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Space Science Hardware

The Hardware That Searches For Dark Matter (hackaday.com) 104

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.
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The Hardware That Searches For Dark Matter

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  • Why not just walk around barefooted in the dark until you stub your toe on it? That's how I usually find things I otherwise cannot detect any other way.

  • by gstoddart ( 321705 ) on Tuesday January 12, 2016 @01:17PM (#51287947) Homepage

    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?

    • Re: (Score:3, Informative)

      by Anonymous Coward

      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.

    • by painandgreed ( 692585 ) on Tuesday January 12, 2016 @02:04PM (#51288305)

      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.

    • by PvtVoid ( 1252388 ) on Tuesday January 12, 2016 @02:36PM (#51288585)

      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!

      • by thegreatemu ( 1457577 ) on Tuesday January 12, 2016 @04:26PM (#51289403)

        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.

        • 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.

          Quite correct. I should have said closely related to the force by which neutrinos interact.

    • by peter303 ( 12292 ) on Tuesday January 12, 2016 @04:00PM (#51289215)
      I googled various interaction probabilities, which are expressed in units called barns:
      http://www.physics.purdue.edu/... [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.
    • by dbIII ( 701233 )

      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?

      No, because it is interacting with us and producing gravitational effects etc - we just can't see it.

  • Xenon molecule, huh? (Score:2, Interesting)

    by Anonymous Coward

    How does that work?

    • Re: (Score:2, Funny)

      It's actually really simple. People who think dark matter exists because it explains why they can't do math properly also think Xenon can bond with itself.
      • by Anonymous Coward on Tuesday January 12, 2016 @03:35PM (#51289039)

        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.

    • 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?

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

        Of fucking course. They calibrate the detector with neutron sources. Do you think they're complete idiots?

      • by lgw ( 121541 )

        Well, really they don't know that it will work. They're building a hyper-sensitive detector for particle interactions along the same lines as neutrino detectors. They can be sure it will detect very infrequent interactions, and they know what neutrinos look like, so they're hoping for anything else.

        Of course, if dark matter doesn't interact with familiar matter* at all, or only does so at higher energy levels than matter drifting through the Solar system, it won't find anything.

        *I almost said "normal matt

      • 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?

        From reading the wikipedia page, I suspect they'll see individual incident(s) (cosmic rays such as neutrons should cause multiple incidents), probably also from not directly above as cosmic rays will be most common from above rather than through the Earth but fairly independent of Earth's sheilding for a WIMP. Then of course make sure that the instruments themselves aren't causing the readings, or other unforeseen causes. From likely incidents, they can figure out an energy to estimate mass and velocity of

    • How does that work?

      Scintillation in liquid Xenon happens when Xe atoms are ionized and temporarily form molecules [arxiv.org] before returning to a neutral state and emitting photons.

      • A more directly relevant reference is here [arxiv.org].

        It's also entirely likely that the person who wrote the summary wrote "molecule" when they meant "atom"...

        • Scintillation in liquid Xenon happens when Xe atoms are ionized and temporarily form molecules before returning to a neutral state and emitting photons.

          Then TF summary is wrong when it says "liquid Xenon sits waiting for a dark matter particle to strike the nucleus of a Xenon molecule".

          It's also entirely likely that the person who wrote the summary wrote "molecule" when they meant "atom"...

          This. TFA doesn't contain the word "molecule," only TF summary does. Would make no sense for this detector to contain macroscopic quantities of exotic polyatomic Xenon molecules.

          • yeah don't anthropomorphize monoatomic xenon, it just hates that. Xenon hexafluoroplatinate however is cool with it.

  • by Anonymous Coward

    Why are they using HDMI cables to transmit the information?

    • They are using the Low Voltage Differential Pairs to transmit some information and single ended bidirectional connections for other data. Let's you use one HDMI port for multiple data sources.

    • Perhaps because they are cheap off the shelf commodity items that carry lots of bandwidth and they didn't have the money to reinvent a wheel? Naw, couldn't be...

  • ... budget for lots of equipment including high end FPGAs to be located in a mine shaft. What shall we say we're doing with it?

    Dark matter! Yeah, that's the ticket.

    Probably looking for illegal grease [slashdot.org].

    • by lkcl ( 517947 )

      ... budget for lots of equipment including high end FPGAs to be located in a mine shaft. What shall we say we're doing with it? .

      actually there's someone out there already working on a way to hack into their network so that they can run bitcoin mining on them. i bet however that they'll probably find that the scientists secretly installed bitcoin mining on them already... ostensibly to help justify the insane cost of the equipment. so now you know the _real_ reason why they haven't found any so-called dark matter....

      • i bet however that they'll probably find that the scientists secretly installed bitcoin mining on them already...

        Makes sense. Liquid xenon would be a great coolant, so you can overclock the crap out of the rigs. They'll be able to convert bitcoin into the toys they really wanted to buy, when they could only get funding for this "dark matter" grailquest. Face it, if you couldn't find a nail with your hammer, a bigger hammer is not likely to be the most effective way to proceed.

      • ... budget for lots of equipment including high end FPGAs to be located in a mine shaft. What shall we say we're doing with it? .

        actually there's someone out there already working on a way to hack into their network so that they can run bitcoin mining on them. i bet however that they'll probably find that the scientists secretly installed bitcoin mining on them already... ostensibly to help justify the insane cost of the equipment. so now you know the _real_ reason why they haven't found any so-called dark matter....

        Yea, until somebody went through and put SETI clients on all of the systems.... No intelligent life found here... Beam us up!

  • by WarJolt ( 990309 ) on Tuesday January 12, 2016 @02:23PM (#51288469)

    processed by FPGAs on custom boards connected using HDMI.

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

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