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Underground Xenon100 experiment closes in on dark matter’s hiding place

The views expressed are those of the author and are not necessarily those of Scientific American.


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Xenon100 detector in a cleanroomA major dark matter experiment has taken a swipe with its technological net in the hopes of catching some of the elusive particles that make up the universe’s missing mass, and once again that net has come up empty. But in swiping and missing, the Xenon100 experiment has closed in a bit tighter on where dark matter—the invisible stuff theorized to outweigh the ordinary matter in the universe by a factor of five—might be hiding.

Xenon100, a tank of liquid xenon deep underground in Italy, has been designed to identify the rare instances when ambient dark matter particles would recoil off regular matter, a subtle collision that should generate a tiny flash of ultraviolet light and a slight electric charge from ionization effects. The detector’s 62 kilograms of cryogenic xenon are shielded from non–dark matter contamination by layers of copper, polyethylene, lead, water—and a thick slab of rock overhead that keeps out almost all cosmic rays.

Even so, the detector gets some deceptive signals, for instance stray neutrons from the occasional radioactive decay of atoms within the experimental apparatus and electrons from radioactive krypton contaminating the liquid xenon itself. And after 100.9 days of collecting data in 2010, Xenon100 recorded only three hits that looked like dark matter, an insignificant amount compared to the expected background of about 1.8 such events. In other words, researchers from the project report, "the observation of 3 events does not constitute evidence for dark matter"; those three hits could easily be background noise. The Xenon100 collaboration, led by Elena Aprile of Columbia University, reported the latest results from the experiment in a paper posted online April 13 at arXiv.org, a physics preprint Web site.

So Xenon100 has not found any evidence for dark matter—at least not yet—but that nondetection helps rule out certain parameters for the stuff, in terms of the putative dark matter particle’s mass and how often it bumps into ordinary matter. The kinds of dark matter particles now ruled out at the 90 percent confidence level include those that were candidates to explain the signals recorded by two other detectors, CoGeNT in Minnesota and DAMA in Italy, casting doubt on those signals as genuine dark-matter hits. The data even rule out part of the regime where Europe’s Large Hadron Collider (LHC) might look for evidence for dark matter as a so-called supersymmetric particle, a hypothetical heavyweight partner to one of the standard elementary particles.

Even though the most recent swipe at dark matter came up empty, the Xenon100 researchers are already back on the hunt. The detector is now taking more data, they report, with fewer false positives thanks to reduced krypton contamination in the detector.

Photo: Xenon100 collaboration





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  1. 1. ineamer 9:15 pm 04/14/2011

    This is intended for comments on the above Dark Matter article, not for sales pitches.Am I wrong?

    Link to this
  2. 2. rloldershaw 10:57 pm 04/14/2011

    Everyone says that "WIMPS" are the "leading dark matter candidate".

    This is a bit bizarre. Over the last 35 years particle physicists, astrophysicists and cosmologists have searched deep space, the Moon, the bottoms of mines , Antarctica, accelerator debris, etc. for "WIMPS".

    Result: Not a single "WIMP" or axion, or "sparticle", or magnetic monopole, or boojum has ever been detected. Nor has any "extra dimension" beyond the 4 we know of.

    Perhaps nature is repeatedly giving us the less-than-subtle hint that the dark matter is NOT in the form of unobservable imaginary particles.

    A far better candidate for the galactic dark matter is "primordial" planetary-mass and stellar-mass Kerr-Newman ultracompact objects, i.e., black holes and singularities. Gravitational microlensing experiments have turned up evidence for such a population of objects. Stellar-mass black holes are known to exist in well-observed binary systems.

    We also know that there are billions of pulsars, isolated neutron stars, RRATs, magnetars, and gamma-ray burst sources out there in the Milky Way Galaxy and they might be viewed as excited states of the ground state black holes.

    Why do we obsess about "WIMPS" when there is no observational evidence for them, and their theoretical "motivation" is more than a bit dubious?

    Robert L. Oldershaw
    www3.amherst.edu…~rloldershaw
    Discrete Scale Relativity; Fractal Cosmology

    Link to this
  3. 3. phalaris 2:15 am 04/15/2011

    Other blogs don’t get so regularly polluted with this spam: why of all institutions can’t SciAm keep it out?
    Didn’t understand the "expected background of 1.8 events" in the article, compared to 3 possible hits.
    Am I missing something – like a timebase?

