10 ms·

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 par

by rpz 5y ago

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?

ISL 5y ago
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.
z5h 5y ago
We don’t. E.g. https://en.m.wikipedia.org/wiki/Sudbury_Neutrino_Observatory https://en.m.wikipedia.org/wiki/Sudbury_Neutrino_Observatory.
zbendefy 5y ago
Results can be better controlled this way
_Microft 5y ago
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.
- drclau 5y ago
  > if you want to observe neutrinos after they have travelled only a few dozen meters Genuinely curious here, why is the travelled distance important?
  - icegreentea2 5y ago
    If you want to measure neutrinos transforming, then knowing the distance to source lets to calculate rate of transformation.
    - frutiger 5y ago
      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.
      - misnome 5y ago
        How would you know the two different neutrinos interactions came from the same source/distance without producing them yourself?
        candiodari 5y ago
        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.
kurthr 5y ago
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.
gumby 5y ago
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”.
- _tom_ 5y ago
  Not "ten to fifteen", but "ten to the fifteenth" A bit larger. :-)
- solidgol 5y ago
  She said 10 to the 15. 10^15
  - dnautics 5y ago
    To be fair I also heard 10-15 on the first pass and then had to think about it for a second.
    - _tom_ 5y ago
      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". Easy to get confused.
      - mkl 5y ago
        "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".
deleted 5y ago
[deleted]
bryan0 5y ago
From TFA: > 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.
misnome 5y ago
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.
- 0kto 5y ago
  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 https://www.mpi-hd.mpg.de/lin/research_dc.en.html
akira2501 5y ago
> given their apparent abundance? 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 https://en.wikipedia.org/wiki/MINOS
- antognini 5y ago
  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.

by rpz 5y ago

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?

ISL 5y ago

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.

z5h 5y ago

We don’t. E.g. https://en.m.wikipedia.org/wiki/Sudbury_Neutrino_Observatory https://en.m.wikipedia.org/wiki/Sudbury_Neutrino_Observatory.

zbendefy 5y ago

Results can be better controlled this way

_Microft 5y ago

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.

drclau 5y ago
> if you want to observe neutrinos after they have travelled only a few dozen meters Genuinely curious here, why is the travelled distance important?
- icegreentea2 5y ago
  If you want to measure neutrinos transforming, then knowing the distance to source lets to calculate rate of transformation.
  - frutiger 5y ago
    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.
    - misnome 5y ago
      How would you know the two different neutrinos interactions came from the same source/distance without producing them yourself?
      - candiodari 5y ago
        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.

kurthr 5y ago

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.

gumby 5y ago

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”.

_tom_ 5y ago
Not "ten to fifteen", but "ten to the fifteenth" A bit larger. :-)
solidgol 5y ago
She said 10 to the 15. 10^15
- dnautics 5y ago
  To be fair I also heard 10-15 on the first pass and then had to think about it for a second.
  - _tom_ 5y ago
    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". Easy to get confused.
    - mkl 5y ago
      "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".

deleted 5y ago

[deleted]

bryan0 5y ago

From TFA: > 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.

misnome 5y ago

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.

0kto 5y ago
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 https://www.mpi-hd.mpg.de/lin/research_dc.en.html

akira2501 5y ago

> given their apparent abundance? 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 https://en.wikipedia.org/wiki/MINOS

antognini 5y ago
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.