The Super Kamiokande had a terrible engineering event where the delicate sensor bulbs shattered, and the pressure delta from one shattering caused neighbors to shatter, in a chain reaction that destroyed large amounts of sensors.
>Neutrinos come in three different “flavors” (electron, muon, and tau) and can oscillate, or switch, between them. To do so, neutrinos must have mass
Why? What actually is "Neutrino oscillation" and why does it require the neutrino have mass? My already feeble understanding of particle and quantum physics always breaks down at these sorts of points.
How are we sure that the neutrino is in fact a single particle that should use the same sort of mathematical machinery as all others? Am I even asking a question that means something? I know literally every physicist ever graduated has spent time thinking everything in physics is wrong and tried poking at such ideas, so I guess I'm more interested in what those kids end up finding that brings them back to "No this makes more sense" of neutrinos in the standard model.
For a particle to "oscillate", it must "experience" time. All massless particles travel at the speed of light. As a consequence of special relativity, they don't "experience" time.
Therefore, neutrinos must be traveling slower than light, and they must have mass.
Oh, duh. I can follow that logic now. The meat of my question is much dumber though:
How do we know the "Same" neutrino is oscillating? We don't even have concrete understanding of how they would have mass, and different existing concepts of how it could be are problematic.
There's so much the standard model isn't sufficient for, in terms of explanations and predictions and categorization, that it always feels odd to me when we shove another weird thing into the "Particle" bucket.
It's also a dumb complaint though. A lot of deficiencies probably come down to simply not having enough good data to distinguish different ideas. It's hard to get good data with something that "Barely interacts" with anything else, by definition.
Also maybe my complaint is entirely semantic, that a naturally unfinished or incomplete theory is presented as "We know". If you model a scientific theory developing over time, are we still so early that neutrino oscillation could actually be entirely different? Or do we actually have the data to demonstrate that "No, a singular neutrino absolutely changes to different flavors over time, nothing else could cause the effects we see in X, Y and Z demonstrations"?
Like, I have sky high confidence that the standard model captures and predicts things like electrons and protons and quarks extremely well, so it always feels dissonant when we see things get weird like this, but also nature doesn't promise us coherent rules, just consistent ones. Reality could very well be full of crummy edge cases.
Just sucks that I'll be dead before we really figure most of this stuff out.
Also WTF even is time.... Why does something that is in one state, sometimes, be in a different state.... Is it even real? You can travel through space because you can have a spacial velocity, and that velocity can change through forces acted upon you, but is it even possible for there to be an analogous set of forces that can change your "Time velocity"....
I'll have to buy my physicist friend a drink so I can have him laugh at me for weird, half baked philosophy questions that aren't really valid.
One can make separate detectors for electronic, muonic and tauonic neutrinos, because they take part in different reactions.
Given a flux of neutrinos, e.g. coming from the Sun or from a nuclear reactor, one can use the different kinds of neutrino detectors to estimate the total flux and the fractions of the three kinds.
With another set of detectors put somewhere else, at a great distance along the direction of propagation of the neutrino flux, i.e. where those neutrinos arrive later, one can measure again the fractions of the total flux.
If one measures different fractions, and knowing the propagation time between the 2 locations, one can conclude that oscillations exist and measure the frequency of the oscillations.
Nonetheless, this is much easier said than done, All neutrino experiments have extremely poor signal-to-noise ratios and all their results are affected by great uncertainties.
The theory about the existence of the neutrino oscillations had been originally proposed as an explanation for the fact that the flux of neutrinos coming from the Sun was about 3 times smaller than predicted by the modelling of the fusion reactions inside the Sun.
Later, experimental results from measuring the fractions of the different neutrino kinds at distant locations appeared to support the oscillation hypothesis.
I'm not a physicist so take it with a grain of salt:
> If you model a scientific theory developing over time, are we still so early that neutrino oscillation could actually be entirely different? Or do we actually have the data to demonstrate that "No, a singular neutrino absolutely changes to different flavors over time, nothing else could cause the effects we see in X, Y and Z demonstrations"?
