT, Kamiokande II detected 12 antineutrinos; IMB, 8 antineutrinos; and Baksan, 5 antineutrinos; in a burst lasting less than 13 seconds.

 

[Vanadium 50]

Both are generated.

And you should know that Wikipedia articles vary in quality.

 

[Ken G]

We can put an upper limit on the excess of neutrinos over antineutrinos in a supernova, assuming lepton number is conserved. An early step in a core collapse (massive star) supernova is converting protons and electrons into neutrons and neutrinos, with the same number of neutrinos as electrons because they are leptons. So although there are also other reactions that can produce both neutrinos and antineutrinos, the net excess of neutrinos has to be no more than the number of electrons originally in the stellar core (about 10^57 of them). That's rather a lot, given that there is only about 10^44 eV released in the supernova, so I would think it is predominantly neutrinos over antineutrinos.

 

[Vanadium 50]

Yes, there are more neutrinos expected than antineutrinos. They are from the "neutronization" phase. However, we have yet to see a single neutrino - only antineutrinos, because in 1987 that's what detectors were available.

 

[swampwiz]

Yes, there are more neutrinos expected than antineutrinos. They are from the "neutronization" phase. However, we have yet to see a single neutrino - only antineutrinos, because in 1987 that's what detectors were available.

So, because there is no observation data on neutrinos, it's existence is still only theoretical?

 

[Vanadium 50]

So, because there is no observation data on neutrinos, it's existence is still only theoretical?

What are you talking about? There is plenty of observational evidence for neutrinos, including two Nobel prizes. What we haven't seen are supernova neutrinos, because in 1987 the detectors in use were sensitive to antineutrinos.

 

[swampwiz]

OK. so what you're saying is that if a Milky Way star goes supernova now, we would have neutrino detectors that would measure it?

 

[Vanadium 50]

Swampwiz, swampwiz, swampwiz, please stop putting words in my mouth. Its a slow way to learn and it's as annoying as heck.

I didn't say anything about today's capabilities. In principle I could look up the running experiments, and find out their sensitivity to neutrinos and compare that to expected SMe fluxes, but that's a lot of work. And if you aren't willing to do that, why should I?

 

[Ken G]

The question that remains on the table is whether the expectation that a core collapse should produce a whole lot of regular neutrinos, outnumbering the antineutrinos, has been experimentally verified yet or not. This is actually kind of hard to tell for non-experts like myself, because many articles do not distinguish between neutrinos and antineutrinos when they talk about neutrinos from supernovae. Often the (anti)neutrinos are detected by looking for Cerenkov radiation, but that is not directly made by the neutrinos, since they don't directly interact with light. The (anti)neutrinos have to interact with something first, which then makes the Cerenkov radiation, but that other thing might require it to be an antineutrino. I don't know why it couldn't be an electron neutrino interacting with a neutron to create a proton and electron, that flavor of "inverse beta decay" would seem like a natural way to make high-energy electrons that could produce Cerenkov radiation.

It is true that the well-known neutrino detectors KamiokaNDE and KamiokaLAND detect a different type of inverse beta decay that is an electron antineutrino interacting with a proton to make a neutron and a positron. For some reason (maybe the 1.8 MeV reaction threshold is lower than for neutrinos interacting with neutrons, or maybe the cross section is just higher), this reaction is more prevalant than the first type of inverse beta decay involving regular neutrinos, so as far as I can tell, it might still be true that regular neutrinos from supernovae have not been detected directly, and remain a theoretical expectation. It would be a shame if this opportunity to verify lepton number conservation on the astronomical scale has still gone unrealized.

 

[Hornbein]

Swampwiz, swampwiz, swampwiz, please stop putting words in my mouth. Its a slow way to learn and it's as annoying as heck.

I didn't say anything about today's capabilities. In principle I could look up the running experiments, and find out their sensitivity to neutrinos and compare that to expected SMe fluxes, but that's a lot of work. And if you aren't willing to do that, why should I?

Kindly consider the possibility that you might know the answer offhand. How is the questioner supposed to know it whether or not it is easy for you? I suggest you give them the benefit of the doubt.

