The probable boring answer, mentioned near the end, is that at a detection rate of 500 events per 15 years of experiment runtime, statistical power comes slowly.
But, as noted, that already happened. Six sigma is six sigma. To demand better than that amounts to special pleading.
It looks like people just wish this would go away, like the failure of the galactic rotation curves. What is strange is that this might be a place to park dark matter, catnip nowadays.
Usually when this sort of thing happens in cosmology, astrophysics, or particle physics, it is because most physicists hate working with the mathematics required, and hope somebody else will tell them they were right to ignore it.
Coming up with potential "tracebacks" is how theoretical physicists make papers and get citations. And then you use them to design a proper experiment that will shed light to other aspects of this anomaly.
> But for some reason this process ain't starting.
or, 2.5 years, one of which was a global pandemic, is not enough time to design, begin, and publicize a series of experiments that take a few decades to produce data in quantity to continue investigating new physics.
Again: the next step is not more experiments. The experiments are done, and confirmed the discrepancy. The next step is theorizing, which can be done in isolation, relatively unaffected by pandemic. That is what is now neglected.
After hypotheses have been shown to be plausible, i.e. account for the result correctly, yet consistent with other results, it will be time to design experiments to choose among them.
So, no, the silence is not consistent with normal process AIUI.
There are theories. Such as the discrepancy being due to sterile neutrinos (also explaining dark matter). Confirming or rejecting through data is currently underway, is not at six sigma, and won’t be for years due to rareness and difficulty of detecting events. So it isn’t being talked about since we are in the lurch.
We have sketches of hypotheses. What we lack are detailed predictions; and refereed papers to exposite them, and to compete. Absent such predictions, it doesn't much matter how much more data you collect.
Given the absurdly low rate of detection, it feels like a focused experiment designed explicitly to maximise detection is warranted. Difficulty of neutrino experiments aside, this is physics! If we want more detection events we just need to make more particles and/or build a bigger detection apparatus. I don’t necessarily think there’s much benefit to a “larger volume” detector in this particular experiment (MiniBooNE) due to beam width and spreading rate, but I don’t see how having one or two more identical detectors lined up along the generated neutrino‘s path wouldn’t help. Building a more “powerful” neutrino production system, even as simply as just having an accelerator with a higher particle flux would likely increase this factor.
If people are arguing over possible issues in the experiment design (which there do appear to be concerns here, but grain of salt as IANAP) then dialling the experiment parameters up will should make any experiment specific effects more obvious as there will now be two sets of data points that can be correlated by the experiment parameters to fill out such effects.
Once again(!!), the problem is not detection. It took 15 years to get what we got, but we got enough. (Why is this so hard to grasp, even with it explained over, and over, and over?)
The problem is that what we did get demonstrates conclusively that there are new physics to elucidate. There are various possibilities for how those will turn out, with different results to be determined, and to design experiments to check on. Right now it doesn't look like anybody knows how to design those experiments, or what even to look for.
And, weirdly, there have been no headlines about this, and apparently no papers published proposing details of the new physics and their implications for what we might attempt to measure, or expect to find if we succeeded in measuring.
I’m aware it’s not an issue of detection. I was more speaking to the ancillary issues surrounding the result. The last thing we need is to wait 15 more years while people argue over experiment design validity before anything gets done to even begin investigation into the new physics that may be hidden here.
Any new higher flux neutrino (or anti neutrino) source will be just as useful for subsequent studies into this phenomena. The nice thing about neutrinos in this case is they go through basically everything. So nothing stops a new particle source being constructed specifically for multiple such experiments. Unless for some reason distances from source to detector are such a sensitive value in the potential experiment setups that every subsequent experiment needs to be exactly 20 or 30 meters from the particle source, barring that we could easily setup a line of different experiments a hundred meters long or more! If subsequent experiments hope to produce results with sufficient statistical detail to elucidate the new physics this points towards they are likely to be significantly aided by a higher flux source.