We are still early in the sense of "why neutrinos have mass" but the evidence for neutrino oscillation itself is very strong. The classic experiment is measuring the neutrino flux coming off of our sun: the total neutrino flux matches solar-model expectations but without neutrino oscillation, the electron neutrino flux does not, and the missing fraction depends on distance divided by energy.
The T2K experiment has measured the oscillation of a muon neutrino beam over about 300km and the Daya Bay experiments measured electron anti-neutrino oscillation from nuclear reactors over a distance of several kilometers. At this point the evidence required to overturn neutrino oscillation would have to be extraordinary.
> Like, I have sky high confidence that the standard model captures and predicts things like electrons and protons and quarks extremely well, so it always feels dissonant when we see things get weird like this, but also nature doesn't promise us coherent rules, just consistent ones. Reality could very well be full of crummy edge cases.
My understanding is that the mathematical machinery created to explain quark flavors is also used to explain neutrino flavors, so we're not dealing with a unique snowflake in physics.
With the caveat that it’s been almost 30 years since I worked on neutrino oscillation, and I was just a student at the time, the way to detect neutrino oscillation is to look at the ratio of different kinds of neutrinos. If those ratios are different from the ratios calculated to be produced by the reaction at hand (eg a collision in a particle accelerator), then you know some of the neutrinos changed flavor.
I used to think this was the explanation, but I was told by a particle physicist that this is actually not correct. Unfortunately I don't remember the correct argument (and I'm not sure I understood fully it in the first place)
It's kind of useful to only think of these things as "particles" in a nominal sense. You need to adopt a "quantum imagination". I tend to think of them as a wave or field of probabilities of energy. It sounds weird, but similar to "spin", "flavour" describes a particular relationship between waves or fields of probabilities of energy as it moves through space over time.
A simplified summary: The discovered mass emerges out of this relationship between detection and probability.
This is making it sound way more complicated than it is. Sibling comment is much better. Thing changes over time -> thing experiences time -> it's not going the speed of light -> it has mass.
And a nuance that is easily lost: the different neutrino eigenstates/flavors travel at different velocities, because they have different mass, so the probability of detecting one flavor depends on the distance travelled.
Essentially each neutrino travels in three different "waves", but is still one unit in a constant superposition between the three.
I'm no physicist, this is just how I understood this thread; but AIUI it's better to think of the neutrino being in all three eigenstates simultaneously, and if any of the three "bump into something" then it collapses to just the one flavor.
That would be a wild thing to accomplish, given that neutrinos are created at relativistic speeds, have virtually zero mass, and don’t react with anything but gravity and the weak force.
Whatever gravity “trap” you make is going to pale in comparison to the gravity wells around us (earth, the sun, etc) and the weak force scales exponentially with energy. So a slow-moving neutrino would interact even less with the weak force than a relativistic one.
Probably! And even potentially some orbiting the planet. They’d be part of the cosmic neutrino background, neutrinos around since the big bang which have had their velocity red-shifted to Newtonian speeds.
But they interact with things even less than relativistic neutrinos. Even a light-year of lead wouldn’t be enough to register a hit from one.
And even if you did have light-years of super dense detector material lying around, the energy deposited by such a neutrino is so tiny that you wouldn't be able to register the hit.
Speed is relative, so if you accelerated away from the sun at 0.999999c hoping to hang out in a slow-moving stream of solar neutrinos, you’d remove the energy needed for them to interact with the weak force.
They’d be all around you, but you would have exponentially smaller chances of interacting with them.
From the (captions under photographs) in the article:
'Located 2.1 kilometers underground in the Creighton mine in Ontario, Canada, the Sudbury Neutrino Observatory’s detector (left) was filled with “heavy” water, which features deuterium in place of hydrogen atoms. Its findings provided evidence that neutrinos can change, or “oscillate,” between different flavors.'
"China’s Jiangmen Underground Neutrino Observatory (JUNO), seen here under construction in 2023, is currently the world’s largest neutrino detector. It began collecting data in August 2025; one of its main goals is to determine the outstanding mystery of how heavy each flavor of neutrino is."