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  4. 4. jtdwyer 2:20 am 04/15/2011

    It should be noted that even if a WIMP were detected, it would not be proof that WIMPs account for the perceived excessive rotational velocity and gravitational lensing of galaxies. That result is merely a discrepancy between observed gravitational effects and their estimates obtained by applying the highly specialized, empirically derived laws of Planetary Motion to galaxies composed of billions of stars and other masses. The perceived requirement for dark matter may simply be compensation for the error produced by inappropriately oversimplified gravitational evaluations of extremely complex masses.

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  5. 5. tharriss 7:35 am 04/15/2011

    I don’t understand why SciAm can’t add that little graphics box of mixed up letters you have to type before entering a comment, that would cut way back on the auto advertisements in the comments…

    It does seem more and more dubious that dark matter is the correct explanation.

    Link to this
  6. 6. jtdwyer 10:49 am 04/15/2011

    SA Editors: I think tharriss’ authentication suggestion is an excellent one – please investigate!

    Link to this
  7. 7. SpoonmanWoS 2:03 pm 04/15/2011

    Dr. Oldershaw, your observations would carry more weight if you were able to provide some actual evidence for your hypothesis. A quick google of you turns up lots of blog comment posts where you’re quick to deride the work of pretty much every physicist’s work, but very little work of your own that has any value. By "very little", I mean it approaches zero.

    When doing further research into the "dark corners" of the Internet, your name turns up more frequently…specifically in the realms of wacko theories and fringe loonies. There are many discussions of you when you look at the deletion history in Wikipedia. For example:

    "Wikipedia has specific and longstanding practices to deal with fringe physics theories. Right now, the article suggests that 10 or so people, over the last 100 years or so, have independently thought, "Whoa, isn’t an atom sort of like a mini-solar-system, and a solar system sort of like a mini-galaxy?" This doesn’t particularly differentiate your theory from the hundreds of few-adherent physics theories we delete all the time. The article does not even begin to suggest that there’s a coherent school of thought behind the "fractal" idea; it’s just a list of people who have, at one time or another, said, "Whoa! Fractals!" Oldershaw, for example, has published dozens of unrefereed papers and gotten exactly one obscure citation. "

    Now, I assume you probably consider yourself akin to Galileo, railing against the "establishment" to get your ideas recognized. I must point out the significant difference between yourself and Galileo:

    Galileo was fighting against the church, which is duty-bound to never accept conflicting points of view from mere mortals as their precepts were handed to them, in their opinion, by their god. On the other hand, you’re railing against the scientific community which, certainly, can be inflexible to change at times. The difference being, with sufficient evidence it DOES change. I point you to plate tectonic theory, first proposed by Alfred Wegener. Wegener was correct, but his theory was entirely based at the time on how things looked similar. It wasn’t until Runcorn & Carey came along with EVIDENCE that the theory was taken seriously. I would point out that the same could happen to you, it just requires evidence.

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  8. 8. Cramer 3:59 pm 04/15/2011

    I’m assuming what they meant by "expected background of 1.8 events" is the number of events from unknown origins that are not dark matter (e.g. background noise) over a 100 day period. They definitely determined the expected number of events over a 100 day period from known and unknown sources. The known sources were, for example, stray neutrons from radioactive decay from Xenon isotopes or other known contaminants (they can’t 100% purify their liquid xenon–too expensive or impossible).

    For unknown events they determined the number to be 1.8 per hundred days; and it probably had a relatively large degree of uncertainty, for example, 1.8 +/- 1.2 with a confidence level of 90%. This number and confidence level would be related to the confidence level of the known events. A longer duration for the experiment would increase the confidence levels, especially for known events. In other words, the confidence level of the number of radioactive decays of a xenon isotope over a period of one hour or one day would be low.

    They probably had a certain number of events (all experimentally unknown), but they explained a statistically calculated number of those events coming from known events. The remainder was three events. They deemed this as most likely coming from unknown events that were not dark matter events. If the remainder had been six events, they may have deem that it was likely that some of these six events were dark matter.

    You could look up how many neutrons events they expected: maybe it was 10; maybe it was 100; I don’t know. And I don’t know how they came up with 1.8, except only through purely statistical methods.

    [wow, that was much more than I thought I would write, sorry]

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  9. 9. rloldershaw 6:42 pm 04/15/2011

    "Spoonman",

    If you go to http://www3.amherst.edu/~rloldershaw you will find the science you claim does not exist.

    For example, a publication list of scores of published papers. Including definitive predictions published in the Astrophysical Journal.

    Also, an easy-to-read list of 38 successful predictions and retrodictions of fundamental properties of nature. Details are in the "Selected Papers" section.

    Also, reviews and discussions of Discrete Scale Relativity at various levels of technical detail.

    Also, some pretty pictures if that is your strong suit.

    Clearly, research skills are not your strong suit.

