Deep in the ice at the South Pole, the IceCube Neutrino Observatory sits and waits for high-energy particles to pass in its midst. However, another detector, DM-Ice, is situated among IceCube’s strings, partnering with its technology for a different purpose: the search for dark matter. Currently, the only detector to make a strong claim to have seen a dark matter signal is DAMA, at the Gran Sasso National Laboratory in Italy. DM-Ice aims to carry out a definitive test of DAMA’s claim.
Among the established WIPAC community are two enthusiastic physicists who can reveal some of the mystery behind the search for dark matter. Reina Maruyama of Yale University has been the principal investigator of DM-Ice since its inception in 2010, during her time as a WIPAC researcher. Among her team of collaborators is Matt Kauer, a postdoc at WIPAC, who is currently working to advance the development of the detector to its full scale.
Q: Can you explain to us what dark matter is?
Matt (M): Dark matter makes up approximately 27% of all matter and energy in our universe right now. But, in fact, no one really knows what this matter is, where it comes from, or what it interacts with. There are a handful of theories explaining what dark matter could be, but we have yet to confirm its origin and nature.
Reina (R): From the luminosity of stars, we can infer their mass. From measuring the speed of rotation of stars, we can also infer their mass and the mass of objects that they rotate around. The second measurement gives us masses much higher than the first, which leads us to believe that there must be much more mass out there than we can see. We call this dark matter. There are other observations that point toward the existence of this invisible mass, like seeing light from distant stars bent around invisible objects. The question is, what is this matter. One of our favorite hypotheses is the so-called WIMP (Weakly Interacting Massive Particle) model. If the dark matter we see out there is made of WIMPs, we might be able to see their interaction with ordinary matter, even if this happens very occasionally.
Q: What is the goal of DM-Ice?
R: DM-Ice really started with a request from the dark matter community. For the last 15 years, the DAMA collaboration has claimed that they have observed dark matter. Their signature is coming from an annual modulation in the number of observed dark matter induced interactions in the DAMA detector, due to the orbit of the Earth around the Sun. The flux of dark matter from the galactic halo on Earth should be higher in early June, when the rotational speed of our planet is added to that of the Sun with respect to the galaxy. In early December, when these two velocities are in the opposite directions, the dark matter signature should be smaller. Since DAMA announced these results, there have been ongoing discussions in the community about whether what DAMA is seeing is really a dark matter signature or just some background fluctuation.
M: DAMA is located in Italy, under the Gran Sasso Mountain, while DM-Ice is buried in the Antarctic ice at the South Pole. From our location, we have a reversed phase of environmental backgrounds with respect to the Northern Hemisphere while the dark matter signature is the same in both hemispheres. Thus, seeing an annual modulation with DM-Ice that’s consistent with DAMA’s dark matter signature would be a smoking-gun confirmation.
R: Right. DAMA’s results have been out there for a very long time, and there are many concerns that the dark matter community has expressed about them. Although the DAMA collaboration has tried to address every concern that people have raised, the truth is that there are many things that can vary annually. We’re trying to look for a very, very small signal, and there are many possibilities that could mimic the signature that DAMA sees.
Q: A deployment at the South Pole is never a simple task. What’s the story behind DM-Ice?
R: The idea came when IceCube was being deployed, back in 2009. Having a detector in the Southern Hemisphere is a great choice, since it allows a cross check of systematics. Francis Halzen (the IceCube PI) and I talked a lot about it, and finally I agreed to at least take a look and assess if it was feasible. I first thought it was the craziest idea, but then I went to the Pole for work related to IceCube and saw what it’s like to work there. And I saw how fantastic this team was, and I came back thinking that this was actually possible. And that’s what we did; we put together a prototype for DM-Ice, starting from scratch.
M: Quickly obtaining NaI crystals for the detector seemed challenging, but there was an elegant solution. The NAIAD experiment was an old dark matter experiment from the early 2000’s in the Boulby mine in the U.K. The experiment had been decommissioned but the crystals were still in storage at the mine, so we talked with them, and they shipped us two of their crystals for DM-Ice. Those are the crystals now taking data at the South Pole.
Q: And all this happened very fast, didn’t it?
R: Yes. DM-Ice was designed and built in nine months and we deployed the prototype during the next polar season at the end of 2010, the final IceCube construction season. The result of this intense year of work is what’s operating at the South Pole now.
M: It’s pretty amazing. The teams at PSL (Physical Sciences Laboratory), WIPAC, and in general the IceCube community, made this possible. They supported us with the design and manufacturing of the material components and electronics. The logistics of getting an experimental apparatus to the South Pole requires a lot of coordination. We work with IceCube and WIPAC to maintain the data acquisition electronics at the South Pole.
