How physicists track and trap the elusive neutrino
quantamagazine.org
quantamagazine.org
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.
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.
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.
> 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.
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.
A simplified summary: The discovered mass emerges out of this relationship between detection and probability.
Essentially each neutrino travels in three different "waves", but is still one unit in a constant superposition between the three.
Maybe this one? https://www.youtube.com/watch?v=eBT1-dV1BTM
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.
They’d be all around you, but you would have exponentially smaller chances of interacting with them.
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.
"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.
“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]”
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.
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).