Science is a team sport and the sport of neutrino flux anomaly research currently has measures its innings/quarters/sets in decades. It may take two or even more rounds of subsequent experiments to fully uncover what’s going on here… not because we’re waiting for the next great mind… but because our puny particle accelerators aren’t making enough neutrinos for us to study this phenomenon faster than the pitch drop experiment!
I get you: It would take another 20+ years to test any new hypothesis. So, publishing yours, even if you think you're right, doesn't feel too urgent, and wouldn't advance your career anytime soon. The Nobel would come at best 40 years later, and then only if you were right and they were generous.
So to fire anybody up, we need to be able to test hypotheses in a lot less than 20 years.
I'm surprised Sabine did not bring her usual drama and controversy to this one and talk about the fact that there is, shall we say, "extreme tension" between the results from LSND/MiniBooNE and those of other experiments.
There was a 2 sigma result, that has later been confirmed by a 4.9 sigma result from a new experiment with a new detector, giving now a combined 6 sigma confidence.
That is distinct from the other 4.7 σ and 3.8 σ results mentioned in TFA, which combined yield a 6. AFICS nobody has suggested this one may be combined with anything.
So, my question remains open: why did a 2 σ result merit publication?
I don't have the background to interpret these figures, and based on a quick search for MiniBooNE mentions in the paper, the authors don't seem to say that MiniBooNE is an outlier relative to all the other experiments. Where does the paper talk about the tension?
EDIT: based on the comments below, it sounds like the tension is between appearance-based and disappearance-based experiments, and isn't necessarily about a single experiment being an outlier relative to others?
They do mention it, it's at the end of their conclusions:
"The results explicitly show the strong tension between null results from disappearance searches and appearance-based indications for the existence of light sterile neutrinos." p7
So yes they are saying there is tension between disappearance and appearance experiments, which is which?
"In this analysis, the measurement of muon (anti)neutrino disappearance by the MINOS experiment is combined with electron antineutrino disappearance measurements from the Daya Bay and Bugey-3 [16] experiments using the signal confi- dence level (CLs) method [17, 18]. The combined results are analyzed in light of the muon (anti)neutrino to electron (anti)neutrino appearance indications from the LSND [8] and MiniBooNE [9] experiments." p3
So yes they are saying there is tension between MiniBooNE results and MINOS, Bugey-3. I've no insight into the results either but GP's characterization of this paper was accurate.
uh,they do say "Regions of parameter space to the right of the red contour are excluded." you can see for yourself where mosto of the miniBoone resylts lie. By the way, why not link the preprint? https://arxiv.org/abs/1607.01177
The existence of a contradiction with other experiments was also something that was reported at the time:
> Despite the affirmation of LSND’s 20-year-old results, physicists have not concluded that there is a fourth neutrino species. The findings directly contradict those by a diverse set of experiments, including the Main Injector Neutrino Oscillation Search, the Daya Bay Reactor Neutrino Experiment, and the IceCube Neutrino Observatory
She has a very empirical way of looking at things which is somewhat uncommon in theoretical physicists and appeals to interested outsiders who haven't been "inducted" into string theory or what have you. E.g. we might be able to do without dark energy, so why put so much emphasis on dark energy? We don't even see one superparticle at LHC, but grad students are still being pushed into stringy shit – why not cut our losses? Or on the flip side, here's a 6 sigma result!!! – why don't her peers seem to care?
The other side would be someone like Luboš Motl, who used to be quite popular too. His main point is that the mathematical completeness and coherence of string theory means it's the only game in town. Which, that might be a fair point, for all I know, but I'm a maths grad and I can only vaguely appreciate what he's going on about most of the time. It just needs a lot of study to get to grips with these theories well enough to judge them in the absence of confirmatory empirical evidence.
Why do we need a particle accelerator to run experiments on neutrinos, given their apparent abundance? Is there any other way to observe neutrinos without a particle accelerator?