The second caption implies that some "flavors" of neutrino may be heavier than other flavors. The first caption says that neutrinos oscillate between different flavors. If the first caption is correct, then wouldn't each flavor of neutrino be just as heavy as the others?
Some real physicist tell me what I’m getting wrong here:
neutrinos have mass but the magnitude is so infinitesimally small, and the oscillation smaller still, such that you’re bumping up against some fundamental properties of the universe and the oscillation is “borrowing” and “returning” some tiny amount of mass/energy from the vacuum.
Not a physicist, but here are a couple Wikipedia excerpts I found:
From page “Conservation of mass”:
“In general, mass is not conserved. The conservation of mass is a law that holds only in the classical limit.”
From page “Neutrino oscillation”:
“Neutrino oscillation arises from mixing between the flavor and mass eigenstates of neutrinos. That is, the three neutrino states that interact with the charged leptons in weak interactions are each a different superposition of the three (propagating) neutrino states of definite mass. Neutrinos are produced and detected in weak interactions as flavour eigenstates[a] but propagate as coherent superpositions of mass eigenstates.[23]”
Maybe it isn’t changing mass at all, and what’s changing is just the probability distribution resulting from a hypothetical flavor measurement. If we could measure the mass (collapse to mass eigenstate), I think I understand we would see no oscillation.
In a great coincidence, just months after being put into operation, a new type of detector meant to study if protons are stable, detected instead the neutrinos coming from the supernova in a way that could be individually timestamped. No other supernova has been visible with the naked eye ever since, or is likely to be seen in our lifetimes.
The neutrinos arrived 3 hours before the supernova was first seen in the sky. This could mean that neutrinos travel faster than light: that they negative mass. It has not been ruled out yet, but it would take extraordinary evidence for any physicist to admit to a faster-than-light particle.
The core of supernovae are predicted to take around 4 seconds to explode, and during that time they release 99% of the energy that was binding the star together as neutrinos. Once this has happened, however, it takes several hours for the explosion to be visible outside.
If they had 0 mass, you'd expect the burst of neutrinos to last for about 4 seconds. The detected spread was of around 6 seconds.
It was likely that the model for supernovae was missing something. But perhaps neutrinos are slowed down as they interact with other particles on the way here from the supernova. Massless particles always travel at the speed of light, only massive particles can be slowed down. Perhaps neutrinos have mass.
In order to test the basic theory of nuclear fusion in stars, the "Homestake experiment" was set up to count neutrinos from the Sun.
The experiment consists of a big tank of perchloroethylene. Electron neutrinos (and only electron neutrinos) are expected to collide with the Cl-37 atoms to form Ar-37 in the reverse reaction to that of radioactive beta decay. Then the individual Ar-37 atoms can be separated into a gas and counted.
According to the brightness of the Sun, it was expected that around 50 such Ar-37 atoms would be detected every 100 days. Only 17 were observerd, on average. A third of the neutrinos went missing.
A new round of experiments was made, culminating in the fantastically sophisticated Super-Kamiokande, which is sentitive to all other "flavours" of neutrinos, and also capable of detecting the direction they are coming from.
It was confirmed that the missing 2/3 of the electron neutrinos expected to come from the Sun had somehow transformed into the other flavours. What's more, muon neutrinos coming from cosmic ray decay in the atmosphere were detected half as frequently in an upwards direction, coming from the other side of the Earth, than in the downwards direction.
The conclusion is that neutrinos must change flavour as they move through space.
Particle accelerators were built in several sites, pointing towards the Super-Kamiokande across the earth's crust, sending a beam of neutrinos of a known kind to measure this "oscillation".
Since they have mass, the artificial neutrinos are slightly dispersed by their passage through the Earth's crust. Their arrival time forms three different overlapping peaks, corresponding to three different masses.
The proportion of each kind changes along the path in a sinusoidal fashion, but the mass peaks remain.
The current interpretation is that the mass and the flavour of neutrinos are (almost) "conjugate" properties of the particles, in a similar way to the well-known uncertainty between position and momentum, or between time and frequency in Fourier analysis.
