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The Hardware That Searches For Dark Matter (hackaday.com)

Posted by timothy on from the secret-underground-layer dept.

szczys writes: Deep in a gold mine in South Dakota, the Large Underground Xenon experiment waits in the darkness for a tiny flash of light that signals that dark matter actually exists. So far we theorize that it does exist, and have gone to great lengths to build hardware to detect dark matter. Very cold, very pure liquid xenon sits waiting for a dark matter particle to strike the nucleus of a xenon molecule, producing a distinct pattern of photons through scintillation. An array of photomultiplier tubes detect the photons, whose pattern is processed by FPGAs on custom boards connected using HDMI. The experiment has generated a list of properties not possessed by dark matter; running for several years no evidence of the particles interacting with the xenon have been found. But when the data collection concludes this year, a much larger version of the impressive hardware will be built.

21 of 104 comments (clear)

  1. Temba ... his arms wide ... by gstoddart · · Score: 3, Interesting

    Hmmm .... so I'm going to have to stretch my little monkey brain and hopefully someone more knowledgeable can chime in ...

    I see reference to WIMPs in the article, so in some ways do we consider Dark Matter to be kind of like a neutrino? All around us but not generally interacting with us?

    So instead of there being vast tracts of stuff we simply can't figure out where it is, it's spread throughout?

    The overall underground detection mechanism sounds like the Sudbury Neutrino Observatory, or that Ice Cube deal in the Antarctic (Russia?)

    I've always thought it wasn't assumed to be floating around us but not interacting, but I ain't no particle physicist.

    Is Dark Matter more like neutrinos than not? Or entirely different, but with enough commonality to confuse a layman?

    --
    Lost at C:>. Found at C.
    1. Re:Temba ... his arms wide ... by Anonymous Coward · · Score: 3, Informative

      Neutrinos are light and very fast. Dark matter if it exists as predicted, would be heavy and very slow. Their common property would be rarely interacting with 'normal to us' matter.

    2. Re:Temba ... his arms wide ... by painandgreed · · Score: 4, Informative

      I see reference to WIMPs in the article, so in some ways do we consider Dark Matter to be kind of like a neutrino? All around us but not generally interacting with us?

      So instead of there being vast tracts of stuff we simply can't figure out where it is, it's spread throughout?

      WIMPs, Weakly Interacting Massive Particles is what they suspect dark matter to be. It interacts very weakly, possibly only via gravity which is almost undetectable in the scale of individual particles. Thus it tends to pass right through everything. They are assumed to have some cross section so can possibly interact with themselves and other particles if they hit directly head on which is what this experiment seems to be trying to detect. It's thought when they interact with themselves, they annihilate, so they do not slow each other down and do not form disk shapes such as solar systems or galaxies. Otherwise, they float around, only affected by gravity, so the form a spherical cloud called a halo around other gravitational objects such as solar systems and galaxies in which they fall into orbit.

    3. Re:Temba ... his arms wide ... by PvtVoid · · Score: 3, Informative

      I see reference to WIMPs in the article, so in some ways do we consider Dark Matter to be kind of like a neutrino? All around us but not generally interacting with us?

      Yes. The idea is that Dark Matter particles interact via the same force (the Weak Force) as neutrinos. The difference is that Dark Matter must be a much heavier particle than neutrinos in order to explain the astrophysics, because neutrinos don't clump enough under gravity.

      There's a reason why this model is taken seriously: it's possible to calculate how such particles are created very early in the Big Bang. It turns out that if the particle responsible for Dark Matter was created in thermal equilibrium in the very early universe, its abundance today depends only on its interaction strength (not, for example, its mass). If you calculate that interaction strength corresponding the Dark Matter abundance implied by astrophysical measurements, you get exactly the same interaction strength as neutrinos. A pretty amazing coincidence!

    4. Re:Temba ... his arms wide ... by ClickOnThis · · Score: 3, Insightful

      Why do they need fast FPGA detectors, if they never detect anything? Really slow detectors would be just as good at generating no results.

      If there are in fact no results, then they need to be sure of it. Hence fast detectors and FPGA processors are used.

      A failed experiment can still be an important one. The Michelson-Morley experiment was a failure, yet it started a scientific revolution that led to Einstein's theory of relativity.

