Dark Matter Day 2019: A conversation with DM-Ice scientist Matt Kauer
DM-Ice, or Dark Matter-Ice, is searching for dark matter in the Southern Hemisphere. WIPAC led the design and deployment of the 17 kg crystal prototype detector and first analysis of background data with DM-Ice. Former WIPAC professor Reina Maruyama, now at Yale University, leads this project. Currently, team members contribute to detector operations and data analysis, and WIPAC houses the data center for the DM-Ice collaboration.
In celebration of Dark Matter Day, celebrated internationally every year on October 31, we spoke to Matt Kauer, an assistant researcher on DM-Ice who is stationed at WIPAC.
How can DM-Ice detect dark matter?
Matt: DM-Ice consists of a large optically transparent crystal of sodium iodide (NaI) with two photomultiplier tubes mounted to both ends of the cylindrical crystal. If dark matter were to collide with the NaI, a flash of light would be created. This is known as a “direct detection” method, where we would directly observe dark matter interacting with the NaI crystal. The photomultiplier tubes can convert the flash of light into an electrical signal that in essence tells us about the mass and energy of the dark matter particles.
Why the South Pole?
Dark matter could be detected anywhere, but there are many advantages to placing your detector in the South Pole ice. The extremely deep (almost two miles) glacial ice shields DM-Ice from cosmic rays such as atmospheric muons. The ice is also a great neutron moderator and helps absorb free neutrons before they reach our detector. Another key advantage is the ice is very pure with very low concentrations of radioactive elements and radon. Muons, neutrons, and radioactive elements can mimic dark matter interactions and create a false signal. One major design goal of a great detector is to minimize these “backgrounds,” and the South Pole ice provides the most ideal location for a dark matter experiment.
What makes DM-Ice unique or different from other dark matter experiments?
The main feature of DM-Ice that sets it apart from other dark matter experiments is that it's the first Southern Hemisphere direct-detection experiment. Dark matter interactions are predicted to be most frequent in June and least frequent in December. In the Northern Hemisphere, many environmental backgrounds are also most frequent in June. For example, muon rates, thermal neutrons, and radon all have a temperature dependence that is highest in summer. This makes it very difficult to know whether your detector is seeing dark matter or seeing backgrounds. In the Southern Hemisphere, summer is December through January so the environmental backgrounds are most frequent in those months and least frequent from June to July. This makes it easier to for a detector in the Southern Hemisphere to distinguish between dark matter and backgrounds.
How would finding dark matter impact physics? Or science more broadly?
One of the big open questions in science right now is “What is dark matter?” We observe gravitational effects in our own galaxy, other galaxies, and clusters of galaxies that require much more matter than we observe from our Standard Model particles, atoms, and molecules. So the term “dark matter” was coined to suggest some form of matter that we have not yet seen or discovered but that must exist to explain what we observe. Finding dark matter and then learning more about this exotic substance would greatly enhance our understanding of how the universe grew and evolved and how galaxies formed and evolve.
When did DM-Ice start? How has the search for dark matter changed since then?
DM-Ice was deployed and started taking data in January 2010. The detector is now frozen into the ice so the fundamental search for dark matter using DM-Ice has not changed much in the past nine years.
Below: Members of the DM-Ice experiment—Reina Maruyama, Walter Pettus, Zack Pierpoint, and Matt Kauer—at the Boulby Underground Lab in the UK in 2014. This time-lapse shows the mounting the photomultiplier tubes to the NaI crystal and getting it ready to be lowered into the lead “castle” where lead and copper are used to shield the NaI from external radiation. Photos by Matt Kauer.