Science & Tech

Searching for Dark Matter a Mile Underground

To cap off the year in science, the European journal Nature named the Large Underground Xenon Dark Matter (LUX) experiment­­­ — a search for dark matter in which faculty of the physics department at UCSB contributed instrumentally to the design — as a major breakthrough in their “365 Days: 2013 in review.”

The experiment is situated 4,850 feet underground at the Sanford Underground Research Facility in Lead, South Dakota, the same location where Raymond Davis Jr. attempted to catch neutrinos — subatomic particles emitted from the sun — in the early 1970s.

The sensitivity required by the apparatus to detect dark matter make any trifling disturbance by cosmic rays, or even traces of radiation emanating from the surrounding rock, a hazard for generating false positives.

The experiment features 368 kilograms liquid xenon, a halogen similar to the element helium. UCSB physicists constructed and filled the water tank which holds the LUX experiment. It consists of an array of buckets submerged in ultrapure water.

Vice-chair of the UCSB physics department Harry Nelson worked as a principal investigator of the UCSB LUX team and said that the researchers compared light and ionization energies to determine whether dark matter was present.

“When dark matter hits a xenon atom, it shakes off a few electrons in addition to emitting light,” Nelson said. “We want to compare the amount of light that is emitted to the amount of ionization that is liberated and by doing that comparison we can sort out if it was dark matter or just a trace of potassium for example.”

These electromagnetic signatures can then be picked up by the experiment’s 122 photomultiplier tubes. The presence of dark matter thus far has only been inferred by the gravitational pull it has on conventional matter, contributing nearly a quarter to the “stuff” which constitutes our universe.

According to an article in Nature, Eugenie Samuel Reich said that although the 166 events detected by the experiment are consistent with background levels of radioactivity, “LUX has now set a limit on dark matter particle interactions that is two to five times more stringent than its closest rival, an experiment called XENON-100 in Gran Sasso National Laboratory near L’Aquila, Italy,” Reich said.

Despite the no-show of the elusive dark particle in the upshot of the LUX experiment, scientists are confident in the clear result and the high precision of the apparatus. More importantly, it tightens the constraints for the remaining candidates of dark matter. This included a controversial rule-out of a potential class of subatomic particles called Weakly Interacting Massive Particles (WIMPs) which previously garnered support after another experiment, the Cryogenic Dark Matter Search, claimed to have recorded three events suggesting their presence.

Professor Nelson is now stepping down from his current role as chair of the LUX executive board to lead the construction of a new experiment called the Lux-Zeplin (LZ), a proposed 30 million dollar extension of the LUX design which is already designed to boast one thousand times more sensitivity.

“Now we have a lot of experience and we think we know how to be a factor of one hundred less radioactive than we currently are,” Nelson said.

 

A version of this story appeared on page 8 of Wednesday, January 15, 2014′s print edition of the Daily Nexus.

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