Deep inside an abandoned iron mine in northern Minnesota, physicists may have spotted the clearest signal yet of dark matter, the mysterious stuff that is thought to make up 90 per cent of the mass of the universe.
The Cryogenic Dark Matter Search (CDMS) collaboration has announced that its experiment has seen tantalising glimpses of what could be dark matter.
The CDMS-II experiment operates nearly three-quarters of a kilometre underground in the Soudan mine. It is looking for so-called weakly interacting massive particles (WIMPs), which are thought to make up dark matter.
The experiment consists of five stacks of detectors. Each stack contains six ultra-pure crystals of germanium or silicon at a temperature of 40 millikelvin, a touch above absolute zero. These are designed to detect dark matter particles by looking at the energy released when a particle smashes into a nucleus of germanium or silicon.
The problem is that many other particles – including cosmic rays and those emitted by the radioactivity of surrounding rock – can create signals in the detector that look like dark matter. So the experiment has been carefully designed to shield the crystals from such background "noise". The idea is that when the detector works for a long time without seeing any background particles, then if it does see something, it's most likely to be a dark matter particle.
Signal or noise?
When the CDMS-II team looked at the analysis of their latest run – after accounting for all possible background particles and any faulty detectors in their stacks – they were in for a surprise. Their statistical models predicted that they would see 0.8 events during a run between 2007 and 2008, but instead they saw two.
The team is not claiming discovery of dark matter, because the result is not statistically significant. There is a 1-in-4 chance that it is merely due to fluctuations in the background noise. Had the experiment seen five events above the expected background, the claim for having detected dark matter would have been a lot stronger.
Nonetheless, the team cannot dismiss the possibility that the two events are because of dark matter. The two events have characteristics consistent with those expected from WIMPs (PDF).
The CDMS-II team is planning to refine the analysis of their data in the next few months. In addition, they have begun building new detectors in the mine, which will be three times as sensitive as the existing setup. These "SuperCDMS" detectors are expected be in place by middle of next year.
Signs from space
Despite the reservations, there is a palpable sense that an incontrovertible detection of dark matter is imminent. Space-based telescopes like PAMELA have seen particles that could be coming from the annihilation of dark matter in our galaxy. Similar sightings have been made by a balloon-based experiment called ATIC. Soon, the Large Hadron Collider will be starting to smash protons together in the hopes of creating dark matter.
Dan Tovey at the University of Sheffield, UK, who works on the LHC's ATLAS detector, says that while the CDMS results are not statistically significant, they are bound to generate excitement at the LHC. "I'm sure that people will be looking at [these results] with a lot of interest," he says.
He points out that even if direct detection experiments like CDMS find evidence of dark matter, the LHC will have to create them in order for us to understand the underlying physics. For instance, the theory of supersymmetry predicts a kind of dark matter that will be the target of searches at the LHC.
"The really exciting aspect of all this is that if you see a signal in a direct-detection dark matter experiment and a signal for supersymmetry at the LHC, you can compare those two observations and investigate whether they are compatible with each other," says Tovey.
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I know this article necessarily needs to skimp on the dull detail but as it's worded it seems to me to be an experiment inherently incapable of giving a positive result.
You think you've found what you're looking for because you believe you've excluded other things that can give the same result and also equipment noise; how can you ever be fully certain you have? Or how can you be certain a positive result is not some unknown phenomena unrelated to dark matter? Couldn't 0.8 or 2 events in 1 year be quantum tunnelling or spaghetti monsters?
It is not really any different from cuttin edge research in any field. You make the best measurements you can, you eliminate (or account for) as many possible sources of noise and error that you can think of, you analyse the results and work out how to improve the measurement. Eventually, you come up with repeatable, consistent results that either fit your initial hypothesis or cause you to form a new one...
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Such is the messy world of science. You come up with a hypothesis that predicts something. You set up an experiment that controls for noise and try to limit environment variables not being tested for as best you can. Then you see if you're predictions match real world data. If it does then you publish a paper and hope other people repeat your experiment. If enough people repeat it and get the same results then we assume it's true. At least until we find the spaghetti monster (I.E. A better theory that predicts even more closely the data seen in the experiment and/or accounts for that data as well as other phenomenon observed by other experiments). I don't think any good scientist is ever really certain of anything, but we shouldn't let that stop us from applying what we think we know. If there was bet whether an apple would fall down when I drop it on Earth, I would take that bet. Based on what I've seen it always falls down, but for all I know it might fall up tomorrow. Gravity is just a theory based on observation (A lot of observation). In any event you are right that this is particularly tricky stuff to prove, but it seems like we're slowly getting there.