There are lots of experiments that do not require particle accelerators but if you want to observe neutrinos after they have travelled only a few dozen meters, you need a source at that distance.
Why can't you sample at one point on the earth's surface and sample again at that point's antipode? You would know the distance that was travelled, and have measurements at both points.
Well that's not the problem. The problem is natural sources are too big. The distance difference between you and the top of the sun and the middle is like 100x the length of the equator. Neutrino sources in the sun have very irregular shapes, so that too changes the distance. And you have zero control over it all. All you'd ever see is the equivalent of a soft shadow, you'd never get a point source.
The second thing you might want to do: "filter" neutrinos ... well that just doesn't work at all. Nothing stops (a decent amount of) neutrinos. So you can't filter them and create a known point with, say, only electron neutrinos like you can with light. There are also no known astronomical objects that filter neutrinos. They fly right through a star, so that's no good either.
Third you could filter observations. But again you hit the filter problem. You can focus neutrinos measurements (even if you can't focus the neutrinos themselves), but that lowers the amount of neutrinos you measure further. And you're only measuring like maybe 50 per cubic meter of water, so you don't have much to work with.
I guess you could wait for things to get especially good for your measurement but you'll be waiting a long time.
I wouldn’t need to. By sampling hundreds of thousands of events, I would know the odds of getting a particular neutrino from a particular source is the same.
Neutrinos mainly come from the sun, and the sun is massively larger than the earth. You can't draw any conclusions from measurements at two points at a distance that is much smaller than the size of the source itself.
> the three types of neutrino-flavors mix into each other. That means, if you start with, say, only electron-neutrinos, they’ll convert into muon-neutrinos as they travel. And then they’ll convert back into electron neutrinos. So, depending on what distance from a source you make a measurement, you’ll get more electron neutrinos or more muon neutrinos.
I am not an expert on this, so take it with a grain of salt:
Neutrinos are not directly detected. What happens is that they interact with matter in a medium and produce muons or electrons that are moving faster than the speed of light in that medium. That's only possible because the speed of light in said medium is less than the speed of light in vaccuum by the way. These fast-moving, charged particles lead to the generation of a cone of cherenkov radiation, not unlike a supersonic object is producing a mach cone. From that cone, the direction of the particle motion can be inferred.
One could also keep the detector running while switching the source on and off to establish a baseline and calculate the excess of detected particles while the source was switched on.
This is somewhat correct, but talking about only one class of experiment (Cherenkov detectors).
Indeed it’s the child-particles of the interactions that are observed, so, electrons muons and Tau particles. Depending how you are measuring, (many approaches) these interactions are generally separable - and usually you can get some directionality out of your measured event - so you know the direction it came from.
Combining the direction with timing - with an accelerator you know exactly when the pulse of neutrinos were produced - this means you can exclude a very large amount of background.
Some background does get through, but you can measure it with e.g. the source turned off as you suggest, or the times when you know the source isn’t otherwise producing (like in between pulses) to get a background rate. You then generate enough data from the source to effectively “drown out” the background.
Much the same can be done with reactor sources - very often when building an experiment with a reactor as a source, there will be some government-level agreement to allow knowledge of what power the reactor is being run at, as well as shutdowns.
Along with Cherenkov, there are and have been scintillator, chemical-energy-deposition, Photographic film and even ocean-audio based methods of detection. Probably some more exotic ones I am forgetting also.
(I did a PHD in neutrino physics, although been out of the field a few years)
I don’t know if any of them ever ended up getting off the ground, but the theory was that you could pick up the step-function shockwave from underwater interactions with a giant array of microphones. There were some test arrays in the Mediterranean IIRC, but cursory searches seem to imply that optical underwater arrays seem to have been pursued instead.
It always seemed a neat idea, that you could instrument absolutely gigantic volumes with.