 

[Ken G]

Perhaps also the key is that if the neutrino is antimatter, it can create antimatter (like a positron) that can annihilate and this helps with the detection of the original antineutrino. It would certainly be ironic if a particle as ghostly and otherworldly as a neutrino is easier to detect when it is even more otherwordly antimatter. Experiments at Fermilab detect both types of neutrino, so it's not like regular neutrinos have not been well established, but there is still a lot about them that is not known (such as their mass). There is even this cryptic quote from Fermilab (https://www.fnal.gov/pub/science/particle-physics/experiments/neutrinos.html#:~:text=Fermilab's NOvA experiment sends a beam of neutrinos,whether neutrinos and antineutrinos oscillate at different rates.): "Neutrinos could have other strange properties as well. They could turn out to be identical to antineutrinos, their antimatter counterparts." So oddly, it might end up that the distinction we are discussing might not even exist!

 

[Cerenkov]

Kindly consider the possibility that you might know the answer offhand. How is the questioner supposed to know it whether or not it is easy for you? I suggest you give them the benefit of the doubt.

And it works the other way round too.

At best the resident experts will have to make assumptions about what the questioner does and doesn't know and understand from a paucity of information.

Cutting each other a little slack might help.

 

[Vanadium 50]

The question that remains on the table is whether the expectation that a core collapse should produce a whole lot of regular neutrinos, outnumbering the antineutrinos, has been experimentally verified yet or not.

I don't think this is in question. We know the star's state changes from half neutrons and half protons to one that is mostly neutrons. That means you expect a pulse of neutrinos. Further, it's that neutrino burst that drives the explosion.

Could we be wrong? Sure, but being wrong in just one thing won't do it - we need to be wrong with a lot of nuclear physics and astrophysics.

Perhaps also the key is that if the neutrino is antimatter

I don't get this at all. Neutrinos are matter, just like electrons and antineutrinos are antimatter just like positrons. The 1987 detectors happened to pick processes that were more sensitive to antineutrinos, for a number of reasons, few of which had anything to do with SNe.

 

[Ken G]

I don't think this is in question. We know the star's state changes from half neutrons and half protons to one that is mostly neutrons. That means you expect a pulse of neutrinos. Further, it's that neutrino burst that drives the explosion.

The central premise of all science is that everything remains in question until it is observationally demonstrated. I'm sure you could, as easily as I, list all the things scientists were extremely confident about that turned out to be wrong! Of course we act on our best current knowledge, but it is always important to keep track of what has actually been observationally demonstrated, and what is still an expectation. I'm not saying we've never observed a neutrino over antineutrino excess from supernovae, merely that I haven't yet seen anything that comes right out and says this has been observed.

Could we be wrong? Sure, but being wrong in just one thing won't do it - we need to be wrong with a lot of nuclear physics and astrophysics.

There certainly have been similar reactions observed in laboratory settings, but one thing astronomy provides is phenomena on a whole different scale. It is always possible there are things going on out there which we have not yet been able to produce in laboratories. Not likely, but science also includes what is possible. The best discoveries are always the biggest surprises! I certainly agree that the idea that electrons are going to make electron neutrinos in a supernova seems like a pretty strong expectation. But what if it is discovered that over astrophysical scales, neutrinos can oscillate into antineutrinos? Remember that the whole phenomenon of neutrino oscillation was not an expectation until it was discovered in an astrophysical setting. (And yes, lepton number conservation goes a lot deeper than neutrino oscillation, I'm just saying, who knows.)

I don't get this at all. Neutrinos are matter, just like electrons and antineutrinos are antimatter just like positrons. The 1987 detectors happened to pick processes that were more sensitive to antineutrinos, for a number of reasons, few of which had anything to do with SNe.

Perhaps it wasn't clear that I am talking about why it was chosen to detect antineutrinos, and suggesting that it could well be because antimatter annihilates with matter, and we have matter in our detectors. Why would that need to have something to do with SNe?

 

[Vanadium 50]

The central premise of all science is that everything remains in question until it is observationally demonstrated.

You'll find that that position soon gets you wrapped up in knots. How do we know that stars beyond our sun are powered by nuclear fusion? Maybe they are powered by some kind of energy non-conservation and our sun is the only one powered by fusion. Prove otherwise! In fact, maybe stars are closer and weaker, and they exhibit annual parallax because they are moving in a 365 day cycle that just so happens to mimic it. And so on.

I would instead say that the better approach is to assume that the physics out there is the same as the physics here until shown to be otherwise. In the case of weak interactions, we have explored them at both lower and higher energies than found in stars and don't see any reason to presume it is any different in stars than in the laboratory.