As a neutrino moves through space, its mass can be measured more and more accurately from the dispersion in arrival times. The better we know the mass of the particle, the more uncertain we are of its flavour. If because of the great distances the mass is well known, as is the case with the solar neutrinos, then the three flavours are almost completely entangled.
On short distances the flavour is well determined but the masses are uncertain.
Could a physicist comment on this? I've been reading on the CNO cycle [1], where it says neutrinos produced in the beta-decay steps can have any share of the resulting energy. Does it mean the Sun is shrouded in a rarefied cloud of such low-energy neutrinos which failed to achieve escape velocity?
This is an interesting question, but it would be very difficult to verify.
Only neutrinos with an extremely low kinetic energy would have velocities much lower than that of light, and the fraction of such neutrinos from the total flux of neutrinos would be very small. If they did not escape Sun, most would be probably captured sooner or later by nuclei from the gases of the Solar atmosphere (i.e. chromosphere or corona).
There is a good exhibit on this at the Miraikan in Odaiba, Tokyo. Detecting things and proving we detected what we detected we previously couldn't is always a fascinating exercise, especially whilst so much matter is still unrecognised.
HN automangled the title, should have a “how” at the beginning. The change makes the headline sound like this is news, but it’s just a description of neutrino detectors.
Wonderful pictures!
The Super Kamiokande had a terrible engineering event where the delicate sensor bulbs shattered, and the pressure delta from one shattering caused neighbors to shatter, in a chain reaction that destroyed large amounts of sensors.
https://www.youtube.com/watch?v=YoBFjD5tn_E
Unrelated:
>Neutrinos come in three different “flavors” (electron, muon, and tau) and can oscillate, or switch, between them. To do so, neutrinos must have mass
Why? What actually is "Neutrino oscillation" and why does it require the neutrino have mass? My already feeble understanding of particle and quantum physics always breaks down at these sorts of points.
How are we sure that the neutrino is in fact a single particle that should use the same sort of mathematical machinery as all others? Am I even asking a question that means something? I know literally every physicist ever graduated has spent time thinking everything in physics is wrong and tried poking at such ideas, so I guess I'm more interested in what those kids end up finding that brings them back to "No this makes more sense" of neutrinos in the standard model.
Disclaimer: I'm not a physicist.
For a particle to "oscillate", it must "experience" time. All massless particles travel at the speed of light. As a consequence of special relativity, they don't "experience" time.
Therefore, neutrinos must be traveling slower than light, and they must have mass.
Oh, duh. I can follow that logic now. The meat of my question is much dumber though:
How do we know the "Same" neutrino is oscillating? We don't even have concrete understanding of how they would have mass, and different existing concepts of how it could be are problematic.
There's so much the standard model isn't sufficient for, in terms of explanations and predictions and categorization, that it always feels odd to me when we shove another weird thing into the "Particle" bucket.
It's also a dumb complaint though. A lot of deficiencies probably come down to simply not having enough good data to distinguish different ideas. It's hard to get good data with something that "Barely interacts" with anything else, by definition.
Also maybe my complaint is entirely semantic, that a naturally unfinished or incomplete theory is presented as "We know". If you model a scientific theory developing over time, are we still so early that neutrino oscillation could actually be entirely different? Or do we actually have the data to demonstrate that "No, a singular neutrino absolutely changes to different flavors over time, nothing else could cause the effects we see in X, Y and Z demonstrations"?
Like, I have sky high confidence that the standard model captures and predicts things like electrons and protons and quarks extremely well, so it always feels dissonant when we see things get weird like this, but also nature doesn't promise us coherent rules, just consistent ones. Reality could very well be full of crummy edge cases.
Just sucks that I'll be dead before we really figure most of this stuff out.
Also WTF even is time.... Why does something that is in one state, sometimes, be in a different state.... Is it even real? You can travel through space because you can have a spacial velocity, and that velocity can change through forces acted upon you, but is it even possible for there to be an analogous set of forces that can change your "Time velocity"....
I'll have to buy my physicist friend a drink so I can have him laugh at me for weird, half baked philosophy questions that aren't really valid.
One can make separate detectors for electronic, muonic and tauonic neutrinos, because they take part in different reactions.
