New Results from the CDMS II Experiment

It’s no secret that astronomers claim that most of our universe is made of dark matter that cannot be readily detected. From Fritz Zwicky’s observations of the Coma clusters in the 1920’s which suggested that additional mass would be necessary to hold the cluster together, to the flat rotation curves of galaxies, to lensing in such places as the Bullet Cluster, all signs point to matter that neither emits nor absorbs any form of light we can detect. One possible solution was that this missing matter was ordinary, but cold matter floating around the universe. This form was called Massive astrophysical compact halo objects, or MACHOs, but studies to look for these came up relatively empty. The other option was that this dark matter was not so garden variety. It posed the idea of hypothetical particles which were very massive, but would only rarely interact. These particles were nicknamed WIMPs (for weakly interacting massive particles). But if these particles were so weakly interacting, detecting them would be a challenge.

An ambitious project, known as the Cryogenic Dark Matter Search, has been attempting to detect one of these particles since 2003. Today, they made a major announcement.

The experiment is located a half-mile underground in the Soudan mine in northern Minnesota. The detector is kept here to shield it from cosmic rays. The detectors are made from germanium and silicon which, if struck by a potential WIMP, will become ionized and resonate. The combination of these two features allow for the team to gain some insight as to what sort of particle it was that triggered the event. To further weed out false detections, the detectors are all cooled to just above absolute zero which prevents most of the “noise” caused by the random jittering of atoms thanks to their temperature.

Although the detector had not previously found signs for any dark matter they have provided understanding of the background levels to the degree that the team felt confident that they would be able to begin distinguishing true events. Despite this, false positives from neutron collisions have required the team to “throw out roughly 2/3 of the data that might contain WIMPs, because these data would contain too many background events.”

The most recent review of the data covered the 2007-2008 set. After carefully cleaning the data of as many false events and as much background noise as possible the team discovered that two detection events remained. The significance of these two detections was the result of today’s conference.

Although the presence of these two detections from 8/5 and 10/27 2007, could not be ruled out as genuine dark matter detections, the presence of only two detections was not statistically significant enough to be able to truly stand out from the background noise. As the summary of results from the team described it, “Typically there must be less than one chance in a thousand of the signal being due to background. In this case, a signal of about 5 events would have met those criteria.” As such, there is only a 1:4 probability that this was a true case of a detection of WIMPs.

Astronomer turned writer, Phil Plait put it slightly more succinctly in a tweet; “The CDMS dark matter talk indicates two signals, but they are not statistically strong enough to say “here be dark matter”. Damn.”

For more information:

Collaboration’s Website

Liveblogging of Conference by Cosmic Variance

21 Replies to “New Results from the CDMS II Experiment”

  1. I asked once before but didn’t get a response, maybe someone could enlighten me. My thinking is that the Universe is packed full of Photons since every star has been emitting them since they have begun to shine. Now the question is does a photon have any mass at all? When we look up at the sky we see darkness but in reality space is filled with light since stars radiate light in all directions all the time.

    So doesn’t that radiant pressure push against other stellar bodies for the same reason a Solar Sail would work causing the whole universe to expand and more over wouldn’t all those photons account for a pretty good sized chunk of the mass in the universe we don’t see because it’s all contained in photons?

    Am I off in left field or does this hypothesis hold water?

  2. Clatonium: Even 1:4 is infinitely more than anything previously reported. What this is confirming is that we are finally reaching detectors sensitive enough to reach the observational threshold. In that light, this is quite an important announcement.

    Silver: photons have no mass, but they DO have momentum. They can push things around (see Yarkovsky effect) but you CAN’T push the universe. There’s nothing to push against. Furthermore, space isn’t “filled” with photons. Sure there’s lots out there, but you can measure the density of photons in a unit volume. In interstellar space, it’s many, many orders of magnitude lower than what’s coming off my computer monitor.

    Long story short, yes, you’re off in left field. Sorry to burst your bubble.

