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Those hard to find WIMPs

Written By: - Date published: 4:00 pm, December 21st, 2009 - 25 comments
Categories: science - Tags:

There have been some new results from the search for WIMPs (Weakly Interacting Massive Particles) at the Cryogenic Dark Matter Search (CDMS) in Minnesota. This showed up in The Economist science section last week with an excellent summary of what they were looking for and why in the post “An early Christmas present?.

Around a quarter of the universe is thought to be made up of dark matter, which, as the name suggests, neither gives off nor reflects light. (The balance, once the small amount of visible matter is subtracted, is made of even more mysterious stuff known as ‘dark energy’.) However, dark matter does make itself known through its gravity. This, indeed, is why astronomers believe it must be there. Some galaxies rotate so fast that they should be throwing off their outermost stars. Only the gravitational pull of these galaxies’ unseen halos of dark matter holds those stars in. Observations of the bending of light around clusters of galaxies, as well as the way that galactic structures formed in the early universe, also suggest that there is much more to reality than meets the eye.

Well the CDMS got one. But as non-scientists often complain, science often isn’t clearcut.

Science is a gradual affair, often of finding out what is impossible and eliminating it. Whatever remains may be true. There is probably no area quite as arcane at doing this as in astrophysics. To find these beasts which barely interact with other matter, they have what is effectively an elimination experiment running. What it is eliminating are possible theories. To do this as the CDMS says in their summary (PDF – which is readable by humans)

The Cryogenic Dark Matter Search (CDMS) experiment, located a half-mile underground at the Soudan mine in northern Minnesota, uses 30 detectors made of germanium and silicon in an attempt to detect such WIMP scatters. The detectors are cooled to temperatures very near absolute zero. Particle interactions in the crystalline detectors deposit energy in the form of heat, and in the form of charges that move in an applied electric field. Special sensors detect these signals, which are then amplified and recorded in computers for later study. A comparison of the size and relative timing of these two signals, heat and charge, allows the experimenters to tell whether the particle that interacted in the crystal was a WIMP– or one of the numerous known particles that come either from radioactive decays or from space in the form of cosmic rays. These background particles must be almost entirely filtered out if we are to see a WIMP signal. Layers of shielding materials, as well as the half-mile of rock above the experiment, are used to provide such background suppression.

The CDMS experiment has been searching for dark matter at Soudan since 2003. Previous data have not yielded evidence for WIMPs, but have provided assurance that the backgrounds have been suppressed to the level where as few as one WIMP interaction per year could have been detected.

But the result is another elimination of the impossible, with a hint that they may have actually seen something in the 2007-2008 data sets.

In this new data set we indeed see two events with characteristics consistent with those expected from WIMPs. However, there is also a chance that both events could be due to background particles. Scientists have a strict set of criteria for determining whether a new discovery has been made. The ratio of signal to background events must be large enough that there is no reasonable doubt. Typically there must be fewer than one chance in a thousand of the signal being due to background. In this case, a signal of about five events would have met those criteria. We estimate that there is about a one in four chance to have seen two backgrounds events, so we can make no claim to have discovered WIMPs. Instead we say that the rate of WIMP interactions with nuclei must be less than a particular value that depends on the mass of the WIMP. The numerical values obtained for these interaction rates from this data set are more stringent than those obtained from previous data for most WIMP masses predicted by theories. Such upper limits are still quite valuable in eliminating a number of theories that might explain dark matter.

Now this is a heck of a result, because WIMPs have been postulated as a being around for many decades without being observed, thought maybe not to be observable, and possibly not even been more than a theory. If the theories are correct, then they probably are going through you all of the time. They should be able to increase the probabilities of observation or non-observation in 2010 because

This is precisely what CDMS experimenters (and many other collaborations worldwide) are now in the process of doing. By summer of 2010, we hope to have about three times more germanium nuclei sitting near absolute zero at Soudan, patiently waiting for WIMPs to come along and provide the perfect billiard ball shots that will offer compelling evidence for the direct detection of dark matter in the laboratory.

However, I’m sure that this would never satisfy some of the science skeptics here as being science. After all it is all based on probabilities and elimination of possibilities, the statistical elimination of outliers and background noise, and the correction of raw data to account for instrumentation variance. I’ll bet that they even write peeved e-mails about each other and the data they’re working in.

Which is exactly what is happening in the field of climate change as well as through most science. No big results, just steadily removing the impossible and leaving the possible visible. It is a pity that small minds don’t understand how science works.

25 comments on “Those hard to find WIMPs ”

  1. Rich 1

    The existence of dark matter wouldn’t contradict any dearly held wingnut worldviews or damage the finances of any multinational corporations.

    Anthropomorthic climate change, on the other hand, means that governments need to take expensive action to prevent flooding and starvation. Since government action (unless it involves killing brown people) is anathema to neo-liberals, the science *must* be wrong by definition.

  2. ieuan 2

    So science is a process of ‘probabilities and elimination of possibilities, the statistical elimination of outliers and background noise, and the correction of raw data to account for instrumentation variance’

    Rather than say a whole new theory replacing the existing generally accepted theory because it is a much better fit for the observed data, say something like ‘plate tectonics’ in the 1950’s.