Given a flux of neutrinos, e.g. coming from the Sun or from a nuclear reactor, one can use the different kinds of neutrino detectors to estimate the total flux and the fractions of the three kinds.
With another set of detectors put somewhere else, at a great distance along the direction of propagation of the neutrino flux, i.e. where those neutrinos arrive later, one can measure again the fractions of the total flux.
If one measures different fractions, and knowing the propagation time between the 2 locations, one can conclude that oscillations exist and measure the frequency of the oscillations.
Nonetheless, this is much easier said than done, All neutrino experiments have extremely poor signal-to-noise ratios and all their results are affected by great uncertainties.
The theory about the existence of the neutrino oscillations had been originally proposed as an explanation for the fact that the flux of neutrinos coming from the Sun was about 3 times smaller than predicted by the modelling of the fusion reactions inside the Sun.
Later, experimental results from measuring the fractions of the different neutrino kinds at distant locations appeared to support the oscillation hypothesis.
I'm not a physicist so take it with a grain of salt:
> If you model a scientific theory developing over time, are we still so early that neutrino oscillation could actually be entirely different? Or do we actually have the data to demonstrate that "No, a singular neutrino absolutely changes to different flavors over time, nothing else could cause the effects we see in X, Y and Z demonstrations"?
We are still early in the sense of "why neutrinos have mass" but the evidence for neutrino oscillation itself is very strong. The classic experiment is measuring the neutrino flux coming off of our sun: the total neutrino flux matches solar-model expectations but without neutrino oscillation, the electron neutrino flux does not, and the missing fraction depends on distance divided by energy.
The T2K experiment has measured the oscillation of a muon neutrino beam over about 300km and the Daya Bay experiments measured electron anti-neutrino oscillation from nuclear reactors over a distance of several kilometers. At this point the evidence required to overturn neutrino oscillation would have to be extraordinary.
> Like, I have sky high confidence that the standard model captures and predicts things like electrons and protons and quarks extremely well, so it always feels dissonant when we see things get weird like this, but also nature doesn't promise us coherent rules, just consistent ones. Reality could very well be full of crummy edge cases.
My understanding is that the mathematical machinery created to explain quark flavors is also used to explain neutrino flavors, so we're not dealing with a unique snowflake in physics.
With the caveat that it’s been almost 30 years since I worked on neutrino oscillation, and I was just a student at the time, the way to detect neutrino oscillation is to look at the ratio of different kinds of neutrinos. If those ratios are different from the ratios calculated to be produced by the reaction at hand (eg a collision in a particle accelerator), then you know some of the neutrinos changed flavor.
Same! I worked on SNO’s neutral current detectors at LANL (as a student so mostly grunt work). Which project?
I used to think this was the explanation, but I was told by a particle physicist that this is actually not correct. Unfortunately I don't remember the correct argument (and I'm not sure I understood fully it in the first place)
I remember some of the videos in this playlist doing a very good job explaining neutrino oscillations, and what it means, but I don't remember which ones: https://www.youtube.com/playlist?list=PLCfRa7MXBEsqOlL_g6wTM...
Maybe this one? https://www.youtube.com/watch?v=eBT1-dV1BTM
It's kind of useful to only think of these things as "particles" in a nominal sense. You need to adopt a "quantum imagination". I tend to think of them as a wave or field of probabilities of energy. It sounds weird, but similar to "spin", "flavour" describes a particular relationship between waves or fields of probabilities of energy as it moves through space over time.
A simplified summary: The discovered mass emerges out of this relationship between detection and probability.
This is making it sound way more complicated than it is. Sibling comment is much better. Thing changes over time -> thing experiences time -> it's not going the speed of light -> it has mass.
And a nuance that is easily lost: the different neutrino eigenstates/flavors travel at different velocities, because they have different mass, so the probability of detecting one flavor depends on the distance travelled.
Essentially each neutrino travels in three different "waves", but is still one unit in a constant superposition between the three.
So neutrinos are changing mass and velocity constantly?
I'm no physicist, this is just how I understood this thread; but AIUI it's better to think of the neutrino being in all three eigenstates simultaneously, and if any of the three "bump into something" then it collapses to just the one flavor.
