A joint Fermilab/SLAC publication

Fermilab physicist transforms dark energy lens into dark matter detector

11/09/10
Juan Estrada

Juan Estrada

A Fermilab physicist recently transformed technology used to search for signs of dark energy into the best detector in the world for spotting low-mass dark matter particles.

The White House honored that physicist, Juan Estrada, on Friday with the Presidential Early Career Award for Scientists and Engineers.

Estrada spends most of his time building and testing imaging sensors for the Dark Energy Camera. But he has set himself apart by using the same technology in a side project to search for dark matter.

“Juan is relatively new to the dark matter field,” said Dan Bauer, deputy head of Fermilab’s Center for Particle Astrophysics. “You’d think it’d be enough of a challenge to work on DECam. But he somehow had spare time to think of new ideas.”

Brenna Flaugher, Estrada’s supervisor, joked, “I don’t think he sleeps.”

Estrada credited the technicians and engineers from the dark energy search who gave their spare time and expertise to his dark matter search project, called DAMIC for Dark Matter in CCDs.

“Fermilab was in a good position to take advantage of this opportunity because of all the work we had already done for DECam,” he said.

The president gives the Early Career Award each year to 100 young researchers “whose early accomplishments show the greatest promise for strengthening America’s leadership in science and technology and contributing to the awarding agencies' missions.” With the grant money from the award, Estrada will be able to build a larger, more advanced prototype of his dark matter detector.

Estrada started at Fermilab as a student from the University of Rochester working on the DZero experiment. As a 32-year-old postdoc, he earned a Wilson Fellowship, which gave him up to five years to study whatever he wanted at the laboratory.

Left to right: Technician Kevin Kuk, physicist Juan Estrada, engineer Herman Cease and physicist Ben Kilminster stand in the NuMI hall next to the DAMIC detector.

Left to right: Technician Kevin Kuk, physicist Juan Estrada, engineer Herman Cease and physicist Ben Kilminster stand in the NuMI hall next to the DAMIC detector.

He had worked on cosmology as an undergraduate in Argentina, so he chose to work on the Dark Energy Survey, much to the surprise of his colleagues.

“Nobody on DZero knew that Juan was a closet astrophysicist,” said Tom Diehl, Estrada’s former DZero colleague who now builds CCDs for the Dark Energy Survey as well.

Estrada builds charge-coupled devices, the same type of imaging sensors found in digital cameras, for the 570-megapixel Dark Energy Camera. These CCDs are specifically designed to capture the light that reaches Earth from extremely distant galaxies and supernovae. The Dark Energy Survey will use data from the camera to search of signs of dark energy, which scientists theorize affects the evolving shape of the universe.

The CCDs need to produce as little background noise as possible so that researchers can distinguish the distant light. Estrada realized that, for this reason, he could use the same CCDs to directly detect elusive dark matter particles. All he had to do was take them somewhere he could shield them from other, distracting particles that constantly rain on the Earth from space.

Lucky for Estrada, Fermilab has just the right place to test his idea: the underground NuMI hall, home to many of the laboratory's neutrino experiments. Estrada built a prototype detector on a shoestring budget, using borrowed parts from other experiments and the help of a small group including local and international high school students.

Snapshots: One-hour exposures of a CCD at sea level (top), at 350 feet underground (middle), and underground with lead shielding (bottom). The straight line signature of cosmic ray muons disappears underground. With shielding, the scattering from background radiation decreases, limiting the wiggly tracks and dots. A dark matter interaction would resemble a white dot on these images.

Snapshots: One-hour exposures of a CCD at sea level (top), at 350 feet underground (middle), and underground with lead shielding (bottom). The straight line signature of cosmic ray muons disappears underground. With shielding, the scattering from background radiation decreases, limiting the wiggly tracks and dots. A dark matter interaction would resemble a white dot on these images.

Estrada plans to collect data from particles that hit the CCDs over the course of a year and compare the data from different seasons. He should see different results for different times of the year because the Earth moves around the Sun while the Sun also moves around the galaxy.

The effect is a bit like playing catch on a train. When you throw a ball toward the front of the train, its overall velocity is equal to the velocity of the train plus the velocity of the throw. But when you throw a ball toward the back, its velocity is equal to the velocity of the train minus the velocity of the throw.

The faster the Earth moves through the galaxy, the more chances dark matter particles have to hit the particle detector. So the detector should encounter more dark matter particles in summer than in winter. In June, the Earth is moving in the same direction as the Sun, which means it is moving a bit faster than it does in the winter, when it swings around its orbit and moves in the opposite direction.

When Estrada began his search for low-mass dark matter, most dark matter experiments were searching for particles with a higher mass.

“The experiment isn’t aimed at a place theory suggests,” Bauer said.

However, results from experiments such as DAMA in Italy and COGENT in Chicago have been consistent with expected measurements from low-mass dark matter particles, which has brought more interest to the low-mass search.

Estrada is working on his next prototype, which he hopes to install in a deeper underground laboratory. In his spare time, of course.

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