Q: When we read about DM-Ice we learn that it’s a sodium iodide detector. How exactly does a sodium iodide detector work?
R: Sodium iodide detectors have existed for the last 50 or more years. This crystal is transparent, dense and has low backgrounds, all of which are important properties when you are trying to look for interactions of yet-to-be-observed particles that very rarely interact. And when they do interact, they could look like interactions induced by well-known particles.
The detection principle is quite simple. We measure dark matter interactions by recording the recoil of target nuclei scattered by a WIMP. When the sodium iodide nuclei get a kick from scattered WIMPs, they would essentially excite electrons in the detector. As the electrons decay back down to their ground state they emit light. Then we collect that light using photomultiplier tubes (PMTs), just like in IceCube, and depending on how many photons come out, it could tell us the energy of that interaction.
Q: What is the difference between a dark matter reaction and just another particle reaction?
M: The amplitude and shape of the interaction are the relevant parameters. With a typical dark matter interaction in DM-Ice, we expect on the order of 100 to 200 photons to be emitted during the nuclear relaxation. This translates to a very small energy range we’re interested in. The shape of the signal, or the time-scale over which the photons are emitted, also provides information about the type of particle interaction being observed.
R: Detecting this collision with a very distinct energy signature would be an indication of dark matter, but on top of that, if we can observe the annual modulation we have mentioned, with the correct phase and correct rate, then we have an additional signature for dark matter.
Q: So, what is the detector’s current status as of 2014?
M: DM-Ice 17 is taking data right now in the ice at the South Pole, mainly as a proof of concept for a full-scale deployment in the ice. We now have 17.5 kilograms of target material from the crystals we inherited from NAIAD, but these crystals are a little too high in internal backgrounds for a competitive analysis. We are currently collaborating with vendors to develop much cleaner crystals for use in the full-scale detector.
R: As Matt says, our prototype is too small and too high in background to really be able to test DAMA, but we have proved that we can deploy and operate a dark matter detector at the Pole. The challenge is now to build the full-scale detector, which would be sensitive enough to see what DAMA sees. We have good teams at Yale, WIPAC, and other places in the U.S., Canada, and the U.K. contributing to our efforts.
M: Here at WIPAC, the DM-Ice team consists of six people, contributing through different analysis and R&D projects geared toward the full-scale 250kg detector. We are, for example, working with two prototype crystals that we have underground at FNAL in Chicago and measuring the potassium backgrounds in those crystals.
Q: What is the near future for DM-Ice?
R: Our job is to be ready when IceCube is ready to drill again at the Pole, hopefully deploying the planned detector extensions. When IceCube drills again, we will have improved DM-Ice detectors that can go in the ice as well. In the meantime, we will run a similar sort of DM-Ice detector in the Northern Hemisphere. It would be a test to reproduce what DAMA found with an independent detector. However, we might just find the same result that DAMA did, without really learning much more about its origin.
The original idea was to put this detector at the South Pole because it is really the ultimate test. If we see the same signature as DAMA, it would be very difficult to attribute it to the seasons. If we don’t see the same annual modulation that DAMA sees, then the scenario of it being dark matter can be ruled out, even if we don’t know the origin of that signal. Basically, we would be able to confirm or rule out DAMA’s claims of a dark matter observation.
Q: Can you tell us more about what we can learn from a northern detector?
R: There are different scenarios that could come from a northern deployment. You see no annual modulation, or you see the same signature as DAMA. If the signature is there, we might be able to test some background hypotheses to figure out what is there aside from dark matter. But we might also end up with a dark matter-only possible scenario, as DAMA did. I think we still have to bring this detector to some other location to verify that the dark matter signature phase stays the same everywhere on Earth to confirm that it’s due to dark matter. In summary, we might learn a few things from a northern run or we might not, but if we go straight to the Southern Hemisphere, then it’s one shot and we would have a definitive answer.
Q: Will WIPAC be an important partner for a northern detector as well?
M: Oh yes! WIPAC and our collaborators at the PSL, also at UW–Madison, contribute far beyond the South Pole expertise and logistics.
R: I would say that’s what is unique about WIPAC and the University of Wisconsin—the existence of a scientific institution coupled with a very good technical and university-oriented engineering center. Being able to build big things at a university is rare, and I think that’s why IceCube was successful and why the DM-Ice demonstrator was possible. Yale also has similar capabilities, and together there is great intellectual and technical support behind DM-Ice.
The team of collaborators working alongside Maruyama and Kauer include distinguished WIPAC physicists Francis Halzen and Albrecht Karle, and engineers Perry Sandstrom and Jeff Cherwinka, as well as students Antonia Hubbard, Walter Pettus, Bethany Reilly, and Zack Pierpoint from the UW–Madison and Yale communities.