(And remember, stellar cores aren't all that hot - megakelvins, not terakelvins. Hot enough to fuse protons, but cold enough that it takes billions of years on average)

I very much want to see the neutronization neutrino pulse, because it will allow comparison with models. But I have no doubt that it exists at all:

1. You need neutrinos to balance the equations
2. You need neutrinos to drive the explosion
3. You need neutrinos to produce secondary nuclei like ⁵⁶Ni.

because antimatter annihilates with matter

If I had a tank of cold neutrinos as my detector, maybe. But "annihilate" is a less good term than "interact" when it comes to neutrinos. Anyway, it turns out that, all other things being equal, neutrinos are about twice as likely to interact with matter as antineutrinos.

These experiments were looking for proton decay. A side effect was that they are more sensitive to antineutrinos than neutrinos in the relevant energy range.

 

[malawi_glenn]

But what if it is discovered that over astrophysical scales, neutrinos can oscillate into antineutrinos? Remember that the whole phenomenon of neutrino oscillation was not an expectation until it was discovered in an astrophysical setting.

Neutrino oscillations were proposed / predicted (about 10 years earlier) before the "solar neutrino problem" was a "thing" . But, most physicists were reluctant to use it as an explanation.

 

[Vanadium 50]

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

If that happens, the inverse process must happen as well. So you'd get half neutrinos and half antineutrinos. More relevantly, most of the neutrinos are produced in the neutronization phase, which lasts under a second. If these turned into antineutrinos, IMB would have seen a very different distribution than they did.

But that's a statement about neutrino propagation, not about neutrino production.

 

[Ken G]

You'll find that that position soon gets you wrapped up in knots. How do we know that stars beyond our sun are powered by nuclear fison? Maybe they are powered by some kind of energy non-conservation and our sun is the only one powered by fusio. Prove otherwise!

It's not the goal to prove it, proofs are for mathematicians not scientists. Instead, we make predictions, and test them-- but we cannot test everything, so there are some things we must take as assumed. One is that if we do a test today, we don't need to repeat it tomorrow. But note this is not so much an assumption as a tested hypothesis-- it has been observed to hold, we just can't prove it will hold tomorrow (because we never prove anything). But there's a difference between saying, we have already tested something in one context and we think it should hold in a similar context without having to test everything, from recognizing something that hasn't actually been tested in the context of interest. That's the issue with neutrinos from supernovae-- if no one has observed (and I don't know if this is true) neutrinos from a supernova, only antineutrinos, then we have something that has not been tested and we should at least recognize that.

In the same vein, I would note that there was considerable satisfaction when antineutrinos were observed from SN 1987A, even though this was certainly expected. How can we claim that it was exciting to detect antineutrinos from a supernova as a check on our theories, but we know they will be outnumbered by regular neutrinos so we don't have to check that?

I would instead say that the better approach is to assume that the physics out there is the same as the physics here until shown to be otherwise. In the case of weak interactions, we have explored them at both lower and higher energies than found in stars and don't see any reason to presume it is any different in stars than in the laboratory.

I completely agree, that's basic Occam's Razor-- we hold to the simplest version until found otherwise, since we have simple brains and cannot overcomplicate things. But we still want to know if the simplest version will hold, so we test it whenever we can. We have tested that it works to assume the same physics "out there" as "down here" (which came as a big shock when first discovered), so that's a tested hypothesis but it could have limitations. (Again, neutrino oscillations were not known from the laboratory, as they require large scales.) I recall a person who claimed that the purpose of the mission Gravity Probe B was to "verify that GR is correct." I say no-- the purpose was to "test GR," a subtle but important difference.

I very much want to see the neutronization neutrino pulse, because it will allow comparison with models,. But I have no doubt that it exists at all:

1. You need neutrinos to balance the equations
2. You need neutrinos to drive the explosion
3. You neeed neutrinos to produce secondary nuclei like ⁵⁶Ni.

I agree that these are all good indirect ways to predict the neutrino flux. We also had good ways to predict the gravitational wave signal of various astrophysical systems, and I'm sure many GR theorists were just as confident that gravitational waves were a real thing, but it was still a big headline when gravitational waves were actually directly detected. We get surprised a lot, just not most of the time.

If I had a tank of cold neutrinos as my detector, maybe. But "annihilate" is a less good term than "interact" when it comes to neutrinos.