For anyone wondering about the title, by "trap" they mean "detect destructively": there are no stable neutrinos in a bottle in this article.
That would be a wild thing to accomplish, given that neutrinos are created at relativistic speeds, have virtually zero mass, and don’t react with anything but gravity and the weak force.
Whatever gravity “trap” you make is going to pale in comparison to the gravity wells around us (earth, the sun, etc) and the weak force scales exponentially with energy. So a slow-moving neutrino would interact even less with the weak force than a relativistic one.
Are there a bunch of slow neutrinos sloshing around near the center of the earth?
Probably! And even potentially some orbiting the planet. They’d be part of the cosmic neutrino background, neutrinos around since the big bang which have had their velocity red-shifted to Newtonian speeds.
But they interact with things even less than relativistic neutrinos. Even a light-year of lead wouldn’t be enough to register a hit from one.
And even if you did have light-years of super dense detector material lying around, the energy deposited by such a neutrino is so tiny that you wouldn't be able to register the hit.
What if you make the trap move at relativistic speed?
Speed is relative, so if you accelerated away from the sun at 0.999999c hoping to hang out in a slow-moving stream of solar neutrinos, you’d remove the energy needed for them to interact with the weak force.
They’d be all around you, but you would have exponentially smaller chances of interacting with them.
Indeed, which is why its so annoying the editor chose to put "trap" in the title!
Damn it. Heart rate sky-rocketed from excitement.
From the (captions under photographs) in the article: 'Located 2.1 kilometers underground in the Creighton mine in Ontario, Canada, the Sudbury Neutrino Observatory’s detector (left) was filled with “heavy” water, which features deuterium in place of hydrogen atoms. Its findings provided evidence that neutrinos can change, or “oscillate,” between different flavors.'
"China’s Jiangmen Underground Neutrino Observatory (JUNO), seen here under construction in 2023, is currently the world’s largest neutrino detector. It began collecting data in August 2025; one of its main goals is to determine the outstanding mystery of how heavy each flavor of neutrino is."
The second caption implies that some "flavors" of neutrino may be heavier than other flavors. The first caption says that neutrinos oscillate between different flavors. If the first caption is correct, then wouldn't each flavor of neutrino be just as heavy as the others?
They are weirder than you think. They interact with flavor, but these are a superposition of three mass states and it all oscillates. https://en.wikipedia.org/wiki/Neutrino_oscillation
I also wonder this!
Some real physicist tell me what I’m getting wrong here:
neutrinos have mass but the magnitude is so infinitesimally small, and the oscillation smaller still, such that you’re bumping up against some fundamental properties of the universe and the oscillation is “borrowing” and “returning” some tiny amount of mass/energy from the vacuum.
Not a physicist, but here are a couple Wikipedia excerpts I found: From page “Conservation of mass”:
“In general, mass is not conserved. The conservation of mass is a law that holds only in the classical limit.”
From page “Neutrino oscillation”:
“Neutrino oscillation arises from mixing between the flavor and mass eigenstates of neutrinos. That is, the three neutrino states that interact with the charged leptons in weak interactions are each a different superposition of the three (propagating) neutrino states of definite mass. Neutrinos are produced and detected in weak interactions as flavour eigenstates[a] but propagate as coherent superpositions of mass eigenstates.[23]”
so it’s changing mass/energy with itself? and the whole thing says constant? I guess that makes sense.
Maybe it isn’t changing mass at all, and what’s changing is just the probability distribution resulting from a hypothetical flavor measurement. If we could measure the mass (collapse to mass eigenstate), I think I understand we would see no oscillation.
In 1987 a supernova was observed in the sky.
In a great coincidence, just months after being put into operation, a new type of detector meant to study if protons are stable, detected instead the neutrinos coming from the supernova in a way that could be individually timestamped. No other supernova has been visible with the naked eye ever since, or is likely to be seen in our lifetimes.
https://sci-hub.st/10.1103/PhysRevLett.58.1494
The neutrinos arrived 3 hours before the supernova was first seen in the sky. This could mean that neutrinos travel faster than light: that they negative mass. It has not been ruled out yet, but it would take extraordinary evidence for any physicist to admit to a faster-than-light particle.