  3. Why do I get this funny feeling the same breed of scientist who attempted to push the geocentric model (and its bizarre wheels upon wheels mechanism) down people’s throats centuries ago have somehow managed to reincarnate into today’s “dark matter” physicist???

  4. 2stepbay: You are making a false equivalence.

    Dogma played a big role when it came to geocentric model. The so called “scientists” were not willing to dump the theory when data didn’t fit their model. They kept on pulling one epi-cycle after the other from their a**.

    20th century is filled with many great instances of scientists predicting existence of particles which were not yet detected. Some examples are Neutrons and Neutrinos. It is extremely likely that the same is true of Dark Matter. Otherwise you wouldn’t see so many scientists working on it.

    If you want to convince your self that scientists are not forcing any thing down anybody’s throat, you could start by going through the evidence presented in the following series of fantastic blog post’s over at

    1. For darkmatter:

    Scroll down on that page to see the 4 part series.


    2 For dark energy:

    One more four part series.

  5. Timely, I was curious about the release.

    And the result is worth the wait, it is quite intriguing to finally see something that could be a signal.

    there is only a 1:4 probability that this was a true case of a detection of WIMPs.

    Um. I must confess I haven’t thought of looking at it like that before. Guess that is why I have a hard time wrapping my head around that one.

    A bayesian a priori estimate could be 40 % (2 in 5) probability of a signal, so that isn’t it. Good, because that isn’t informative anyway. [Which is presumably why I have avoided such hypotheses.]

    Perhaps it is 25 % of the estimated signal distribution tail? (That is, 2 events makes 400 % of 1:1000 chance of background noise, or 99.6 % “certainty” when you are aiming for 99.9 %.) Of course I will need to look at the references, but any help here would be appreciated.

    [Truly OT, seems I haven’t got enough coffee yet so I’m grumpy:

    Is it just me that gets annoyed by national perversity of dating systems?

    No, I don’t mean how to get the girls. But as opposed to the Fahrenheit/Celsius vs Kelvin problem, datings are contingent on nationality _and there is never any way to be 100 % certain about ~ 50 % of the dates in isolated cases_.

    For instance, I would use the YYMMDD standard: “070805”. Presumably it is international and (internal) internet standard, and for sure it puts the most significant numbers first as in all other observations. For reasons of math practicality and physics universality.

    Others here [Sweden] would use the everyday DDMMYY standard: “5/8 -07”. Presumably because it resolves significance based on local contingency, from day towards year. Fair enough.

    But IMO it takes a slow and/or perverse mind 😀 to insist on (an undeclared) MMDDYY standard: “8/5 2007”.

    I can see how you would want to resolve dates based on months in the day of horse carriages. That would be about the period to send letters or people around, tour and retour.
    But now? (o.O)]

  6. @ ST:

    You have already gotten the professional answer, as I understand it. But perhaps I can try to expand on it:

    It is actually the other way around, the expansion explains why the universe isn’t filled with light heating you up to ~ 10^3 – 10^4 K as the surface of most stars. Even an infinite universe dilutes radiation if it expands. Look up “Olber’s paradox”. (Apparently a misnomer, already the old Greeks…)

    As for mass. Photons have no invariant mass as described. They have relativistic mass from their energy of course. That mass acts to halt the universe expansion btw.

    I don’t know enough to say if a radiation dominated universe would expand forever (albeit for reasons of general relativity, not classical mechanics as you suggest). I can just copy the claim that any (invariant) mass will eventually dominate over radiation energy in what would happen in an expanding universe, exactly as it happened in our universe. Dark energy, which in turn decides now, is energy with a vengeance, I’m sure. 😀

    @ Popisfizzy:

    Good one! [Too bad it was a EU universe or sumthin’ equivalent believer, that jumped at the opportunity to “promote your personal theories.”]

  7. the Fahrenheit/Celsius vs Kelvin problem

    D’oh! It’s “the Fahrenheit/Celsius vs kelvin problem”, of course.

