Signs of Dark Matter From Minnesota Mine 158
thomst writes "Juan Collar, team leader of COGENT, an experimental effort to detect WIMPs (Weakly Interacting Massive Particles), recently presented a paper detailing 15 months of data collected via a pure germanium detector located deep in a Minnesota mine which seems to confirm similar results reported by a European effort called DAMA/LIBRA. The results are particularly intriguing, because they appear to show a seasonal variation in the density of WIMPs that accords with models which predict Earth should encounter more WIMPs in Summer (when its path around the Sun moves in the same direction as the Milky Way revolves) than in Winter (when it goes the opposite direction). The most interesting thing about the COGENT experiment is that the mass of the WIMP candidates it records is significantly less than most particle physicists had predicted, according to popular models. (Ron Cowen wrote an earlier article about COGENT last year that goes into a lot more detail about how COGENT works, what its team expects it to find, and why.)"
The Soudan Mine can be toured (Score:5, Interesting)
The lab is located in a lower (the lowest?) level of the Soudan Mine. This mine is also a state park and you can tour the mine.
The tour (when I took it, about 9 years ago) took you down to the same level as the lab, which I think is the lowest level of the mine or within a level or two of the lowest level.
You ride a mine cart to a room where extraction of iron ore took place, hear some details about early mining, including a lights-out experience where they show you what it was like with nothing more than old-fashioned arc lamps on the miner's helmets.
Before you leave this level, you get to go into the lab area and get a look around. I don't think you go much past the entry way, but it's neat anyway.
The mine had a fire recently and I don't know if the tours are back in operation, but I believe they have every intention of continuing with them once they fix whatever happened.
Re:The Soudan Mine can be toured (Score:4, Interesting)
The problem with seasonal variation... (Score:5, Interesting)
I did my physics undergrad at the University of Minnesota Duluth, and they graciously let me play around in the mine on occasion. I don't do much particle physics anymore so I'm not particularily equipped to judge their results, but I can say that all kinds of seasonal errors can be introduced in these experiments. Cosmic rays have a seasonal variation for example. Another that happens at Soudan for sure and possibly in Italy is seasonal variation in background radiation. The air circulation at Soudan is largely passive, and there is lots of radon gas seeping from the rocks. In the cold winter the exchange is excellent, but in the summer the circulation is terrible and you get anywhere from 5 to 10 times the radon background in the cave (air in the cave is warmer than outside in the winter and cooler in the summer, you can do the math).
I'm not saying either of those are the cause of this, but there is good reason to squint hard at anything claiming "seasonal evidence" when the claim is extrordainary (in the sense that it is way off from any model). Scientists should be skeptical of this, especially since they are claiming a result before theory suggests a result should even be possible.
Re:The Soudan Mine can be toured (Score:5, Interesting)
They charged the miners for candles, dynamite, and pretty much every other supply. See, (if I recall correctly from when I took the tour several years ago) the miners were not actually employees -- they were independent contractors. The company sold them supplies, let them into the mine, and then bought whatever ore they hauled out. This was mostly done to screw them. They could spend 18 hours a day hauling iron out of the mine for the company and yet not turn a profit.
Also, incidentally, my group also took a full tour of the lab while we were down there -- I think they are happy to give tours, they just aren't regularly scheduled. We had called a few days before and one of the grad students working there met us and showed us around. Sadly, I was not struck by any super-power-granting science beams.
Not so clear cut (Score:5, Interesting)
There's a huge controversy right now in the field. The DAMA/Libra experiment has been claiming an 8-sigma excess for years which they say is consistent with dark matter, but they keep getting excluded by other experiments, most notably CDMS and Xenon. Every time their favored region is excluded, they come up with a new way to reanalyze their data to make it consistent again. But they have not ever released any of their data to the community (and hold patents on the type of crystal they use for their detector) so it's impossible to directly verify.
CoGeNT first released hints of a low-energy excess which could be consistent with DAMA-type dark matter about a year ago. I was at the APS conference earlier this week where Collar released the seasonal modulation results which make it seem even more likely that they see the same thing as DAMA. However, just the next day, CDMS presented an analysis of their low energy data which is below their normal dark matter threshold (because the rate of background events in that region is quite high and poorly understood). They showed that, even if they didn't account for the known sources of background, the rate in their detector is inconsistent with CoGeNT's. As many people rightly point out, CoGeNT is seeing an exponential signal near threshold, which is what you'd expect to see in just about any detector with or without dark matter present.
The whole situation is muddled even further by politics and personalities. Collar is respected as a scientist, but is also generally agreed to be an asshole. When he announced the annular modulation result, he spent 25 minutes of his talk attacking xenon on mostly pointless grounds, then had only a single slide showing the important result of the modulation. He finds tiny holes in other's analyses, but doesn't often present a very convincing picture of his own.
tl;dr: The community is far from agreeing that what he and DAMA have seen are in fact WIMPs. CDMS and Xenon tend to have better established analysis programs and pay more attention to their systematics, and they still rule out both DAMA and CoGeNT. However, I think everyone at this point agrees they are seeing something interesting, just likely not WIMPs.