I guess the sound might be very characteristic because variations in the process of generation might be limited (amount of energy delivered / type of particle created?) and I would expect it to sound a lot like a sharp "ping" if I had to take a guess.
Temperature, salinity and currents would affect propagation and dispersion but deep enough in the sea there might be a stable enough environment to be able account for that. A very interesting idea at least!
If everything was as expected, we don't, but the issue is that there appears to be something different about the particles coming from the accelerator (sterile neutrinos? or simply shorter distance?) since these results haven't been seen in other experiments. Of course the fact that the results could be separately replicated (and combined to give the 6sigma confidence) is another useful aspect.
If there are sterile neutrinos... they are a possible dark matter particle.
She mentions this in her transcript. First of all they aren’t as common on earth as other particles (she says “10-15 have passed through you while listening to this paragraph”. How many photons hit you in that period of time? If you can make a bunch at once (as a side effect of another high energy interaction) you can look at them in a known place at a known time.
Secondly, she mentions LSND which was a big tub of liquid that would interact with neutrinos that passed through it. Flashes from those interactions would be picked up by photo detectors (hence “scintillation”). The most famous such setup in a mine in Japan, but there are several such “neutrino telescopes”.
That's because she actually said "ten to the fifteen" not "ten to the fifteenth", so you have to decide if you autocorrect by removing the "the" -> "ten to fifteen" or by adding the "th" -> "ten to the fifteenth".
"Ten to the fifteen" is also correct, and in my experience more common. It's short for "ten to the power of fifteen", whereas "ten to the fifteenth" is short for "ten to the fifteenth power".
That's a good and subtle question, with multiple relevant answers:
1) As _Microft says in a sibling reply, the reason is because for this type of experiment, the researchers were studying short-baseline oscillations. A curious property of neutrinos is that they oscillate -- we interact with them through their "flavor", but they propagate through space in a mixture of flavor eigenstates, so a neutrino that is created as an electron-type neutrino can interact later as a muon-type (or tau-type) neutrino.
LSND and MiniBooNE study short-baseline oscillations, where L/E (the distance travelled divided by the particle energy) is very small, a few meters on a ~1MeV neutrino. By comparison, almost all the (vast number!) neutrinos that pass through each of us each second come from much farther away. Most of them come from the Sun, so their origin is distributed over a sphere 100x the diameter of the Earth (and, to make matters worse, something called the MSW-effect scrambles things further). So, people who study these effects either build an accelerator or snuggle up next to a reactor.
In principle, one could use the existing neutrinos from the Sun to do this sort of experiment, but one would need a material capable of blocking all of, say, the electron-type neutrinos from the Sun. If such a thing existed, we could place a detector a few meters away from that filter and watch for electron-type neutrinos to reappear. Alas, the only known material perhaps capable of achieving such a task exists for a fraction of a second inside core-collapse supernovae -- the extremely dense infalling matter is so dense that it can actually trap some of the outgoing neutrinos.... until the neutrino pressure blows it apart.
2) In this specific case, the experiments were originally done with muon anti-neutrinos. The easiest way to get those is with a particle accelerator.
3) One can also attempt to get at this sort of thing (though the approach has different sensitivity/systematics) through reactor-neutrino experiments, as one of the downvoted posts below suggests. For the shortest baselines, though, there's probably really nothing like a beam-experiment.
4) (an aside) One thing that Hossenfelder doesn't really address in the article: There are still systematic-uncertainty questions around LSND and Mini/MicroBooNE. The modern experiment has addressed many of them, but not all. The experiments are difficult and the anomalies have their quirks. Yes, the combined result reaches a statistical power of ~6-sigma, but I don't think you'll have to look too far to find credible experimentalists who have concerns about the result.
This is in contrast to something like the Higgs, where the discovery was fairly clean and simultaneously confirmed by two groups. LSND was huge news back in the day, but the difficulty/expense of replication and the substantial experimental challenges mean that the situation remains fairly murky.
