After its first run of more than three months, operating a mile underground in the Black Hills of South Dakota, a new experiment named LUX has proven itself the most sensitive dark matter detector in the world. Among the dozens of scientists involved in the research collaboration is University of Rochester's Professor Frank Wolfs and his colleagues.
The results from the first-run were announced today at a seminar at the Sanford Underground Research Facility (Sanford Lab) in Lead, S.D.
"After seven years of designing, building, testing, and commissioning it is exciting to see that we have been able to create the world's most sensitive dark matter detector," said Wolfs. "This was our goal."
Wolfs explained that the Large Underground Xenon experiment, LUX, will be searching for elusive and mysterious subatomic particles that are believed to comprise more than 80 percent of the mass of the universe, But so far, dark matter particles, which are neutral and don't emit light, have eluded direct detection. Wolfs points out that the difficulty in detecting dark matter, even though it is such a large part of the universe is what makes this "an exciting and challenging area of research."
LUX scientists are looking for evidence of collisions between dark matter particles—called weakly interacting massive particles, or WIMPs—and atoms of xenon inside the LUX detector. The detector will now be run for 300 days during which the researchers will be looking for evidence of these collisions. Wolfs explains that "even if our next run fails to detect dark matter, our results will start to rule out various theoretical WIMP models." Theories and results from other experiments suggest that WIMPs could be either "high mass" or "low mass." The test results announced today show that LUX's sensitivity to either of these cases is a significant improvement on existing detectors, but the LUX results are inconsistent with low-mass WIMP hints seen by other experiments.
"The universe's mysterious dark sector presents us with two of the most thrilling challenges in all of physics," says Saul Perlmutter of DOE's Lawrence Berkeley National Laboratory (Berkeley Lab), a winner of the 2011 Nobel Prize in Physics for discovering the accelerating expansion of the universe. "We call it the dark sector precisely because we don't know what accounts for most of the energy and mass in the universe. Dark energy is one challenge, and as for the other, the LUX experiment's first data now take the lead in the hunt for the dark matter component of the dark sector."
One of the components of the detector is the electronic trigger key system, which Wolfs and two graduate students have been focusing on. The firmware for the electronics was developed by Eryk Druszkiewicz, a Ph.D. student in electrical and computer engineering. Mongkol Moongweluwan, a Ph.D. student in physics, carried out extensive testing of this firmware to ensure it would work properly for LUX.
"The trigger decides whether any signal seen in the detector is something worth analyzing," Wolfs explained. "It selects only those event that fit certain criteria so we don't have to use resources recording and analyzing too much unnecessary information, which becomes a problem if you're trying to run these experiments for a very long time."
The digital signal processing electronics that the detector uses were first developed by Dr. Wojtek Skulski, senior engineer at SkuTek Instrumentation and a visiting scientist at the University, and who has collaborated with Wolfs for over 15 years. Skulski started his own company, SkuTek Instrumentation, specializing in the sort of electronics needed for experiments like this.
"This is only the beginning for LUX," said Dan McKinsey of Yale University, co-spokesperson for LUX with Brown University physicist Rick Gaitskell. "Now that we understand the instrument and its backgrounds, we will continue to take data, testing for more and more elusive candidates for dark matter."
"LUX is blazing the path to illuminate the nature of dark matter," said Gaitskell.
The constant rain of cosmic radiation from space can drown out the faint signals of WIMPs interacting with matter, which is why LUX is located 4,850 feet underground in the Sanford Lab, where few cosmic ray particles can penetrate. The detector is further protected from background radiation emitted by the surrounding rock by immersion in a tank of ultra-pure water.
At the heart of the experiment is a 6-foot-tall titanium tank filled with almost a third of a ton of liquid xenon, cooled to minus 150 degrees Fahrenheit. If a WIMP strikes a xenon atom it recoils from other xenon atoms and emits photons (light) and electrons. The electrons are drawn upward by an electrical field and interact with a thin layer of xenon gas at the top of the tank, releasing more photons.
Light detectors in the top and bottom of the tank are each capable of detecting a single photon, so the locations of the two photon signals – one at the collision point, the other at the top of the tank – can be pinpointed to within a few millimeters. The energy of the interaction can be precisely measured from the brightness of the signals.
"LUX is a complex instrument," says McKinsey, "but it ensures that each WIMP event's unique signature of position and energy will be precisely recorded."
LUX's biggest advantage as a dark matter detector is its size, a large xenon target whose outer regions further shield the interior from gamma rays and neutrons. Installed in the Sanford Lab in the summer of 2012, the experiment was filled with liquid xenon in February, and its first run of three months was conducted this spring and summer, followed by intensive analysis of the data. The dark matter search will continue through the next two years.
Wolfs will be giving a talk on these results at a general colloquium on Nov. 20, 4pm, at the University's River Campus.
The Sanford Lab is a state-owned facility, and the U.S. Department of Energy (DOE) supports its operation. The LUX scientific collaboration, which is supported by the National Science Foundation and DOE, includes 17 research universities and national laboratories in the United States, the United Kingdom, and Portugal.