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"it seems to me to be an experiment inherently incapable of giving a positive result." - Phil
There is no such thing as an experiment capable of giving a positive result, if you want to be pedantic. There will always be some chance, however small, of unknown phenomena, spurious quantum effects or interference by alien practical jokers, spaghetti monsters or God(s). Or something nobody ever thought of. You can never be fully certain you have excluded everything. That is why we have to apply statistics and Occam's razor and why experiments have to be replicated. And the key to the scientific method is, you can never truly be certain of anything. That is what makes science self-correcting.
I wish they would teach this in school, rather than teaching "science" as a list of rote "facts".
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It was a pretty sloppy article. More concrete reporting, less hyperbolae please.
What energy range(s) was the experiment looking at, how does this differ from the energies that the LHC is looking at, what are their plans to improve the filtering of noise, what will they do next?
Come on guys, a little more meat please!!
What is even more strange to me in all considerations of dark mater is: if the dark matter are consisted of WIMPs due to the weakly interaction of dark matter particles except gravitational field, I could except that Dark matter is everywhere around us inside us, within is, passing through normal matter all the time. Gravity could influence dark matter particles to form clump together forming large areas of dark matter but in that case I would except that although it interact weakly, since 90% of the Universe is consisted of DM; we could detect interactions in a much lager number, measuring a typical "noise" from such collisions. At the end, according to the theory, 90% of the Everything is made from dark matter, right?
If gravity influence on the dark matter the same way as on the normal matter, we should except that galaxies are formed in the parts where dark matter clumps more, thus also attracting normal matter to clump together. So, we do not need to search for the DM in the vast Universe, the best place to search for it is here on the Earth.
Maybe this will help, gravity only influences dark matter, dark matter is radiance less, it is mass. Darkmatter that is visible has the radiating energy component incorporated in the system to compose atoms. Mass(which is dark) + radiation(which is bright) = atom. Not all mass has radiation completing the system, this is dark matter. Not all radiation has mass, this is light. Mass is dark, a blackhole is dark, however, light following the contours of space of the dark hole produces a visible system. A star. Without mass systems would not hold. Gravity. Dark matter is not all around us it is an intricate part of all we can see, and lays in areas we can not. Large clump of darkmatter, no matter how many times more massive than a hydrogen mass it is still a singularity, a blackhole. Interaction of darkmatter is not weak, wimps are massive, the interaction is observed as weak as they are not visible, they are the inverse of radiance circuits search for in tests, they are weakly interactive with our methods of searching. What we see is the opposite of what is there if we have a complete system, mass is the curving of space, this is not visible, light is the radiance in space, it is visible, and will travel space through all its curvings. I hope this helps explain weakly interacting mass particles.
I am sorry, "V", but your answer didn't help. What's more, it is a bunch of confusing and contradictory statements, some of the completely wrong.
Dark matter (further in text DM) is a mass?
Current theory predicts Higs boson to be a particle that carries mass in all other particles. So even the dark matter could have Higs boson as the mass carrier in its composition. You are suggesting that the DM is consisted of Higs bosons only?
large clump of DM is still a singularity? Where is the proof, for this? Or, at leas any observation confirming this? We cannot for sure say that the singularity even exist in the Universe, or it is just an abstraction of our mathematics. We estimate that the center of the black hole is a singularity, but we do not have any proof, even for this. Large clump of DM do not necessarily has to be a black hole.
>Interaction of DM is not weak, wimps are massive, the interaction is observed as weak as they are not visible
This is not an argument. How many particles were discovered or predicted that are not visible but still have weak or strong interactions which could be observed or could be visible? Where is an evidence or observation for this statement?
Dark matter is not all around us it is an intricate part of all we can see, and lays in areas we can not.<
Wrong! Astronomers theorized DM to explain orbital moving of galaxies- why galaxy is not falling apart due to the orbital speed. Calculating orbital speeds and total estimated mass of the galaxy, it seems that without DM, galaxies are rotating too fast, but still not falling apart. DM could solve the equations, but has to have very strange properties in the same time: to have weak interactions (so it is not simply observable), to comply to gravity as the normal matter, but in the same time still not condensing towards the center of the mass (galaxy cores), rather as the halo around the galaxy (which is contradictory to the normal gravity laws).
So such WIMPs are not actualy WIMPs, to produce halo around galaxy, they have to have strong interactions between themselves, repulsive in nature, which are in balance to the gravity interactions, preventing DM to clump into the galaxy center, repulsing wimps to produce halo, but still keeping DM under the gravity influence so not falling apart and dispersing from galaxy.
Very strange and contradictory. Dark matter and wimps are raising more questions than offering answers. And what's more, it produces 90% of our Universe?
Of course, we have to search more to confirm eventually this hypothesis, but with such a lot of contradictions, we should reconsider also our current theories and search for new completely different views on our Universe.
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