A New Piece to the Dark Matter Puzzle

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For twenty five years I’ve been working on the “dark matter problem”—the question of what makes up roughly 90% of the mass of our Milky Way galaxy as well as every other galaxy. This past week saw intriguing new experimental results that may be telling us something profound about this question.

Last week I was at the American Physical Society meeting in Anaheim at Hyatt Regency Disneyland (well that’s effectively what it is). It was a very large conference with thousands of people, commemorating the 100th anniversary of Rutherford’s discovery of the nucleus inside atoms. My main reason for being there was to attend the Executive Board Meeting prior to the scientific meeting. My friend and colleague Juan Collar ran up to me in the hotel lobby, gave me a big hug, and told me the latest about his dark matter experiment, the Coherent Germanium Neutrino Technology (CoGeNT) collaboration. “We’re drowning in annual modulation!” he said, “and we’re going to have to use your analysis method to enhance the significance.” Annual modulation, the variation of the dark matter signal with the seasons, is the closest thing experimenters have to a “smoking gun” for dark matter, and it is an idea my collaborators, and I first proposed in a paper we wrote in 1986. So to me, this was extremely exciting news!

The dark matter problem is the longest outstanding problem in all of physics. Zwicky first postulated its existence in the 1930s and it still hasn’t gone away. Ordinary atomic matter only constitutes 4% of the universe and the rest of it remains a mystery. Most of us believe that the dark matter that dominates our galaxy consists of as yet unidentified subatomic particles. There would be billions of these flying through our bodies every second, yet they interact so weakly we don’t notice. In fact, we think they are Weakly Interacting Massive Particles, or WIMPs. Dark matter particles fly through the Galaxy at a million miles per hour and the trick is to build a detector that responds to the rare interactions of WIMPs as they traverse the detector.

In direct detection experiments like CoGeNT, dark matter particles scatter off of nuclei in the detector, and the detector records that the interaction has taken place. CoGeNT is made of germanium about the size and shape of a hockey puck. Dark matter experiments have to take place deep underground, to eliminate competing signals from atmospheric particles. CoGeNT is housed in the Soudan mine in Minnesota, about 700 meters underground. It was chugging along taking data until a fire on March 17th of this year shut down all the experiments in the mine. So Juan and collaborators decided to analyze the data they already accumulated in the 15 months before the fire to see what they could find.

The signal they were looking for was annual modulation, an idea Andrzej Drukier, David Spergel, and I proposed as we were sitting around a table at the Harvard Smithsonian Center for Astrophysics in 1986. The Sun is moving around the center of the Milky Way Galaxy, into the wind of WIMPs. But as the Earth circles the Sun, the relative velocity between the WIMPs and the dark matter detectors varies with the time of year. The reason annual modulation is so powerful as a “smoking gun” for dark matter is that you don’t expect other potential sources of signal, like radioactivity in the detector, to have this kind of seasonal variation. Only WIMP scattering is expected to fluctuate with the time of year in this particular way. The predictions we made in our work were that the signal peaks in June and is minimal in December. So the trick is to pull out this variation in the signal. The Italian Dark Matter Experiment (DAMA) underneath the Apennines near Rome, Italy, has been seeing such a modulation for a decade now. The scientific community has been skeptical about their results for a variety of reasons, including the fact that other experiments, especially CDMS and XENON, have been unable to reproduce these results and, in fact, claim to rule them out.

The latest twist is that CoGeNT is now seeing signs of annual modulation too. This could be a confirmation that they really are seeing dark matter. The reason it’s so exciting is that the detection of annual modulation in two different experiments, made of two different detector materials and on two different continents, would be amazing. The analysis will take some time, as Juan’s group implements the more detailed analysis laid out in our second paper. But I can hardly wait to see it! I can also hardly wait to get my hands on the data so we can do our own analysis to see what we think.

Right now, the CoGeNT signal is still preliminary, even with a greater than 99% chance of being real—but scientists aren’t convinced until the data are overwhelming. The weird thing is that a third experiment, the Cold Dark Matter Search (CDMS), is made of the same material as CoGeNT (germanium) and doesn’t see any signal. Although CDMS is geared to higher masses, they were able to reanalyze their data down to these lower energies and see no evidence of WIMPs whatsoever. So one of either CoGeNT or CDMS has to be reinterpreted!

The XENON experiment doesn’t see anything either, but then it’s a different detector material and also was not designed for these low WIMP masses; Laura Baudis presented limits from this experiment which also claims disagreement with CoGeNT. CoGeNT group leader Juan Collar told me that he’s a Spanish guy caught in between two Italian women: Rita Bernabei from Rome is the group leader for DAMA and Elena Aprile (Prof at Columbia University) is the group leader for the XENON experiment. All are strong personalities.

The real test will come from the South Pole, where the ICECUBE/DEEPCORE experiment is doing a test run with NaI (same material as DAMA) imbedded deep down into the ice. Right now the amount of material is too small, but the experiment is working beautifully and in the future the group can expand to large crystals. Looking for annual modulation in a different hemisphere—the Southern hemisphere—and in a place where there is no temperature variation, should do the trick of deciphering whether or not the dark matter has indeed been found.

Catch Katherine Freese in person at the WSF11 event “The Dark Side of the Universe.” Learn more about her lab’s work here »

Juan Collar is professor of physics at the University of Chicago, Elena Aprile is a professor at Columbia University, Laura Baudis is professor of physics at the University of Zurich, and Rita Bernabei is a professor at the University of Rome.