    Robert L. Oldershaw
    http://www3.amherst.edu/~rloldershaw
    Discrete Scale Relativity; Fractal Cosmology

    Link to this
  10. 10. Thim 7:28 pm 04/15/2011

    I agree with you:First of all Theoretical Physics (TP) has to be corrected by getting rid of wrong theories like relativity. Cosmology suffers a lot from thos errors in TP

    Link to this
  11. 11. SpoonmanWoS 8:06 pm 04/15/2011

    Actually, I had already been to your site and reviewed your documentation. I wasn’t impressed, which is why I did further research just to make sure I wasn’t misinterpreting. That’s why I tracked your reputation and citations to find out how well received you were. I always do my research before opening my yapper. :)

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  12. 12. outsidethebox 12:25 pm 04/16/2011

    This current "dark matter" and "dark energy" theories remind me of nothing so much of the "string theory" nonsense that we went through a decade ago. We were told for years that was going to be the ultimate answer to everything. But I’m sure they’ve got it right this time.

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  13. 13. Thim 6:06 pm 04/16/2011

    As long as these theories are built on wrong theories such as relativity theory only nonsense will be predicted.If Relativity Theory is removed from Theoretical Physics (Cosmology) everything will be fine!
    Hartwig Thim(www.ime.jku.at)

    Link to this
  14. 14. NoToe 6:45 pm 04/17/2011

    Let me ask this:
    Dark Matter is needed to explain the orbital velocities of galaxies and clusters. Simply they are traveling through time and space faster than they should be.
    Dark energy is needed because the galaxies and clusters in the universe are expanding away from each other faster than they should be.
    Both are in reference to matter and how it moves through time and space.
    Both are dealing with gravitational forces.
    What is the common theme of dark matter, dark energy, gravity, and matter?

    Link to this
  15. 15. reasonable2 8:28 pm 04/17/2011

    Are we looking for a "graviton"? Just asking….

    Link to this
  16. 16. NoToe 5:01 am 04/18/2011

    What is the common theme of dark matter, dark energy, gravity, and matter?Time, one way or another all have an association with time.   

    Dark matter is needed to produce a greater force of gravity to justify the orbital velocities of galaxies and clusters, the rate at which matter is traveling through time. 

    Dark energy is needed to explain why galaxies and clusters are accelerating at faster time rates away from each other.  

    The problem is dark matter and dark energy create two separate opposing forces both of which are universal, the expansion force of the universe and the universal force of gravity.   

    The unusual part is galaxies and clusters seem to be self contained units maintaining gravitational forces and orbital velocities even while a universal opposing force is exerted upon them.   

    My logic is to not create more pieces to the puzzle making it harder to solve.   

    The problem boils down to an expanding force and a collapsing force. 

    These two problems needed to be in play for the universe to begin. A singularity needed to have a collapsing force to collapse into itself and expand out creating this universe. 

    The problem is the expansion force has to exceed the collapsing force in order for it to keep expanding. This means the collapsing force needs to be an accumulative force. 
    This would rule out a single singularity that contained all the generic matter in the universe. 
    Instead it could have started out as a much weaker force from an infinite amount of singularities.   As long as the collapsing force repeated itself the end results leading to a big bang could exist. 

    The collapsing force could still exist and is happening now. 
    As each collapsing force collapses in as a sphere it pulls on more surrounding singularities.  Collapsing in, each time creates a larger void to expand into.  This creates a density problem in space and creates dark energy. 

    The never ending repeating collapsing force creates the allusion of dark matter. 

    Galaxies and clusters are self contained unites because they use more energy as they accelerate with the expansion force, each new collapsing force has more energy. 
    A balancing act was created in the beginning but never perfect.    
    The two opposing forces create gravity.           

    Link to this
  17. 17. bucketofsquid 10:20 am 04/19/2011

    A couple of divergent comments:
    First – The forum software that SciAm uses isn’t very good and they do a poor job of preventing spam. After reporting spam I then tried to log in to the forum to post a comment. Instead of allowing me to log in I instead was thanked for reporting abuse. In other words, the function of the log in link was subverted by the report abuse link functionality. How do you even manage to do that?

    Second – My actual comment on the article; Every time I hear or read about dark matter or dark energy it always comes down to "our math is bad". We already know that the formulas and behavior of matter and energy are different at the quantum level. Why is it so hard to believe that perhaps they behave a bit differently at the galactic level? I personally subscribe to the theory put forth by JTDwyer, that the formulas being used are over simplified and are not accounting for gravitationl affects acroos galactic distances. I won’t claim that I have any evidence based reason for my belief but that just puts me on the same level as the "dark" supporters. It reminds me of the early quest for flight when the formulas used had little to do with reality but were eventually replaced by more realistic formulas that actually allowed flight worthy designs.

