Buried deep underground in Black Hills, S.D. lies a cavern with a titanium tank the size of a phonebooth. Over the next year, researchers hope this vessel will detect dark matter, the presence of which physicists have long been aware, but which has never been directly observed.
The tank was filled with xenon gas Monday, one of the final steps of preparing to turn the detector on. The detector is being run through the Sanford Lab as part of the Large Underground Xenon experiment, a massive multi-institution effort to detect dark matter. Data collection is scheduled to begin in January.
Modern physics only explains about 5 percent of the universe, the part that is made up of normal matter, or the things that we can see. The remaining part is made up of dark matter and dark energy, said Richard Gaitskell, professor of physics, who is one of many researchers working on the LUX experiment.
"It is extremely embarrassing to admit that we don't know what 95 percent of (the) universe is made of," Gaitskell said.
The ideas of dark energy and dark matter were born of astrophysical observations. In the 1990s, images of distant supernovae from the Hubble Space Telescope showed the expansion of the universe was accelerating, not slowing as was expected due to the effects of gravity. Scientists conceived the idea of dark energy as the underlying cause of this accelerated expansion and determined that it may make up 72 percent of the universe based on its effects, according to an article on NASA's website.
The other 23 percent is made up of dark matter, so called because it does not take on the form of things we see. Scientists have determined that dark matter is not like the dark clouds of normal matter because those clouds contain baryon particles, and no baryonic radiation has been detected with dark matter. Instead, dark matter is believed to consist of exotic particles like Weakly Interacting Massive Particles.
The LUX experiment is looking for interactions of these WIMPs to find dark matter. The search is driven by the knowledge that this matter exists without an understanding of what it is.
"We're looking for something nobody has ever seen that we strongly suspect is there because of indirect observations," said Simon Fiorucci, senior associate of physics at Brown and commissioning director at the LUX experiment.
Detecting dark matter would be an "enormous step" in better understanding both particle physics and astrophysics, said James Verbus GS, a member of the Brown Particle Astrophysics Group working with the LUX experiment.
"On the particle level, we'd be detecting a particle outside the standard model," Verbus said, "and on the astrophysics level, dark matter has huge cosmological effects, being critically involved in the formation of galaxies and galaxy clusters."
The xenon-filled tank is encased in over 70,000 gallons of water, which provides shielding from the radiation of the surrounding rocks, and the detector also consists of an array of photomultiplier tubes, Verbus said. If the LUX experiment successfully detects dark matter, the physicists will see two flashes of light.
"When dark matter interacts with xenon, it is like billiard balls scattering," said Kevin Lesko, head of operations at the Sanford Underground Research Facility and senior physicist at the University of California at Berkeley.
"Electrons are shaken off the nucleus," he said. "The recoiling nucleus produces light and the electrons that are emitted drift upward to produce a second flash of light."
The photomultiplier tubes are able to detect these flashes of light, he added.
Brown graduate students work with these photomultiplier tubes and handle the electronics and analysis of the experiment, said David Malling GS, another member of the Brown Particle Astrophysics Group.
The laboratory is placed underground to shield it from cosmic rays from outer space, which would produce unwanted radiation, Verbus said. At the surface, about one cosmic ray per second passes through matter. At that depth underground, about one cosmic ray passes through matter every three or four months, he said. .
"We can make a detector very 'clean' so that the intrinsic radioactivity is very low, but we can't do anything except run away from cosmic rays by going deep underground," Lesko said.
In July, the LUX detector was lowered into the underground cavern, and the team filled the encasement with water last month. Now that the tank is filled with xenon, the team will run final checks before liquefying the gas in January.
At that point, the team will begin data collection, Fiorucci said. When the detector starts running, it will be far more sensitive than previous experiments.
"Within the first 20 hours, it will surpass the sensitivity of all previous dark matter experiments except one," Gaitskell said. "Within two weeks, it will become the most sensitive dark matter experiment of all time."
The longer the experiment runs, the more sensitive it becomes. "Even though the flux of dark matter particles through matter is in the millions per second, the probability of an interaction - even in a detector the size of LUX - is very small," Gaitskell said, adding that interactions could happen as infrequently as once per week or once per year.
The challenges for the team are the uncertainties in running the detector underground. The first time running an experiment can lead to unexpected consequences, Malling said. "We have the advantage of learning from previous experiments."
Observing dark matter in the lab would only be the f
irst step to understanding this exotic form of matter, Lesko said.
"Even if we find it, it is not a definite answer to the question," Fiorucci said. "There is a lot of work still to be done, and it is an exciting time to be in physics."