    See that is the thing about science, it is all open to question, discussion, refinement and possibly even being totally thrown away and replaced by something else.

    Nothing in science is ‘fixed’ or ‘settled’ other than the ‘laws’ of science.

    • lprent 2.1

      The ‘laws’ are up for discussion as well. It all depends on your time frames. Fortunately a lot of those discussions relate to milliseconds during the big bang (itself a theory).

      What a science consensus describes as probable, would to most people be a cast-iron certainty. Certainly that is the way engineers tend to take it.

      • ieuan 2.1.1

        ‘What a science consensus describes as probable, would to most people be a cast-iron certainty. ‘

        Like there being 9 planets in the solar system?

        • lprent

          Depends what you define as a planet. It has always been pretty clear that pluto was a pretty weird ‘planet’. Since they’ve been discovering other bodies in the oort that look a lot like pluto and its moon that clearly aren’t, it is only a matter of time before they classify it as an oversized comet. They’re half-way there now.

        • wtl

          Pluto itself hasn’t disappeared, only what whether we call it a planet or not. Terminology, like language, is always going to change over time.

        • NickS

          @ ieuan
          Go read the article on Pluto on wikipedia, because I’ve already done my cluebat duty on this topic.

          And the reasons for re-classifying the term “planet” are really, really, really simple. Well, mostly, from memory some of the orbital dynamics stuff is a bit less easy to understand, but not that difficult.

      • burt 2.1.2

        What a science consensus describes as probable, would to most people be a cast-iron certainty. AND when there is no actual consensus and there is claimed to be one – most people call that a fraud.

        • ak

          ah there’s burt – you found one Lyn 😎

        • Bill


          Hold a glass above some concrete with the intention of dropping it. A predictable result would be that the glass will shatter. But it’s a prediction and as such contains an element of doubt.

          But what you seem to want is to be told that the glass will break. That it will break into ‘x’ number of pieces and that they will come to rest ‘here’, ‘here’, ‘here’, and ‘here’.

          Nobody can hold your hand and give you that certainty Burt. If you desperately need that degree of certainty, you can always adopt or construct a cosy delusion…..a religion, to wrap yourself up in at night.

          • burt

            If you desperately need that degree of certainty, you can always adopt or construct a cosy delusion ..a religion, to wrap yourself up in at night.

            Yes yes yes, a degree of certainty, someone to tell me how much sea levels will rise and how much temperatures will rise down to the a 1/10 of a degree depending on what emission targets we aim for.

            You hit the nail on the head – things are changing and we don’t know what the key factors are or if it is just normal – the lovers of cuddly blankets want reduction targets translated to temperature changes….

        • quenchino

          burt… have you any idea how extensive and exhaustive the IPCC publication process is? Here is a simple overview.

          If there was ever any human endeavour in all history, that so closely meets the formal definition of ‘consensus’… it is the IPCC climate change reports.

        • NickS


          Please destroy your computer, you are too stupid to use the internet.

          Because less than 20 minutes of mucking around in wikipedia and on climate blogs would show the claim of “no consensus on climate change” to be a big pile of sh*t, and that the claims of no consensus typically rely on non-specialists, and a rather small number of climatologists. It’s as stupid as saying there’s no consensus on evolution when those declaring it to be false are either non-biologists etc, or form a very small percentage of those with relevant degrees, to such an extent actually, they’re outnumbered by Steves that are both biologists etc and accept evolution.

          Of course, this also relies on you being smart enough to understand the definition of the word “consensus”, which typically not 100%, but a clear majority of individuals, or in this case, climatologists and those actively publishing in climatology and related fields.

          And for those with working brains, here’s a survey from ’04;
          The Scientific Consensus on Climate Change
          Naomi Oreskes

          Also: 97% of active climatologists agree that human activity is causing global warming

          • burt

            Here’s a survey from ’04

            Oh yes, one whole year before the most recent IPCC models. Wow, time stood still since ’05 – must have been the decline.

            • NickS


              So in other words, you’re moronic enough not to understand time series sampling sizes for examing trends that you’ve taken a sub-decade slice of temperatures, ignoring fairly obvious f*cking issues:
              (I have others, but it’s 11:30…)

              And the you’ve ignored the blog post on the survey pub’d in EOS, this year.

              Yay for human stupidity!

    • NickS 2.2


      Except of course their referring to particle physics, which oft involves trawling through detector data looking for repeated peaks and then looking for the possible causes, aiming to control for outliers that are due to noise, mis-functions or instrument variation in measurement, and then how those peaks relate to current models of subatomic particles etc. So the description you’re rubbishing slightly, is in the context of particle physics, not rubbish, but the current accepted way of making sense of detector data.

