A joint Fermilab/SLAC publication

Most sensitive dark-matter detector constrains search for WIMPs

07/20/12

No WIMPs here. XENON100 detector, image courtesy of XENON Collaboration

The XENON collaboration announced this week that they detected no signs of potential dark matter particles during the last 13 months. Their results will be used to narrow the search for the unseen particles that scientists think make up most of the matter in the universe.

The collaboration used the most sensitive dark-matter detector in existence, XENON100, for their search. Scientists report the experiment is now 3.5 times more sensitive than it was in 2011.

“The goal of a dark matter search experiment is making a discovery, but being able to place the best limit for these kind of interactions is also a great result,” said Antonio Melgarejo, a XENON100 analysis coordinator from Columbia University. “We have managed to operate our detector in very stable and excellent data-taking conditions for a very extended period of time, and this shows that we are on the right track and that liquid xenon detectors are one of the best detectors in this field.”

The Gran Sasso Laboratory in Italy hosts the XENON100 experiment, which is designed to detect collisions between xenon particles and hypothesized dark-matter particles called Weakly Interacting Massive Particles, or WIMPs. WIMPs are considered the most likely dark matter candidates because of their large mass and their limited interactions with other particles, which render them unable to be seen directly. Their discovery could help explain the composition and influence of the 23 percent of the universe that is occupied by unseen matter.

Although the XENON100 experiment detected no collisions, the data collected allows the refinement of physical models that incorporate WIMPs by ruling out possible masses and rates at which WIMPs interact with other particles. Further data collection will continue to narrow the boundaries within which a WIMP could be detected.

XENON100 uses xenon because it contains very large nuclei that are easier for the WIMP particles to hit than alternative materials. Scientists predict that if a WIMP and xenon nucleus collided, a very faint charge and light signal would be generated. The collisions would be rare and possibly detected by XENON100 just a few times a year.

To further enhance the chance of spotting a collision, the detector is located underground to cut out most of the cosmic radiation that pervades Earth’s atmosphere and could cause distracting signals in the detector. Still, some noise reaches XENON100. It recorded two events during the 225 days it collected data in 2011 and 2012, but they were consistent with background radiation and not considered significant.

Other labs have recorded events that could be connected to dark matter, but no conclusive observation has ever been made. For many years, the DAMA/LIBRA experiment in Italy recorded annual fluctuations in a signal they believed to be connected to large-scale dark matter. In 2010, researchers at the experiment in Minnesota reported they had spotted interactions compatible with the existence of a WIMP. The latest results from XENON add an important perspective to the search.

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