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!