As I said, it's not the neutrinos that annihilate, but when antineutrinos interact they create antileptons, and those annihilate. I think that might be the reason it is easier to detect antineutrinos, but it could be other things too. I'm basically curious as to why it is easier to detect antineutrinos, and I'm curious if we have ever tested that supernovae produce far more neutrinos than antineutrinos.

Anyway, it turns out that, all other things being equal, neutrinos are about twice as likely to interact with matter as antineutrinos.

These experiments were looking for proton decay. A side effect was that they are more sensitive to antineutrinos than neutrinos in the relevant energy range.

Kamioka-type experiments don't look for proton decay, yet as far as I can tell they also detected antineutrinos from supernovae. That's the context I was referring to, Cerenkov emission in water.

 

[Ken G]

Neutrino oscillations were proposed / predicted (about 10 years earlier) before the "solar neutrino problem" was a "thing" . But, most physicists were reluctant to use it as an explanation.

A lot of things are proposed, but when physicists are reluctant to use it as an explanation, that's pretty much the definition of "not expected." Still, I take your point that it was not completely unexpected-- there was some theoretical basis. There's often a complex interplay between theory and observation before a new idea is fully mature.

 

[snorkack]

I don't know why it couldn't be an electron neutrino interacting with a neutron to create a proton and electron, that flavor of "inverse beta decay" would seem like a natural way to make high-energy electrons that could produce Cerenkov radiation.

It is true that the well-known neutrino detectors KamiokaNDE and KamiokaLAND detect a different type of inverse beta decay that is an electron antineutrino interacting with a proton to make a neutron and a positron. For some reason (maybe the 1.8 MeV reaction threshold is lower than for neutrinos interacting with neutrons, or maybe the cross section is just higher), this reaction is more prevalant than the first type of inverse beta decay involving regular neutrinos, so as far as I can tell, it might still be true that regular neutrinos from supernovae have not been detected directly, and remain a theoretical expectation.

You cannot actually invert beta decay
n=p+e^-+ν^~
because it forms 3 particles. You cannot invert positron emission, same reason.
You might invert electron capture because it is a two particle process:
p+e^-⇔n+ν
And you can have a process which is not an inverse of a likely process:
p+ν^~=n+e⁺
the opposite process is improbable because positrons are rare, so are free neutrons, while bound neutrons repel positrons.

 

[Vanadium 50]

. I'm basically curious as to why it is easier to detect antineutrino

Because the detector was set up to be more sensitive. Specifically, they used oxygen as the target. Had they used another nucleus (such as argon), it might have been more sensitive the other way.

 

[malawi_glenn]

I think that might be the reason it is easier to detect antineutrinos, but it could be other things too. I'm basically curious as to why it is easier to detect antineutrinos, and I'm curious if we have ever tested that supernovae produce far more neutrinos than antineutrinos.

Huge neutrino detectors, such as Ice Cube, uses cherenkov light as the signal to detect. Thus, as long as you get a muon or anti-muon that travels very fast - you can detect them and thus infer interactions from muon-neutrinos and muon-antineutrinos. Drawback: they can not distinguish between those interactions (afaik).

When was the last time we had a supernovae nearby (I mean approx 100 k ly) that we could test?

You cannot actually invert beta decay

Inverse beta decay is standard nomenclature for the process $\bar \nu_e + p \to e^+ + n$
https://en.wikipedia.org/wiki/Inverse_beta_decay

 

[Ken G]

You cannot actually invert beta decay
n=p+e^-+ν^~
because it forms 3 particles. You cannot invert positron emission, same reason.

Well, formally speaking, all processes must have an inverse (it's a thermodynamic requirement), but your point is that some are prohibitively unlikely to find conditions where they are in equilibrium as in the case you mention. Nevertheless, the other processes that start out with a neutrino often do get called "inverse beta decay," though it is probably just informal language, as per your point that the true version basically never happens.

You might invert electron capture because it is a two particle process:
p+e^-⇔n+ν

Yes, and indeed there are times during core collapse where this process reaches equilibrium.

And you can have a process which is not an inverse of a likely process:
p+ν^~=n+e⁺
the opposite process is improbable because positrons are rare, so are free neutrons, while bound neutrons repel positrons.

Right, so the thermodynamic conditions needed for equilibrium are very hard to achieve in some cases. This leads to some very odd situations, like the "ORCA" process, where a neutron can produce a proton and have an (anti)neutrino escape, and the proton can be turned back into a neutron and another neutrino escapes! Magic, constant neutrino losses without any other change, which play a big role in core collapse.