      --
      If it weren't for deadlines, nothing would be late.
    5. Re:Temba ... his arms wide ... by thegreatemu · · Score: 3, Insightful

      Yes. The idea is that Dark Matter particles interact via the same force (the Weak Force) as neutrinos.

      Everything else you wrote is spot on, but WIMPs have been ruled out as truly Weakly interacting (where Weak with a capital W means "by the exchange of W or Z bosons) for almost a decade now. The original Weak Miracle posited WIMPs to be truly Weakly interacting, but now they are held to be sub-Weak, interacting most likely through Higgs exchange, but we kept the name. Really we should rename them wIMPs at this point.

      Also I hate whoever decided to use Weak and Strong as formal names for those respective interactions.

    6. Re:Temba ... his arms wide ... by TopherC · · Score: 2

      I don't know the details, but the signals they are looking for are particular distributions of light. There are a lot of background processes that produce light, but in patterns or distributions that can be subtracted out. The experiment is carefully designed to have small backgrounds with respect to the signals they hope for, but I'm sure there is still a lot of data being generated.

  2. Xenon molecule, huh? by Anonymous Coward · · Score: 2, Interesting

    How does that work?

    1. Re:Xenon molecule, huh? by ripvlan · · Score: 2

      I wondered something similar - How do they know it WILL work? Not being a physicist myself - my uninformed-self wonders if they built a hypothetical system to detect a hypothetical material?

      My software side wonders: Did they build a unit-test and confirm the system will work as designed?

      Dark Matter has this history of just slipping by unnoticed. If they don't know the system works are they attempting to prove a negative?

    2. Re:Xenon molecule, huh? by Anonymous Coward · · Score: 4, Informative

      Oh dear, Software know-it-alls...
      Xenon _can_ bond with itself. It's in a field known as "Plasma Physics". It's not that easy, but I've done it.
      First, you need an ECR-IS. Slip in a bit of 4He, ionize it, and set up your initial Cyclotron Resonance spirals of Electrons. Now inject just a sniff of Xenon. (We used 136Xe.) Set up your particle detector downstream of the Analyzer and look for Mass 140- Xenon Helide. Note that this can _only_ exist in the +1 charge state.
      Once you've tuned this out, Analyze for Mass 272- +1(136Xe(2)). The best production that I've seen was ~10e2 per second.
      Note that various species exist roughly in cylindrical shells within the ECR Plasma, depending on mass and charge state, so very fine adjustments of the ECR Magnetic Fields, RF, and Vacuum can be used to selectively optimize for any given shell. (You'll see a lot of atomic Xenons, all the way up to +54, if you have a good enough ECR.)
      We started out with something easier initially- Helium Hydride, on the suggestion of Mike Pryor. One doesn't even need an ECR-IS for this. It's easy to get ~10e14 Helium Hydrides per second with a regular Plasma Source.
      Note that all this actually has practical applications, involving Ion Implantation. On the theoretical side, Astrophysicists are delighted; it's rare that they can actually experiment on something, instead of just observe and calculate.

      Code Monkeys should stay away from commenting on any Physics more complicated that rolling a ball down an inclined plank, and timing it with a stopwatch.

    3. Re:Xenon molecule, huh? by ClickOnThis · · Score: 4, Informative

      See also here. Note the comment about a Xe-Xe bond in Xe(2)Sb(2)F(11).

      It has been known since 1962 that Xenon can form compounds with other elements. Most of them involve Fluorine, because it is a seriously badass element when it comes to stealing electrons. It can even oxidize Oxygen.

      --
      If it weren't for deadlines, nothing would be late.
  3. Re:keep waiting by dissy · · Score: 4, Insightful

    It's a lot more likely that dark matter and dark energy are just math errors that don't take into account proper universe/space expansion and doesn't understand how gravity really works.

    Care to show your math on that? We can then compare it to all the scientists who already did the math a few million times showing the same results you say are wrong.

    There is even a Nobel prize or three in it for you.

  4. How ignorant is ignorant enough? by Tenebrousedge · · Score: 5, Informative

    Dark matter and dark energy have multiple independent lines of supporting empirical evidence. I presume you are willfully ignorant of this. Perhaps you can manage to keep it to yourself next time.

