One of the greatest mysteries in modern astrophysics is dark matter. While it makes up about 80 percent of the matter in the entire Universe, its detection has eluded physicists and scientists since it was first hypothesized. Dark matter has never been directly detected, but the Large Underground Xenon (LUX) dark matter experiment is determined to change that.
Postdoctoral candidate and UCSB researcher Carmen Carmona, who has been a part of the research team since 2009, explained how the experiment is set up.
In a time-projection chamber one mile underground “we have liquid xenon, and on the top there is some xenon gas. So when a particle interacts the xenon, it produces some light and electrons and we can detect this light with photomultipliers, a device that detects light. We have that on the bottom and top of the chamber. By detecting the light and the electrons, we can differentiate between different kinds of particles,” Carmona said.
Carmona and her colleagues are looking for dark matter in the form of W.I.M.P.s, weakly interacting massive particles.
“It’s going to act like a neutron, so neutrons act as a background for us. We’ll study and reduce the neutron interactions rate, but [the neutrons] help us understand the signal we’re expecting,” Carmona said.
These particles rarely interact with ordinary matter except through gravity; dark matter has only been observed through its gravitational effects on galaxies and clusters of galaxies, leading researchers to believe that W.I.M.P.s are the leading candidate for dark matter. However, this is considered indirect evidence since they have not been seen as they do not emit light.
Since it has first started taking data in April 2013, researchers from 18 institutions across the U.S. and Europe, including physics professor Harry Nelson and Carmona, have been working on the LUX detector to make it even more sensitive to the light given off by the xenon atoms.
LUX scientists calibrated their detector using neutrons as a replacement for dark matter particles. The interactions between the neutrons and the xenon atoms allowed scientists to quantify how the LUX detector responds to the recoiling process. Researchers see this process as a game of pool.
With the neutron as the cue ball and the xenon atoms as the solids and stripes, they are able to track the neutron in order to get details of the xenon recoil. This information and technique calibrates the detector to the dark matter signal they expect to see, as the interactions between the two atoms is thought to be similar to the interaction of dark matter and xenon.
LUX scientists have also been able to calibrate the detector’s response to the deposition of small amounts of energy by struck atomic electrons by temporarily injecting the chamber with some radioactive gases.
These improvements, along with advanced computer simulations at Lawrence Berkeley National Laboratory and Brown University, allows scientists to test additional particle models for dark matter that can be excluded from the search.
The LUX experiment is currently on its long run of 300 days and is expected to end by this summer, after which it will be decommissioned to make way for the much larger xenon detector, LUX-ZEPLIN, or LZ.
“The people from LUX joined the collaboration from ZEPLIN III. ZEPLIN is mainly groups from the UK. We joined together to make a bigger detector that is called LZ,” Carmona said.
LZ will contain about seven tons of xenon, about 40 times the initial volume of LUX.
“We’re working on designs and buying the parts,” Carmona said. “We have to do some remodeling of the lab and we’re thinking the LZ detector will be underground in 2018 and then in 2019, we should be able to commission the detector.”
Professor Nelson, who is a spokesperson for LZ, has commented that the LUX experiment has set the foundations for the LZ experiment. LZ should be able to achieve over 100 times the sensitivity of LUX.
Along with Nelson and Carmona, other LUX scientists and researchers here at UCSB include physics professor Michael Witherell, Senior Development Engineers Dean White and Susanne Kyre and graduate students Scott Haselschwardt and Curt Nehrkorn.
A version of this story appeared on p. 18 of the Thursday, Feb. 4 print edition of the Daily Nexus.