Eric Dahl receives DOE award to develop hybrid dark matter detector

Eric Dahl is one of this year’s DOE Early Career Research Award recipients.The award supports his search for dark matter using a new style of particle detector. Photo: Reidar Hahn

Eric Dahl, a joint Fermilab-Northwestern University physicist, recently received an award from the Department of Energy to develop a new style of particle detector for finding dark matter particles, the nearly invisible matter that makes up more than 80 percent of our universe.

Dahl is a recipient of the DOE Early Career Research Award, a prestigious honor given annually to outstanding scientists early in their careers. University researchers receive $150,000 a year for five years to cover summer salary and research expenses.

Dahl has worked extensively on two styles of dark matter detector experiments. Graduating from Princeton University in 2009, he worked on the development of xenon time projection chambers, and he currently works on the PICO bubble chamber experiment, a ton-scale successor to COUPP, which is scheduled to begin taking data in 2017. His new prototype will use elements from both detector types.

“By combining the two technologies, you get the best aspects of both without really losing anything,” Dahl said.

Bubble chambers, such as the SNOLAB-based PICO, contain a superheated fluid that on its own will not boil. When a particle interaction deposits a critical amount of energy in the fluid, however, a vapor bubble is created.

This technology excels at separating the dark matter signal from the most common backgrounds, including gamma and beta decays, but researchers using this type of detector face challenges weeding out alpha decays. They also can’t tell how much energy was deposited in the interaction responsible for a given bubble.

In a xenon time projection chamber, such as the South Dakota-based LUX, particle interactions within the liquid xenon produce two signals: a flash of scintillation light and a delayed ionization signal. The background discrimination is weaker than in the bubble chamber, but the scintillation signal gives the energy of the interaction, which also eliminates alpha decays as a background.

Dahl’s prototype would use liquid xenon as the target of a standard PICO bubble chamber, with additional instrumentation to detect xenon scintillation light. The chamber should have the gamma and beta discrimination of a bubble chamber plus the energy information and alpha discrimination of a xenon TPC. Lower backgrounds mean greater sensitivity to dark matter.

“The more that I thought about it, the more that I thought it would work really well,” Dahl said. “I found myself getting excited about doing the work as I was writing the proposal.”

The award provides Dahl the resources to try to prove that the technology works. If it does, he can then work on acquiring funding for a large-scale detector.

“It really is a nice idea. Personally, I think that Eric is one of the most talented people I know,” said Hugh Lippincott, a Wilson fellow at Fermilab.

Lippincott has worked with Dahl for four years on PICO and feels that Dahl is well suited to the challenge of building the prototype because of his broad skill set.

“He was once a string theorist, but he saw the light and saw that dark matter particle physics is where it is at,” Lippincott said. “But he can still do the calculations, which is extremely useful.”

Dahl’s award comes at a critical time. The Particle Physics Project Prioritization Panel’s (P5) report, which was released on May 22, details a path forward for U.S. particle physics. Dark matter experiments were given priority in the plan.

“We know there is something we don’t understand. Maybe it will be a new particle, maybe it will be something radical,” Dahl said. “We go after these questions because we don’t know what the answer is going to be. Whatever it is, it will teach us something.”