Dark Matter Comes and Goes at April Meeting
By Michael Lucibella
Dark matter was in the air at this year’s APS April Meeting, with researchers in many areas of the hunt saying that there were important developments. Some teams reported they were getting closer, while others saw once promising results fade away.
“It looks like the pot is boiling,” said Leslie Rosenberg from the University of Washington. “A lot is happening in the dark matter sector.”
At the meeting, researchers from the Cryogenic Dark Matter Search said they’ve seen three potential candidates for dark matter particles known as WIMPS. The experiment’s eight silicon detectors recorded the readings at around 8.6 GeV during its 14-month run.
“The data are insufficient to claim discovery of WIMPs… neither are we claiming evidence for them, but further investigation is warranted,” said Blas Cabrera, a physicist at Stanford University. “Analysis favors a WIMP signal with about a three sigma.”
The silicon detectors are located deep underground in the Soudan mine in Minnesota. They’re designed to detect the minute recoil that occurs whenever a WIMP strikes a silicon nucleus.
The results are surprising in a few ways, and Cabrera said that more analysis and data were needed before any conclusions could be definitively drawn. If the results hold up, the relatively light particle would be consistent with the COGENT dark matter experiment, also in the Soudan mine, but at odds with results from the Xenon-10 and Xenon-100 dark matter experiments located in Italy. It is also unclear why such light particles would not have been seen in particle accelerators already.
“You can construct models where you would not have seen such particles in accelerators,” Cabrera said, “But clearly the mass is such that you can access it with accelerators, so you have to ask that question and analyze it in detail.”
The team is also taking data with another experiment in the Soudan mine using nine kilograms of germanium, and is developing a larger, 200-kilogram detector for the SNOLAB facility in Canada.
WIMPS aren’t the only exotic particle that physicists think might explain the missing mass of the universe. Axions have been the focus of much theoretical work since the 1970s, and researchers are preparing an experiment that might be able to confirm the existence of axions.
According to the theory, the particles are ubiquitous throughout the universe, but are very light and weakly interacting. There are as many as 10 quadrillion axions per cubic centimeter on Earth, but they are very difficult to detect with a mass only in the micro-electron volt range.
“We really don’t have a lot of clues about what makes up the dark matter,” Rosenberg said. “An axion could be a perfectly good solution to the dark matter problem.
The WIMP could make up the dark matter completely, and it would be a wonderful solution to the dark matter problem. Or you could have a mix.”
Researchers at the University of Washington are building the Axion Dark Matter Experiment (ADMX) to hunt for them. The experiment employs a strong magnetic field around a resonant microwave cavity to convert the occasional passing axion into a faint radio signal. A sensitive receiver powered by quantum electronics then picks up that radio signal.
“With our radio receiver, you could go to any planet in the solar system and get four bars on your cellphone without any trouble whatsoever,” Rosenberg said.
ADMX is currently in its commissioning phase, and will begin to collect data this summer. The first results are expected sometime next summer.
“In a short number of years, we’ll actually have a very sharp answer, a definitive answer, to the question of ‘Is the axion… the dark matter in our galaxy?’” Rosenberg said. “We’ll either find it and be really happy, or we won’t and we’ll be out of business but we’ll still be really happy.”
At the same time, hopes faded for a different, formerly promising dark matter clue seen late last year. In late October there was a buzz in the astrophysics community about an apparent spike in gamma rays seen by the Fermi Gamma Ray Space Telescope’s large area telescope. However, the team running the satellite said at the conference that, after reprocessing, the spike has all but disappeared.
Researchers independent of the Fermi collaboration pored over the original raw data release and saw a four-sigma spike in gamma rays with energies of about 130 GeV emanating from the galactic center. The thought at the time was they might be signals from annihilating WIMPS of the same mass.
“There is evidence for some type of spectral feature here, but the effect is not large enough to claim any kind of interpretation beyond statistical fluctuations at this point,” said Alex Drlica-Wagner of Stanford University, adding that with reprocessing the feature’s significance dropped off to less than two sigma.
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