> we have three flavors of neutrinos and these mix into each other as they travel….There are natural sources like the sun, and neutrinos that are created in the upper atmosphere when cosmic rays hit. And then there are neutrinos from manmade sources, particle accelerators and nuclear power plants. In all of these cases, you know how many neutrinos are created of which type at what energy. And then after some distance you measure them and see what you get.
Energy ranges, knowing exactly how far you are from the source of the neutrinos, and background exclusion.
If you are too far from the source, the mixing you might be measuring might have happened already happened. You can build a detector right next to the source, and also far away, and correlate the differences.
External interference can be excluded on energy range, direction, and “timing” - you know exactly when the pulse happened in the source and can exclude things outside that window (as the neutrinos all travel at the speed of light to within measurement error).
Things like neutrinos from the sun can and have been measured, but for many things you want to measure you need more careful control.
Since nuclear reactors emit neutrinos as well, there are experiments measuring the neutrino output at two different locations from a commercial power plant and compare the flux of neutrinos taking the mixing into account (as well as all the other sources, of course).
Actually, I did an internship after school with one of the groups: https://www.mpi-hd.mpg.de/lin/research_dc.en.html
That's part of the problem. Which Neutrinos are you measuring? The ones from your experiment, the ones from the Sun, the ones from the environment? They're exceptionally light particles, so effective shielding is difficult.
It's why a lot of these experiments are deep underground. The one I visited was in northern Minnesota in an old abandoned Mine placed half a mile under the surface and positioned in such a way to receive a neutrino beam all the way from Fermilab in Chicago.
The smaller and lighter the phenomenon, the larger your laboratory needs to be come.
The neutrino experiments aren't underground to shield them from background neutrinos. There is no way to shield background neutrinos. They are underground to shield them from other particles that can produce experimental signatures that look similar to neutrinos.
So you're detecting all neutrinos, but you're looking in an energy range where the signal you're interested in rises above the background levels.
Tangential, but for those interested in large-scale neutrino physics projects check out the successor to Japan’s Super-Kamiokande experiment, Hyper-Kamiomande: https://en.wikipedia.org/wiki/Hyper-Kamiokande
> Now, to be fair, neutrino-mixing in and by itself isn’t all that weird. Indeed, quarks also do this mixing, it’s just that they don’t mix as much. That *neutrinos mix is weird because neutrinos can only mix if they have masses. But we don’t know how they get masses.
Could someone explain to a layman why "neutrinos can only mix if they have masses"? Perhaps I don't understand what it means to "mix"
Particles without mass are required to travel at the speed of light. Particles traveling at the speed of light don't experience time in their own frame of reference. For instance a photon is emitted and absorbed at the same instance from its point of view. So they have no time to do anything like change properties.
74 comments
[ 2.8 ms ] story [ 130 ms ] threadIt looks like people just wish this would go away, like the failure of the galactic rotation curves. What is strange is that this might be a place to park dark matter, catnip nowadays.
Usually when this sort of thing happens in cosmology, astrophysics, or particle physics, it is because most physicists hate working with the mathematics required, and hope somebody else will tell them they were right to ignore it.
wanna bet how long it'll take to figure out _how_, at such a low detection rate?
We're at "we've confirmed there's a bug", not "here's the repro, traceback, and by the way if you fix these four lines it'll work again".
But for some reason this process ain't starting.
or, 2.5 years, one of which was a global pandemic, is not enough time to design, begin, and publicize a series of experiments that take a few decades to produce data in quantity to continue investigating new physics.
After hypotheses have been shown to be plausible, i.e. account for the result correctly, yet consistent with other results, it will be time to design experiments to choose among them.
So, no, the silence is not consistent with normal process AIUI.