 

[Ken G]

Huge neutrino detectors, such as Ice Cube, uses cherenkov light as the signal to detect. Thus, as long as you get a muon or anti-muon that travels very fast - you can detect them and thus infer interactions from muon-neutrinos and muon-antineutrinos. Drawback: they can not distinguish between those interactions (afaik).

Ah, so you are saying that even if Ice Cube detected a supernova, we might not be able to tell if we were seeing neutrinos or antineutrinos! The plot thickens.

When was the last time we had a supernovae nearby (I mean approx 100 k ly) that we could test?

Good question, there is a supernova about every 100 years in each galaxy, and we don't have many galaxies that close, basically just one. So I'm guessing there hasn't been one that close in a long time, I believe 1987A is the closest one we've had and we didn't have the detectors to tell. Then again, as you point out, maybe we still don't!

 

[Ken G]

Because the detector was set up to be more sensitive. Specifically, they used oxygen as the target. Had they used another nucleus (such as argon), it might have been more sensitive the other way.

That's interesting, it sounds like you are saying the nuclei interact differently with neutrinos and antineutrinos, so you control which one you see by the nuclei you choose. So the number we see is the product of the cross section times the flux, so we cannot tell if we are seeing one or the other unless we have a handle on the expected fluxes.

 

[Ken G]

I tried ChatGPT on this and it said that all the detectors that saw 1987A neutrinos could not distinguish neutrinos from antineutrinos, so it didn't seem to think it was true that we saw antineutrinos from SN 1987A. Since ChatGPT is not always reliable, I'm not sure if this is correct, but it also said they were all Cerenkov detectors, which is consistent with what malawi_glenn is saying. So if we expect many more neutrinos than antineutrinos, then even if the detector is better tuned for antineutrinos it doesn't necessarily imply that's what was seen.

 

[swampwiz]

Perhaps also the key is that if the neutrino is antimatter, it can create antimatter (like a positron) that can annihilate and this helps with the detection of the original antineutrino. It would certainly be ironic if a particle as ghostly and otherworldly as a neutrino is easier to detect when it is even more otherwordly antimatter. Experiments at Fermilab detect both types of neutrino, so it's not like regular neutrinos have not been well established, but there is still a lot about them that is not known (such as their mass). There is even this cryptic quote from Fermilab (https://www.fnal.gov/pub/science/particle-physics/experiments/neutrinos.html#:~:text=Fermilab's NOvA experiment sends a beam of neutrinos,whether neutrinos and antineutrinos oscillate at different rates.): "Neutrinos could have other strange properties as well. They could turn out to be identical to antineutrinos, their antimatter counterparts." So oddly, it might end up that the distinction we are discussing might not even exist!

OK, this is a stupid question, but here goes. Neutrinos & antineutrinos don't have a charge, so why is any particular one considered regular with the other being anti? Did some physicist just arbitrarily decide it?

 

[malawi_glenn]

Ah, so you are saying that even if Ice Cube detected a supernova, we might not be able to tell if we were seeing neutrinos or antineutrinos! The plot thickens.

Do not take my word for it. I could do some research and ask some people I know that work with Ice Cube. This was just from my basic understanding that you can not tell the difference between a cherenkov cone from a muon and an antimuon. Perhaps there is though. Let me get back to this.

Good question

It was a rethorical question ;) SN1987A is the most recent close enough supernovae.

I tried ChatGPT on this and it said that all the detectors that saw 1987A neutrinos could not distinguish neutrinos from antineutrinos, so it didn't seem to think it was true that we saw antineutrinos from SN 1987A.

Read about the actual experiements yourself instead.
- Kamiokande II
- IMB
- Baksan (BNO)

 

[malawi_glenn]

OK, this is a stupid question, but here goes. Neutrinos & antineutrinos don't have a charge, so why is any particular one considered regular with the other being anti? Did some physicist just arbitrarily decide it?

It has to do with lepton number https://en.wikipedia.org/wiki/Lepton_number

 

[Ken G]

OK, this is a stupid question, but here goes. Neutrinos & antineutrinos don't have a charge, so why is any particular one considered regular with the other being anti? Did some physicist just arbitrarily decide it?