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  18. 18. jtdwyer 3:34 pm 04/19/2011

    Thanks for your kind remarks.

    Just this week I’ve been made aware of an independent, parallel effort that has essentially produced identical results, but theirs are much more technically complete. Please see:

    "Rotating thin-disk galaxies through the eyes of Newton",
    James Q. Feng, C. F. Gallo, (2010),
    http://www.arxiv.org/abs/1007.3778

    Link to this
  19. 19. rwstutler 2:05 am 04/20/2011

    Research is fine, when you get paid for it. Entertainment time is way too precious to waste on crackpots like jtd and rlo, or whatever they are calling themselves these days. Those who seek to criticize the work of researchers based on a magazine article expose themselves. I’m still waiting for the lab rats to come up with more interesting phenomena and data.

    Link to this
  20. 20. rufusgwarren 4:09 pm 04/20/2011

    Is it not time to review our basic assumptions in physics?

    Link to this
  21. 21. rwstutler 12:14 am 04/21/2011

    Why? Are they not the best working approximations currently available?

    Link to this
  22. 22. jtdwyer 7:47 am 04/21/2011

    You’re referring to the ‘phenomenon’ of dark matter, inferred only by the discrepancy between observed gravitational effects and those projected by astronomers’ estimations based on their misapplication of the superseded ‘laws’ of planetary motion? The dark matter that, after more than 40 years has no theoretical basis for its necessary material properties, that has never been directly detected?

    Please at least review the independent research I’ve referenced in the preceding comment before criticizing. I could provide additional references in addition to those listed by the authors of that research – our common conclusions are not based on this article.

    Some crackpots object to anyone who criticizes the works of researchers reported in a magazine without any research of their own. If you do investigate research publications you’ll find that researchers often contradict established prior works, by suggesting the existence of now long unidentified, undetected matter, for example. Personally, I have to wonder about anyone who wold not question such long held but unsubstantiated ‘scientific’ beliefs, much less discredit those who do.

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  23. 23. Zandor 8:16 am 04/21/2011

    Hi everyone, this is my first post here and I find it important to mention that I’m not a professor of any kind not a great physicist or a genious as you’ll fin out, I guess. I’m only being intrigued by modern theoretical physics and cosmology questions so I read and try to understand as much as I can. And english isn’t my first language either so sorry in advance for mistakes.

    I have always been skeptic about dark energy and dark matter, as to my understanding they only represent that something is missing from our current widley accepted and used formulas.
    So one has to introduce a "random" number into the equation for it to be in balance. But this number would not necessarily mean the presence of a whole new entity, it may only be that the already existing numbers aren’t correct, right?

    So why do we want to find actual new particles before rethinking the number of already known ones in the universe.

    I hope you don’t mind me to come up with an uneducated but non the less common sense answer for these dark bits in our equations. So..

    ..As far as I know the only things we can observe in the universe are the ones which emmit light and not just a small amount as our telescopes get better we can see fainter objects and further away with x-ray and IR as well but why do we think that there is no more ordinary matter then we can see (how many exoplanets did we see directly so far)? And what about the black holes (stellar mass and bigger), dust, planets, gasses etc.
    Huge and short lived stars just kept going supernova since pretty much the beginning of time. Shouldn’t we count all the stars we see, determine the sum of their mass and multiply it by 3 or 5 to get a statistically correct close approximation of all the mass in the universe. So yeah, sure, dark metter is all around us… but not as an illusive particle but as very ordinary matter.

    Dark energy? It seems to me that it acts like vacuum. Vacuum of the big nothingness (or higher dimension or call it whatever you want) that our space and time expands in. It’s not a repulsive force it seems to me that it acts like the vacuum of space on a blown up balloon.. makes it expand but not from the inside. And then of course it’s not even a mystery why galactic clusters stay together cause it’s like putting peas into the blown up balloon… they get further apart but don’t tear to pieces.

    Sorry for the long post and tell me if I’m completely nuts or incompetent but this is my opinion and would love to get replys uppon it.

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  24. 24. jtdwyer 12:44 am 04/24/2011

    I suggest you start by reading wikipedia on the two subjects and following its related imbedded links. Please start with:
    http://www.en.wikipedia.org/wiki/Dark_matter
    http://www.en.wikipedia.org/wiki/Dark_energy

    I have my own opinions, but wikipedia is a pretty good source for a consensus view and common alternatives. It’s free, online and available for most subjects.

    Link to this

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