Given the absurdly low rate of detection, it feels like a focused experiment designed explicitly to maximise detection is warranted. Difficulty of neutrino experiments aside, this is physics! If we want more detection events we just need to make more particles and/or build a bigger detection apparatus. I don’t necessarily think there’s much benefit to a “larger volume” detector in this particular experiment (MiniBooNE) due to beam width and spreading rate, but I don’t see how having one or two more identical detectors lined up along the generated neutrino‘s path wouldn’t help. Building a more “powerful” neutrino production system, even as simply as just having an accelerator with a higher particle flux would likely increase this factor.
If people are arguing over possible issues in the experiment design (which there do appear to be concerns here, but grain of salt as IANAP) then dialling the experiment parameters up will should make any experiment specific effects more obvious as there will now be two sets of data points that can be correlated by the experiment parameters to fill out such effects.
The problem is that what we did get demonstrates conclusively that there are new physics to elucidate. There are various possibilities for how those will turn out, with different results to be determined, and to design experiments to check on. Right now it doesn't look like anybody knows how to design those experiments, or what even to look for.
And, weirdly, there have been no headlines about this, and apparently no papers published proposing details of the new physics and their implications for what we might attempt to measure, or expect to find if we succeeded in measuring.
Any new higher flux neutrino (or anti neutrino) source will be just as useful for subsequent studies into this phenomena. The nice thing about neutrinos in this case is they go through basically everything. So nothing stops a new particle source being constructed specifically for multiple such experiments. Unless for some reason distances from source to detector are such a sensitive value in the potential experiment setups that every subsequent experiment needs to be exactly 20 or 30 meters from the particle source, barring that we could easily setup a line of different experiments a hundred meters long or more! If subsequent experiments hope to produce results with sufficient statistical detail to elucidate the new physics this points towards they are likely to be significantly aided by a higher flux source.
Science is a team sport and the sport of neutrino flux anomaly research currently has measures its innings/quarters/sets in decades. It may take two or even more rounds of subsequent experiments to fully uncover what’s going on here… not because we’re waiting for the next great mind… but because our puny particle accelerators aren’t making enough neutrinos for us to study this phenomenon faster than the pitch drop experiment!
So to fire anybody up, we need to be able to test hypotheses in a lot less than 20 years.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...
The 4th figure is the money shot.
That is distinct from the other 4.7 σ and 3.8 σ results mentioned in TFA, which combined yield a 6. AFICS nobody has suggested this one may be combined with anything.
So, my question remains open: why did a 2 σ result merit publication?
I don't have the background to interpret these figures, and based on a quick search for MiniBooNE mentions in the paper, the authors don't seem to say that MiniBooNE is an outlier relative to all the other experiments. Where does the paper talk about the tension?
EDIT: based on the comments below, it sounds like the tension is between appearance-based and disappearance-based experiments, and isn't necessarily about a single experiment being an outlier relative to others?
"The results explicitly show the strong tension between null results from disappearance searches and appearance-based indications for the existence of light sterile neutrinos." p7
So yes they are saying there is tension between disappearance and appearance experiments, which is which?
"In this analysis, the measurement of muon (anti)neutrino disappearance by the MINOS experiment is combined with electron antineutrino disappearance measurements from the Daya Bay and Bugey-3 [16] experiments using the signal confi- dence level (CLs) method [17, 18]. The combined results are analyzed in light of the muon (anti)neutrino to electron (anti)neutrino appearance indications from the LSND [8] and MiniBooNE [9] experiments." p3
So yes they are saying there is tension between MiniBooNE results and MINOS, Bugey-3. I've no insight into the results either but GP's characterization of this paper was accurate.
The existence of a contradiction with other experiments was also something that was reported at the time:
> Despite the affirmation of LSND’s 20-year-old results, physicists have not concluded that there is a fourth neutrino species. The findings directly contradict those by a diverse set of experiments, including the Main Injector Neutrino Oscillation Search, the Daya Bay Reactor Neutrino Experiment, and the IceCube Neutrino Observatory
https://physicstoday.scitation.org/do/10.1063/PT.6.1.2018061...