I believe the definition was to preserve lepton number, where anti-leptons get a negative lepton number. Since this seems to be possible to define to make it work out, we of course are going to go ahead and define it that way. Probably a deeper answer is the standard model predicts which is which, and so as long as the standard model seems to work, we'll go with that. I presume the Fermilab claim that they might end up being their own antiparticles would represent a deviation from the standard model!

 

[Ken G]

It was a rethorical question ;) SN1987A is the most recent close enough supernovae.

Ah, I see. Yes, we have a dearth of data here....

Read about the actual experiements yourself instead.
- Kamiokande II
- IMB
- Baksan (BNO)

That's a bit of work! It will be nice when AI answers can actually be trusted...

 

[malawi_glenn]

That's a bit of work! It will be nice when AI answers can actually be trusted...

Hard work is the key to success and understanding :)

 

[Vanadium 50]

You cannot actually invert beta decay

As @malawi_glenn said, that's what it's called. You don't complain that you drive on a parkway and park on a driveway, do you?

 

[Vanadium 50]

I tried ChatGPT

Which is about as reliable as a fortune cookie,

 

[Ken G]

It's better than that, but it certainly can't be trusted. It can give a very nice answer to a difficult question, but it can also mess up a simple one! It's actually not a bad place to start, but it's a terrible place to finish. Much like everything else about the internet!

 

[Vanadium 50]

That's interesting, it sounds like you are saying the nuclei interact differently with neutrinos and antineutrinos,

They do. If a (anti-)neutrino interacts with a nucelus, a (positron) electron is emitted, and the buclear charge (decreases) increases by one. So event rates are determined by the nuclear properties of the target nucleus and the adjacent nuclei.

As a practical matter, only a few nuclei are considered. You need kilotons of target or so, so it needs to be cheap, and since most detectors rely at least partially on optical signals, it's better if it were transparent. That leaves water (oxygen), argon, in principle nitrogen (but anything nitrogen can do argon can do better), and a few others. Art MacDonald managed to borrow tons of deuterated water from the Canadian government - there is no way he could afford to buy it.

IceCube has a thresholds much, much higher than typical SN neutrinos. Its job isn't so much to study normal SNe, but the few that are thought to drive cosmic ray production, AGNs, etc, It is approximately twice as sensitive to neutrinos as antineutrinos.

 

[Ken G]

Thus capture of an antineutrino (resulting in simultaneous emission of three photons of fixed energies) is a much more distinctive event than capture of a neutrino (resulting in emission of just one electron of no fixed energy).

This is not a hijack, it's an effort to answer the question I raised earlier-- since it was suggested that neutrinos seen from 1987A were antineutrinos, why is it easier to detect an antineutrino than a neutrino. That suggested answer was specific to the electron neutrino, and I believe Cerenkov detectors get all three types of neutrinos because there are decay channels that involve other things than electrons and positrons (and malawi_glenn mentioned muons). But it's a reasonable point, related to what I mentioned earlier about the fact that if you create antimatter in a detector containing regular matter, you will get certain telltale signs of annihilation, which might help signal that an antineutrino entered the detector. However, Cerenkov detectors don't focus on light created by annihilations, they focus on Cerenkov light, so at the moment I'm still not even clear that the Cerenkov detectors do detect more antineutrinos than neutrinos from supernovae, since such sources should emit way more neutrinos than antineutrinos, and the signal we get might be dominated by processes that cannot distinguish.

 

[Vanadium 50]

Good question, there is a supernova about every 100 years in each galaxy,

People estimate 30 for the Milky Way. So we're due,,

One might look at M31 and M33 and try and draw conclusions. The problem is that the last SN in M31 was in 1885 and there has never been one observed in M33.

Thing is M31 has a low star formation rate, and M33, while a little better, is a small galaxy. The Milky Way is thought to have a high star formation rate, but 85% of it is obscured by dust.

 

[Ken G]

Many answers are in a paper in 1987 by Krauss, which is behind a pay wall I believe.

[Another reference to ChatGPT redacted by the Mentors]

It is true what was said in the thread that the neutrinos from 1987A were probably antineutrinos (based on theoretical flux expectations and the fact that the cross section for the antineutrino-nucleus interactions is about 70 times higher than for neutrino-electron scattering, as well as what was said that antineutrinos react better with these nuclei), and it is also true that none of these detections can directly tell neutrinos from antineutrinos, even though the processes that create the Cerenkov light are very different. The big thing that is clear from this paper is that the theoretical expectation is that the total neutrino+antineutrino production rate is quite a bit larger than the excess neutrinos you get from lepton number conservation. (The paper points out that the positive lepton excess comes from deep in the supernova where neutrinos do not easily escape, so perhaps the extra leptons do not generally come out as neutrinos at all, and even if they do, it's not a large fraction of the total.)