    --
    Those who advocate genocide deserve every protection afforded by law, and none afforded by common human decency.
    1. Re:How ignorant is ignorant enough? by lgw · · Score: 3, Informative

      There are many alternative theories about what dark matter is. All the evidence, from multiple independent sources, limits this to "some kind of matter, not moving fast like neutrinos do".

      The failure of the few attempts at detectors doesn't contradict that at all. It simply rules out categories of hypotheses about what sort of particles might make up dark matter. There's really isn't some "settled" idea about that - there are many, competing ideas.

      --
      Socialism: a lie told by totalitarians and believed by fools.
    2. Re:How ignorant is ignorant enough? by Tenebrousedge · · Score: 2

      As lgw said, no one has a settled idea of what dark matter is or where it comes from. We have detected that it exists and have a few constraints on what its properties must be, but there is no "theory of dark matter" per se.

      Look at how long it took to go from Ben Franklin's experiments with lightning to the theory and detection of the electron. Or from Newton's work on optics to the photon. The twentieth century advanced our knowledge of physics phenomenally, so that now we are left with studying phenomenae that are very very hard to perceive: Higgs bosons, gravitational waves, and dark matter/energy. I don't know why precisely you would expect results from these experiments in any sort of short time frame when the phenomenae in question are most notable at the galactic scale. And frankly, you're very foolish to dismiss the concept without having some understanding of the evidence for it.

      --
      Those who advocate genocide deserve every protection afforded by law, and none afforded by common human decency.
  5. Not HDMI by WarJolt · · Score: 2

    processed by FPGAs on custom boards connected using HDMI.

    Just because you use hdmi cables doesn't make it hdmi.

  6. Re:How long is long enough? by PvtVoid · · Score: 2

    If we don't find anything, how long do we keep looking?

    There's a point at which the detectors will get sensitive enough that they start seeing neutrinos, and then the neutrino signal will swamp any Dark Matter signal. This is a couple of generations away in terms of technology development.

  7. Re:Wait... what? by Anonymous Coward · · Score: 2, Interesting

    Lack of results doesn't always mean lack of progress, the lack of results can sometimes be interesting and still help us learn new things. if we already knew how to detect, and all the properties of WIMPS then why bother building the experiment to begin with? we would have nothing new to learn.

    Finding out what properties WIMPS dont have helps refine our current theories. Some of those theories may have predicted we would see some of those properties, but because experimentation rules it out we now know those are dead ends and so can focus our time on other theories. Lack of results allows us to adjust and focus our experiment in ways that are more likely to receive positive results.

  8. Re: keep waiting by truck_soccer · · Score: 2

    Hi, this is the internet. People come here specifically to run their mouths off on subjects that they really shouldn't bother having a thought on. I am frequently guilty of this.

  9. WIMPS million times less detectable by peter303 · · Score: 4, Interesting

    I googled various interaction probabilities, which are expressed in units called barns:
    http://www.physics.purdue.edu/...

    neutron hitting uranium nucleus: 1 barn
    helium nucleus hitting gold nucleus: 100 barns (Rutherford experiment 1911)
    anti-neutrino captured by proton making a neutron: 10E-17 barns (first detected 1956)
    WIMP hitting a xenon nucleus: 10E-21 barns? (year???) need to 10,000 times better than neutrino detector

    Numbers are actually ranges including factors like particle energy and angle.

  10. Re:How long is long enough? by crunchygranola · · Score: 2

    The longer we look without seeing anything, the lower the estimate will be for the density of dark matter. At some point that density may fall well below what is expected from other experiments and theories. At that point one starts to doubt the theories, and to look for ways to revise them. But you need to look long enough to be sure the theories are wrong. Also, it could be that dark matter exists, but has a much lower density than theory predicts. To confirm that, you need to keep looking. Obviously not forever, but as long as you can.

    You don't identify what type of "density" you are referring to - mass density or particle density. For mass density we have a very good idea of what it is from direct measurement of its gravitation - that is not really a matter of theory. Now particle density depends on what the mass of what the particles are. There we have room for lots of uncertainty, and of course there is the even bigger uncertainty about how the interact with known types of matter - regardless of particle density.

    --
    Second class citizen of the New Gilded Age