The other side would be someone like Luboš Motl, who used to be quite popular too. His main point is that the mathematical completeness and coherence of string theory means it's the only game in town. Which, that might be a fair point, for all I know, but I'm a maths grad and I can only vaguely appreciate what he's going on about most of the time. It just needs a lot of study to get to grips with these theories well enough to judge them in the absence of confirmatory empirical evidence.
https://www.youtube.com/watch?v=gYHGdBqI7z4
My sentiment exactly.
It was truly as uplifting as it could be.
They don't do anything, there is no use for them really, they don't even have many properties...
The only real effect they have is making physicists' maths a little more complex (or slightly wrong if they forget them).
Genuinely curious here, why is the travelled distance important?
The second thing you might want to do: "filter" neutrinos ... well that just doesn't work at all. Nothing stops (a decent amount of) neutrinos. So you can't filter them and create a known point with, say, only electron neutrinos like you can with light. There are also no known astronomical objects that filter neutrinos. They fly right through a star, so that's no good either.
Third you could filter observations. But again you hit the filter problem. You can focus neutrinos measurements (even if you can't focus the neutrinos themselves), but that lowers the amount of neutrinos you measure further. And you're only measuring like maybe 50 per cubic meter of water, so you don't have much to work with.
I guess you could wait for things to get especially good for your measurement but you'll be waiting a long time.
> the three types of neutrino-flavors mix into each other. That means, if you start with, say, only electron-neutrinos, they’ll convert into muon-neutrinos as they travel. And then they’ll convert back into electron neutrinos. So, depending on what distance from a source you make a measurement, you’ll get more electron neutrinos or more muon neutrinos.
Neutrinos are not directly detected. What happens is that they interact with matter in a medium and produce muons or electrons that are moving faster than the speed of light in that medium. That's only possible because the speed of light in said medium is less than the speed of light in vaccuum by the way. These fast-moving, charged particles lead to the generation of a cone of cherenkov radiation, not unlike a supersonic object is producing a mach cone. From that cone, the direction of the particle motion can be inferred.
One could also keep the detector running while switching the source on and off to establish a baseline and calculate the excess of detected particles while the source was switched on.
https://en.wikipedia.org/wiki/Cherenkov_radiation
Indeed it’s the child-particles of the interactions that are observed, so, electrons muons and Tau particles. Depending how you are measuring, (many approaches) these interactions are generally separable - and usually you can get some directionality out of your measured event - so you know the direction it came from.
Combining the direction with timing - with an accelerator you know exactly when the pulse of neutrinos were produced - this means you can exclude a very large amount of background.
Some background does get through, but you can measure it with e.g. the source turned off as you suggest, or the times when you know the source isn’t otherwise producing (like in between pulses) to get a background rate. You then generate enough data from the source to effectively “drown out” the background.
Much the same can be done with reactor sources - very often when building an experiment with a reactor as a source, there will be some government-level agreement to allow knowledge of what power the reactor is being run at, as well as shutdowns.
Along with Cherenkov, there are and have been scintillator, chemical-energy-deposition, Photographic film and even ocean-audio based methods of detection. Probably some more exotic ones I am forgetting also.
(I did a PHD in neutrino physics, although been out of the field a few years)
I did not know that "ocean-audio based methods of detection" for neutrinos are even possible and will certainly look that up later.
It always seemed a neat idea, that you could instrument absolutely gigantic volumes with.
If there are sterile neutrinos... they are a possible dark matter particle.
Secondly, she mentions LSND which was a big tub of liquid that would interact with neutrinos that passed through it. Flashes from those interactions would be picked up by photo detectors (hence “scintillation”). The most famous such setup in a mine in Japan, but there are several such “neutrino telescopes”.
A bit larger. :-)
Easy to get confused.