The core of supernovae are predicted to take around 4 seconds to explode, and during that time they release 99% of the energy that was binding the star together as neutrinos. Once this has happened, however, it takes several hours for the explosion to be visible outside.
If they had 0 mass, you'd expect the burst of neutrinos to last for about 4 seconds. The detected spread was of around 6 seconds.
It was likely that the model for supernovae was missing something. But perhaps neutrinos are slowed down as they interact with other particles on the way here from the supernova. Massless particles always travel at the speed of light, only massive particles can be slowed down. Perhaps neutrinos have mass.
In order to test the basic theory of nuclear fusion in stars, the "Homestake experiment" was set up to count neutrinos from the Sun.
https://sci-hub.st/10.1126/science.191.4224.264
The experiment consists of a big tank of perchloroethylene. Electron neutrinos (and only electron neutrinos) are expected to collide with the Cl-37 atoms to form Ar-37 in the reverse reaction to that of radioactive beta decay. Then the individual Ar-37 atoms can be separated into a gas and counted.
According to the brightness of the Sun, it was expected that around 50 such Ar-37 atoms would be detected every 100 days. Only 17 were observerd, on average. A third of the neutrinos went missing.
A new round of experiments was made, culminating in the fantastically sophisticated Super-Kamiokande, which is sentitive to all other "flavours" of neutrinos, and also capable of detecting the direction they are coming from.
https://arxiv.org/pdf/hep-ex/9807003
It was confirmed that the missing 2/3 of the electron neutrinos expected to come from the Sun had somehow transformed into the other flavours. What's more, muon neutrinos coming from cosmic ray decay in the atmosphere were detected half as frequently in an upwards direction, coming from the other side of the Earth, than in the downwards direction.
The conclusion is that neutrinos must change flavour as they move through space.
Particle accelerators were built in several sites, pointing towards the Super-Kamiokande across the earth's crust, sending a beam of neutrinos of a known kind to measure this "oscillation".
https://arxiv.org/pdf/hep-ex/0606032
Since they have mass, the artificial neutrinos are slightly dispersed by their passage through the Earth's crust. Their arrival time forms three different overlapping peaks, corresponding to three different masses.
The proportion of each kind changes along the path in a sinusoidal fashion, but the mass peaks remain.
The current interpretation is that the mass and the flavour of neutrinos are (almost) "conjugate" properties of the particles, in a similar way to the well-known uncertainty between position and momentum, or between time and frequency in Fourier analysis.
As a neutrino moves through space, its mass can be measured more and more accurately from the dispersion in arrival times. The better we know the mass of the particle, the more uncertain we are of its flavour. If because of the great distances the mass is well known, as is the case with the solar neutrinos, then the three flavours are almost completely entangled.
On short distances the flavour is well determined but the masses are uncertain.
Could a physicist comment on this? I've been reading on the CNO cycle [1], where it says neutrinos produced in the beta-decay steps can have any share of the resulting energy. Does it mean the Sun is shrouded in a rarefied cloud of such low-energy neutrinos which failed to achieve escape velocity?
[1] https://en.wikipedia.org/wiki/CNO_cycle
This is an interesting question, but it would be very difficult to verify.
Only neutrinos with an extremely low kinetic energy would have velocities much lower than that of light, and the fraction of such neutrinos from the total flux of neutrinos would be very small. If they did not escape Sun, most would be probably captured sooner or later by nuclei from the gases of the Solar atmosphere (i.e. chromosphere or corona).
There is a good exhibit on this at the Miraikan in Odaiba, Tokyo. Detecting things and proving we detected what we detected we previously couldn't is always a fascinating exercise, especially whilst so much matter is still unrecognised.
Related BBC radio 4. Featuring the lovely Frank Close (I added the lovely).
https://www.bbc.co.uk/programmes/b0106tjc
HN automangled the title, should have a “how” at the beginning. The change makes the headline sound like this is news, but it’s just a description of neutrino detectors.
Rehowed now. Thanks!
Now howed, brown cow?