So a core collapse supernova is now thought to produce similar numbers of neutrinos and antineutrinos, but since the nuclei in the detectors have a much higher cross section for interacting with antineutrinos, and that is also much higher than neutrino-electron scattering cross sections, the theoretical expectation is that all the neutrinos detected were probably antineutrinos, in stark contradiction to ChatGPT (but at least it was right about the different detection channels). The theoretical expectations fit the data well, so it's good evidence that the model passed a test here, but we are just shy of being able to say we directly detected antineutrinos rather than regular neutrinos. (But we are pretty close-- it would be very strange indeed if somehow there were at least 70 times more neutrinos than antineutrinos, that would totally put neutrino physics on its ear.)

I probably should have just done this work in the first place, but there you have it.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

After some cleanup, the thread is reopened.

 

[malawi_glenn]

This is a good read on the Ice Cube (theoretical) performance on supernovae neutrinos if anyone is interested https://arxiv.org/abs/1106.6225

 

[Ken G]

On the issue of why nuclei have larger cross sections for antineutrinos, it seems to me this should not be a nucleus-dependent issue, it should have simply to do with the fact that an antineutrino can interact with a proton in a way not available to neutrinos. It's very much the antimatter aspect, though not so much an issue of annihilation-- a proton has a positive charge, so it can make a positron (or antimuon, or antitauon) when something with negative lepton number bashes into it, hence the antineutrino but not the neutrino. This cross section is way larger than neutrino-electron scattering cross sections (available to both neutrinos and antineutrinos because it is just a scattering by the weak force), so the mere fact that our detectors contain lots of protons and not antiprotons is the reason we detect antineutrinos so much more easily. Core collapse supernovae are copious sources of both neutrinos and antineutrinos, so until we detect a very large number of neutrinos in the Cerenkov detectors (which will require a supernova in our galaxy), we will not detect neutrinos from supernovae.

Alternatively, perhaps we have already seen just as many neutrinos as antineutrinos, and we simply don't have the statistics or the theoretical accuracy to know if we are getting the fluxes wrong by a factor of 2 (because as the Fermilab site above said, we don't really know we have the neutrino physics correct). The Fermilab site intimated that maybe neutrinos will end up being their own antiparticles ("Majorana fermions"), in which case, regular neutrinos should be just as able to make positrons (and antimuons and antitauons) as antineutrinos can. I'm puzzled as to why our laboratory experiments don't already know if that is happening or not, but I guess it's just hard to know what kind of neutrino you have, if you don't know how they differ.

 

[snorkack]

If neutrinos can oscillate to antineutrinos, should we have seen solar neutrinos oscillated into antineutrinos?

 

[Ken G]

Yes you'd think this would have been tested by now, by using a source you think should have few antineutrinos and lots of regular neutrinos, and a detector that mostly detects antineutrinos, and see if you get way more of them than you were expecting. For some reason, this has not yet been done in the Fermilab neutrino experiments, or they would know by now. (I don't think the issue is oscillation, it is simply if neutrinos and antineutrinos are the same particle.)

 

[Vanadium 50]

Reason 1 is that even a neutrino beam contains a few percent of antineutrinos. It's easier to say "create a pure beam" tnan it is to actually do it.

The second reason is that if you allow the decay \pi^- \rightarrow e^- \nu in addition to the normal antineutrino, it goes about 20,000x faster. So even a tiny mixing between neutrinos and antineutrinos would be immediately obvious.

In the past you've argued, yeahbut they didn't measure this exact thing. Science witha capital S says we should do itl, It is hard to get support to mount an experiment that is 100-1000x less sensitive than existing measurements because in some unspecufied way maybe this time thinghs are just different.