1) As _Microft says in a sibling reply, the reason is because for this type of experiment, the researchers were studying short-baseline oscillations. A curious property of neutrinos is that they oscillate -- we interact with them through their "flavor", but they propagate through space in a mixture of flavor eigenstates, so a neutrino that is created as an electron-type neutrino can interact later as a muon-type (or tau-type) neutrino.
LSND and MiniBooNE study short-baseline oscillations, where L/E (the distance travelled divided by the particle energy) is very small, a few meters on a ~1MeV neutrino. By comparison, almost all the (vast number!) neutrinos that pass through each of us each second come from much farther away. Most of them come from the Sun, so their origin is distributed over a sphere 100x the diameter of the Earth (and, to make matters worse, something called the MSW-effect scrambles things further). So, people who study these effects either build an accelerator or snuggle up next to a reactor.
In principle, one could use the existing neutrinos from the Sun to do this sort of experiment, but one would need a material capable of blocking all of, say, the electron-type neutrinos from the Sun. If such a thing existed, we could place a detector a few meters away from that filter and watch for electron-type neutrinos to reappear. Alas, the only known material perhaps capable of achieving such a task exists for a fraction of a second inside core-collapse supernovae -- the extremely dense infalling matter is so dense that it can actually trap some of the outgoing neutrinos.... until the neutrino pressure blows it apart.
2) In this specific case, the experiments were originally done with muon anti-neutrinos. The easiest way to get those is with a particle accelerator.
3) One can also attempt to get at this sort of thing (though the approach has different sensitivity/systematics) through reactor-neutrino experiments, as one of the downvoted posts below suggests. For the shortest baselines, though, there's probably really nothing like a beam-experiment.
4) (an aside) One thing that Hossenfelder doesn't really address in the article: There are still systematic-uncertainty questions around LSND and Mini/MicroBooNE. The modern experiment has addressed many of them, but not all. The experiments are difficult and the anomalies have their quirks. Yes, the combined result reaches a statistical power of ~6-sigma, but I don't think you'll have to look too far to find credible experimentalists who have concerns about the result.
This is in contrast to something like the Higgs, where the discovery was fairly clean and simultaneously confirmed by two groups. LSND was huge news back in the day, but the difficulty/expense of replication and the substantial experimental challenges mean that the situation remains fairly murky.
> we have three flavors of neutrinos and these mix into each other as they travel….There are natural sources like the sun, and neutrinos that are created in the upper atmosphere when cosmic rays hit. And then there are neutrinos from manmade sources, particle accelerators and nuclear power plants. In all of these cases, you know how many neutrinos are created of which type at what energy. And then after some distance you measure them and see what you get.
If you are too far from the source, the mixing you might be measuring might have happened already happened. You can build a detector right next to the source, and also far away, and correlate the differences.
External interference can be excluded on energy range, direction, and “timing” - you know exactly when the pulse happened in the source and can exclude things outside that window (as the neutrinos all travel at the speed of light to within measurement error).
Things like neutrinos from the sun can and have been measured, but for many things you want to measure you need more careful control.
That's part of the problem. Which Neutrinos are you measuring? The ones from your experiment, the ones from the Sun, the ones from the environment? They're exceptionally light particles, so effective shielding is difficult.
It's why a lot of these experiments are deep underground. The one I visited was in northern Minnesota in an old abandoned Mine placed half a mile under the surface and positioned in such a way to receive a neutrino beam all the way from Fermilab in Chicago.
The smaller and lighter the phenomenon, the larger your laboratory needs to be come.
https://en.wikipedia.org/wiki/MINOS
So you're detecting all neutrinos, but you're looking in an energy range where the signal you're interested in rises above the background levels.
Could someone explain to a layman why "neutrinos can only mix if they have masses"? Perhaps I don't understand what it means to "mix"
Particles without mass are required to travel at the speed of light. Particles traveling at the speed of light don't experience time in their own frame of reference. For instance a photon is emitted and absorbed at the same instance from its point of view. So they have no time to do anything like change properties.