From WIMPs to Ice Cubes: Inside the Maruyama Lab at Yale

Sitting behind the desk in her unassuming office at the newly renovated Wright Lab, Professor Reina Maruyama attempts to sum up the magnitude of what dark matter researchers like herself are doing. “We’re trying to figure out what we’re made of, what the universe looks like, how it evolved, how we got here. It’s exciting to get new data and add to that knowledge,” she says. Maruyama is a co-spokesperson for the COSINE-100, an international collaboration conducting a dark matter experiment. She heads one of the dark matter research groups in Yale’s Wright Laboratory, where she and her team work to detect and characterize the mysterious matter that scientists now believe makes up a majority of our universe.

The term “dark matter” comes from the fact that this as-yet-unidentified type of matter neither emits nor interacts with any type of electromagnetic radiation, including visible light. Since it is completely invisible to the electromagnetic spectrum, scientists haven’t observed it directly. However, Maruyama clarifies that existing detectors attempt to turn the interactions of dark matter with other matter into measurable signals.

When asked for her “elevator spiel” that she uses to describe her work, she explains that her group is designing and operating detectors that will record interactions between matter in the detectors and Weakly Interacting Massive Particles (WIMPs). WIMPs, which could potentially make up dark matter, are proposed elementary particles that may interact with other matter such as atomic nuclei via the weak interaction – one of the four known fundamental forces along with gravitational, electromagnetic, and strong interaction. If they exist, scientists believe WIMPs were produced shortly after the Big Bang. Several other candidates for the identity of dark matter have been proposed of which WIMPs are only one possibility. “Invoking a new particle can be uncomfortable, but may be necessary,” says Maruyama.

If dark matter consists of WIMPs, Maruyama says the particles’ interactions with atomic nuclei can be calculated to produce a specific expected time and energy transfer signature. At the Cryogenic Underground Observatory for Rare Events (CUORE), located in Italy, the Maruyama group and their collaborators use highly sensitive instruments to measure changes in the atomic nuclei of detector crystals. The main goal of the experiment is to determine whether neutrinos are also their own antiparticle, Majorana particles. The same detector can also look for dark matter. In order to detect the minute temperature changes that result from weak interactions with dark matter, detectors must have an incredibly low baseline temperature. CUORE, which obtained its first pulse data earlier this year, is the largest detector of its kind ever built and is maintained at only a few thousandths of a degree above absolute zero – so cold that it is actually the coldest cubic meter in the universe.

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The Maruyama group’s other projects are both focused on searching for an annual modulation signature from dark matter. To do so, the researchers examine the motion of a star – our sun – around the galaxy. Various scientists have hypothesized that a halo of dark matter surrounds the entire galaxy. Thus, as the sun goes around the galaxy and the Earth goes around the sun, both end up passing through the halo. Because of the patterns of motion, Maruyama explains, physicists would expect to observe more WIMPs in June than in December. Like CUORE, the COSINE-100 experiment (located in South Korea) uses a highly sensitive detector to measure the energy transferred to nuclei from interactions with WIMPs. A similar detector called DM-Ice is located in Antarctica at the geological South Pole. Replicating the experiment in the southern hemisphere, where seasons are reversed, allows researchers to decouple seasons from experimental results and separate signal from possible sources of background such as temperature and humidity. Both DM-Ice and COSINE-100 experiments operate deep underground in order to minimize background from cosmic radiation.

The detectors at both COSINE-100 and DM-Ice are thallium-doped sodium iodide, a very specific type of crystal. Maruyama and her team chose this crystal for their detectors because they wanted to replicate the methods of an existing Italian experiment called DAMA/LIBRA (DArk MAtter). DAMA first hinted at direct dark matter detection in 1998 and has continued releasing positive results since then. Significant tension has arisen since then in the field of dark matter because no other research group has obtained similar results. However, none of the other research groups used the same type of crystals in their detectors. Maruyama and her team recognize the importance of rigorous replication of technique. The goal of their experiments is two-fold: to look for the same modulation observed by DAMA, and, if that modulation fails to appear, to determine what DAMA might be detecting.

After talking to Maruyama about the large-scale research projects with which she is involved, an obvious question arises: if all major projects are in other locations, why is Maruyama based at Yale, and what do she and her team do while they’re here? Laughing good-naturedly, she explains that her work at Yale mostly revolves around R&D and data analysis. In the modern age of technological interconnectivity, detectors as far away as Antarctica, Italy, and Korea can easily transmit data to servers at Yale, where Maruyama and her team of postdoctoral, graduate, and undergraduate students work to analyze and understand it. They also travel frequently to install/modify/tweak the detectors and to meet with their collaborators – regular face to face interaction is still needed for effective collaboration.

If conversations with the researchers are anything to go by, the Wright Lab at Yale seems to be a uniquely supportive place to work. Teams are close-knit, the work is always interesting, and atmospheres are friendly. Maruyama can’t keep a smile off her face whenever she talks about her team members. “I love the work; I love my group,” she says, describing how she has crafted both a team environment and a physical work space that is open, productive, and friendly.

Reflecting on her work and what the public needs to know, Maruyama notes, “There’s a tendency nowadays toward alternative facts in news, but I want to convey to the world that science is based on facts. All we can say is that we are forming our view of the world based on what we observe, and if evidence and facts point toward something else and stand up to rigorous testing, we accept it.”

Image courtesy of the Maruyama Lab

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