 

[Ken G]

Well, not quite-- even the Fermilab site explicitly dangles the possibility that neutrinos are Majorana fermions! (OK, maybe they are just looking for funding, but it's legitimate scientific curiosity, not something that has been ruled out yet.) As it happens supernovae are expected to be a pretty equal source of both neutrinos and antineutrinos, so if we think we are only detecting antineutrinos, and neutrinos are the same particle, we have an observation that is only a factor of 2 too high. The combined low statistics with theory uncertainties make it difficult to be able to notice that. (25 counts, many of which are thought to be noise, means that if we think only 20 were real counts as Strauss suggests, and we think that means the supernova should have given us 40 of which we could only see 20, it seems to us like all is well. But maybe a repeat of the experiment would have seen 30 by chance, and that's all the supernova gave us because the theory was high by 30%, then we would have been fooled by Majorana fermions). That's why we have to keep track of what we have actually established by observation, to high confidence.

If we do get a supernova in our own galaxy, we'll have plenty of statistics, and maybe we'll have better theory and will be able to use a factor of 2 difference between what is observed and what we expect to discover that neutrinos actually are Majorana fermions, and we would immediately stop saying we knew all along that what we detected before were bona fide antineutrinos. So often this has happened in the history of science, we should know better by now!

 

[snorkack]

On the issue of why nuclei have larger cross sections for antineutrinos, it seems to me this should not be a nucleus-dependent issue, it should have simply to do with the fact that an antineutrino can interact with a proton in a way not available to neutrinos. It's very much the antimatter aspect, though not so much an issue of annihilation-- a proton has a positive charge, so it can make a positron (or antimuon, or antitauon) when something with negative lepton number bashes into it, hence the antineutrino but not the neutrino.

It is a heavily nucleus-dependent issue. Consider oxygen instead of protium...
In case of O-16, F-16 is unbound, meaning that a neutrino has to remove a neutron from O-16 and leave behind O-15 (halflife 2 min) - a threshold of 15 MeV or so. An antineutrino would produce N-16 (halflife 7 s).
In case of O-18, F-18 half-life is 110 min and N-18, while bound, has half-life 0,6 s. Since neutrino absorption in O-18 is inverse electron capture of F-18, it has threshold of about 1,66 MeV.
Etc., etc.. The thresholds and cross-sections are going to depend on the specifics of daughter nuclei.

What is a bias for antineutrino: what do you get when you absorb a neutrino? A fast electron. Which looks much like a fast electron emitted by beta decay.
Absorb an antineutrino? Sure, a positron emitted by antineutrino looks much the same as a positron emitted by positron decay. But positron emitting isotopes are somewhat less common in nature than electron emitting ones. (For example K-40 emits both but far fewer positrons than electrons). It is not so much that antineutrino absorption has higher cross-section but that it seems to have lower background noise of similar looking but different events.

This cross section is way larger than neutrino-electron scattering cross sections (available to both neutrinos and antineutrinos because it is just a scattering by the weak force), so the mere fact that our detectors contain lots of protons and not antiprotons is the reason we detect antineutrinos so much more easily.

Our detectors contain a lot of neutrons, though.

Core collapse supernovae are copious sources of both neutrinos and antineutrinos, so until we detect a very large number of neutrinos in the Cerenkov detectors

There are two basic ways of neutrino/antineutrino interacting.
One is absorption. This has flavour specific information and also receives the whole energy and momentum of the incoming particle.
And the other is elastic scattering. This is flavour unspecific, and while it is constricted in terms of energy and momentum, it does not take the full momentum information of the incoming particle (because it moves on with unknown energy).
Both of these usually produce a rapid lepton. (The obvious exception is events of elastic scattering off baryons, but those are hard to detect anyway). The absorption also produces altered/unstable nuclei.
Detecting the rapid lepton is already the next step. Cherenkov detectors have high energy threshold, but have some directional information. Scintillation has far lower threshold, so catches lower energy events, but seems to lose the direction information.

 

[Ken G]

OK, thanks, now I understand what @Vanadium 50 meant. I did not recognize that an antineutrino knocking a positron off a proton is not really all that different from a neutrino knocking an electron out of a neutron. To the weak force, a proton and a neutron must not look all that different, so one has to consider the more detailed issues you are talking about, and that @Vanadium 50 alluded to earlier. Since we always have lots of water, it will probably always be easier to detect antineutrinos, so it looks like the best scenario for using astrophysical sources to test if neutrinos are Majorana fermions is to wait for a supernova in our galaxy and count on the factor 2 difference in detectable neutrinos to decide the issue, assuming our models can be relied on at the factor 2 level.

 

[PeterDonis]

Moderator's note: Discussion of Majorana neutrinos has been moved to a separate thread:

https://www.physicsforums.com/threads/majorana-neutrinos.1050693/