    [See? Now you have gotten to me too. :-/]

  8. To answer your question, Silver Thread, I checked the Wikipedia entry for “photon” to verify what I remembered from college physics (Please everyone, be kind; I am a layman, not a scientist). The photon is a “massless” particle. Wiki goes on to say that it’s rest mass is 0. Further, it has no electric charge. Now one of the indicators that clued scientists in on the possible existence of dark matter was such phenomena as unexplained gravitic lensing, the speeding up of the expansion of the universe, and other occurrences of effects that had their origins in gravity. Since a photon has no mass and since gravity is dependent upon mass (as I understand it–you physics savvy aficionados jump in here and correct me if I am wrong) I think it highly unlikely that photons are the missing dark matter that scientists are looking for. It was a good thought, though.

  9. I am picking at the paper:

    Much of the statistics involves nuclear recoil data I am not terribly familiar with. The results are based on a 1:4 (23%) probability of getting two background signals in the energy quoted, which is then used in some regression or Bayesian regression analysis to give a 90% confidence of detecting a WIMP at 70GeV/c^2.


  10. This is simple and normal particle physics banter.

    Whenever your experiment fails to find something which is pure theory, the answer is to build up the experiment to be bigger, colder, and faster to keep looking for it. By the way, doing this also increases noise! Our next accellerator will probably have to get bult inside the moon!

    Distilling the atom into its parts is becoming a lot more complicated every day. One of these days, we just might figure it out.

  11. Pedantic Said:
    “the indicators that clued scientists in on the possible existence of dark matter was such phenomena as unexplained gravitic lensing, the speeding up of the expansion of the universe,…”

    The speeding up of the expansion of the universe is indicative of dark ENERGY, not dark MATTER.

    Lawrence: Thanks for posting the paper. I’ve been waiting for it, but it wasn’t up by the time I went to bed.

    Aodhhan: Yes, the noise does increase, but NOT at the same rate as detection capability.

  12. @Torbjorn Larsson OM:

    From the paper:

    “These expectations indicate that the
    results of this analysis cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject either event as signal.”

    This hardly equates to “90% confidence in detecting a WIMP”

  13. I have to go with silver on this one. He makes an intresting point. Altough photons don’t have any mass and we know mass creates gravity. Isn’t said that every action as an opposite reaction? So if mass creates gravity and photons don’t have mass wouldn’t it be right in saying they would have an anti gravity effect?

  14. Thanks for the responses from all of you, I am a layman myself but am no less bound up in the universe than any other human being has ever been.

  15. Well, since E=mc^2 and for photons E=hf (h is Planck’s constant and f the frequency of the photon) one can equal the equations which results in a photon mass of

    m = hf / c^2.

    The important thing is that this is not the rest mass of the photon, which is obviously 0. The photon does only have this mass while it exits, or while it is “on the move”.

    Also one should note that photons are influenced by gravity — or, more precisely, by the curvature of space-time. This leads to the famous effects of gravitational lensing or gravitational redshift (not to confuse with cosmological redshift!!).

    @ King Gaz:

    No, photons have no “anti gravity effect”, as I described above. If they would have one, it would be WELL known!
    Anti-gravity has not been observed yet, although some claimed otherwise — but their experiments couldn’t be verified.

  16. Jon: Depends on what you’re detecting, and what specifically you are looking for, in what you’re studying. Some have more expense (energetically not just monetarily) than others.

    I never go into great detail on a blog. Those interested know where to do their research, or have the needed knowledge. Plus people get bored… there are already enough patzers who ‘google’ things up, cut and paste, and attempt to look like they are the ultimate authority.

    Photons are affected by many things. They are contained, absorbed, reflected, bent, condensed, yada yada.
    The best thing about massless items is you can make them go really really fast!
    If ‘light’ had mass, it wouldn’t go as fast as it does. You can’t just spin anything you want in a particle accelerator…. darn-it.

  17. One simple question: If photons have no mass, how could their movement, caused by gravity or another form of energy, be measured? All exterior objects are but forms or shadows of a previous existence.

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