      Of course, it doesn’t take much thinking to notice you can have some philosophy of science fun here with Kuhnian descriptions of science, per paradigms, and theory-ladeness of observation, but at the same time, there’s also fundamental elements of science here. Such as testing models against reality, which in particle physics, involves looking for peaks (r.e. previous paragraph) in detector data which fit, and then nutting out where the peaks which aren’t explained by current models fits in. And more so, the fact this result, with further confirmation, possibly spells the beginning of the end for MOND as an alternative to dark matter as an explanation for the galaxy rotation problem and the probable hiding place of much the universe’s missing matter.

      Although, there’s obviously further work required to out rule the other dark-matter hypothesis, via further telescope observations and refinements of models, but non-the-less, this is an awesome bit of science, that reminds me why cosmology and physics are almost as beautiful as biology 😛

      And one last thing, yes, there is no certainty in science, but in order to question findings and claims, one must have evidence that is basically scientifically sound and actually points at real flaws within the target, along with being a rational argument, i.e. no formal/informal fallacies present. Which is something CCD’s and other denialists of science, fail time and time again to pull off.

  3. Alethios 3

    Dark matter is something i’ve been fascinated by since high school. Exciting news that perhaps someday soon, we’ll have a much better grasp upon the structure of the cosmos.

  4. Ben 4

    Actually, one thing that the lala-right may take issue with, is that a (more) solid understanding of dark matter, may lead to a better understanding of where we come from, and how it all happened, which they still claim to “know”, as in certainty.

    But for the rest of us, imagine a better understanding of the Big Bang, which the research is ultimately aimed at. By removing an impossiblity, I think it’s worth pointing out, that you get a more accurate theory, not a new one that totally replaces anything that came before.. It is the process of building on existing knowledge, to gain new knowledge, and not – as Burt probably imagines – the process of switching theory-systems like teenage girls switch fashions. The (scientific) theory of the atom, for instance, has been around since the 1800s, that’s not counting philosophical theory, going back th the classical greek era. Since that time we have discovered sub-atomic particles, like quarks and gluons, and we are still in the process of understanding fission, and quantum-level dynamics, and a host of other problems I’m not competent to talk about. But your periodic table is still the same, nothing is thrown away in this process. Knowledge doesn’t have life-cycles. Old knowledge does not die, in order for new knowledge to come into existance. The dynamic is fairly complex, problems solved lead to new models, experiment-results force refinement in a theory, but a short-hand version would be an image of constant expansion, with observation and experimentation going on around the borders, which is where known meats unknown.

    Burt : But if you don’t believe me, just drop your plates, and forget about all that Isaac Newton mumbo-jumbo. It’s 300 years old, it can’t possibly still be accurate.

    Well either way, I’m very interested.

  5. burt 5


    Burt : But if you don’t believe me, just drop your plates, and forget about all that Isaac Newton mumbo-jumbo. It’s 300 years old, it can’t possibly still be accurate.

    He also supposed that all prisms would give a spectrum of exactly the same length .

    He subsequently published many papers in the Philosophical Transactions on various parts of the science of optics, and, although some of his views have been found to be erroneous, and are now almost universally rejected, his investigations led to discoveries which are of permanent value.

    History may be repeating ben, but as Issac would have said; a man must either resolve to put out nothing new, or to become a slave to defend it.

  6. Ben 7

    I thought we’d be disagreeing, glad to see it’s not the case. Unless that supposed to be a rebuttal of some sort? I mean his science was built on the knowledge of the time, yet he correctly observed that a prism could decompose white light into a spectrum of colours, that a lense or another prism could recompose into white light. This was the basis upon which his major discovery in optics (the wave-quality of light, much like sound) was made. He also grasped the second quality, the particle-quality of light (calling them “corpuscules”), which was an embryonic theory of his. Is it your point that today’s understanding (wave-particle duality, the proton, quantum-mechanics) is totally independent of Newton’s? I don’t even think that’s true – but someone more competent in the field would have to make that judgement. My point was the progressive, expanding nature of science as opposed to wholesale replacement nature that you seem to advocate.

    That is to say that today’s understanding is fundamentally an improvement of Newton’s. Young’s and Maxell’s observation of wave-theory would not have been possible without Newton. Planck’s constant was only possible because of Young/Fresnel and Maxwell, Einstein’s photoelectric explanation required Planck’s research.. I hope no part of this progression escapes you.

  7. burt 8


    I hope no part of this progression escapes you.

    Not at all, the only thing that currently escapes me in this is why humans have failed to learn that the current understanding is not by default the correct understanding. AGW supporters quickly tell us that scientists were wrong about predicting an ice age in the 70’s then in the next breath tell us that todays scientists cannot be wrong because we have a better understanding today. If you genuinely think we now know enough about climate science to stop harshly testing and refining current assumptions then you probably studied earth science in the 70’s & 80’s as well.

    • Zaphod Beeblebrox 8.1

      Until we can come up with a better understanding, the current theories are the best by which to base your decisions on. To do otherwise would be to condemn yourself to mysticism and ideology.

      Many of Newton’s theories were proven to have imperfections by Einstein’s Theory of Relativity- but you will ahve difficulty arguing that Newton’s theories were not of benefit to mankind.

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