Scientists hunt for dark matter

A group of Yale faculty and students is currently racing to be the first to detect dark matter.

Scientists began the cooling process of the Large Underground Xenon experiment’s ultrasensitive dark matter detector on Jan. 24. Run in a facility nearly a mile below the Black Hills of South Dakota, the LUX experiment is a collaboration among 16 institutions, including Yale. The cooling of the LUX’s particle detector — the largest to date — is a major step in the experiment’s goal of detecting dark matter. The dark matter detector, a titanium cylinder approximately the size of a phone booth, is housed in a stainless steel water tank that holds 70,000 gallons of deionized water.

“It’s a huge scientific goal to be able to detect dark matter,” said Yale professor and LUX co-spokesman Dan McKinsey. “If we succeed, it would be revolutionary.”

Although dark matter particles have never been directly detected, dark matter is commonly accepted to comprise most of the universe’s mass. Theories suggest galaxies rotate much slower than expected for their given mass, suggesting the presence of undetected matter that slows the speed of rotation, said Masahiro Morii, a physics professor at Harvard and former collaborator on the LUX experiment. McKinsey said the LUX experiment aims to test these theories by looking for evidence of existence of dark matter particles, also known as WIMPs. These WIMPs, or weakly interacting massive particles, are detected through their collisions with nuclei of atoms — in this case, xenon atoms.

The LUX detector contains xenon and photomultiplier tubes that alert researchers to the presence of light. If a WIMP bumps into a xenon atom, the collision will produce two flashes of light. The first is at the point of impact, and the second is caused by electrons released during the collision. A comparison of the two flashes will determine whether or not the particle is dark matter.

Accurate identification of dark matter requires that the detector be shielded from radiation from rocks, the sun and the earth. This radiation releases neutrons that would interfere with the experiment, resulting in false positive identification of a moving particle as dark matter.

“At worst case, a cosmic ray can split a nucleus and release a neutron that would behave like dark matter,” McKinsey said. Running the experiment in an underground water tank shields the detector from the bulk of radiation and interference.

The experiment is currently in its calibration stage to ensure both that the detector is highly sensitive and that the electronics and operating systems function with minimal interference. Ethan Bernard, a physics research assistant who joined the experiment in 2010, said that this calibration will ensure that the system “picks up signals we want it to pick up and ignores signals we want it to ignore.” The team began collecting data earlier this month and expected to run the experiment for at least a year.

Although the LUX experiment is among the most technologically sensitive of those currently under way, no one can say if the actual detection of dark matter will occur. McKinsey said these collisions are very rare, occurring perhaps once per month or even year.

“It’s an experiment,” Morii said. “You can get lucky, or you can go home empty-handed. You just have to keep experimenting.”

The LUX experiment is currently in competition with other efforts under way to detect dark matter in Canada, Italy and China.

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