XENON100 zeroes in on dark matter
Rice physicists help build most sensitive detector yet deployed

BY JADE BOYD
Rice News staff

A team of Rice physicists is celebrating the release of the first experimental results from the new dark-matter detector — the most powerful ever fielded — that it helped design, build and deploy in Italy early this year.

The XENON100 found no hint of dark matter during its first 11 days of data collection, but given the sensitivity of the instrument — and claims of possible dark-matter discoveries made by other research groups in the last six months — the initial findings from XENON100 were significant enough to prompt stories in The New York Times, Nature, New Scientist and other publications.

	XENON100 is designed to act like a 3-D camera and capture images of collisions between dark-matter particles and xenon nuclei.

“The most exciting aspect of the XENON100 detector is its unmatched low background, two orders of magnitude better than its current competitor,” said Rice’s Uwe Oberlack, the William V. Vietti Assistant Professor of Space Physics. “This, combined with the large target mass, makes XENON100 so much more sensitive than any other experiment in the field that even these preliminary results are important. In just two weeks, we were able to exclude some of the potential places that dark matter was previously thought to exist.”

There are two immediate implications of the new findings. First, it casts doubts on the theoretical interpretation that low-mass dark matter particles could have produced signals seen in other experiments. Second, it suggests that the XENON100 team has the best chance of any group in the world to make the initial discovery of a dark-matter particle.

But how can physicists be so sure that dark matter exists if they have never seen it.

The answer is that dark matter — i.e., something that has mass but has yet to be measured by human instruments — must exist because galaxies, including our own, are spinning so fast that they should be flying apart. Physicists realized years ago that there must be some unseen mass that holds the spinning galaxies together, and dark matter is the name that was coined for this unseen glue. Indeed, astrophysical measurements at even larger scales indicate that 80-85 percent of all matter is dark matter.

Physicists theorize that dark-matter particles have not been measured before because they seldom interact with ordinary matter. The XENON100 experiment is designed to catch one of these rare interactions.

XENON100 is housed at Gran Sasso National Laboratory beneath central Italy’s tallest mountain. It uses a time-projection chamber — a copper vessel containing about 350 pounds of pure liquid xenon. The chamber is designed to act like a 3-D camera and capture images of collisions between dark-matter particles and xenon nuclei. The mountain filters out false signals that are created by cosmic rays, and lead and plastic shielding further reduces background signals.

Oberlack said a paper describing the first data from the experiment has been submitted to the journal Physical Review Letters, and the XENON team is already at work on an analysis of the detector’s first few months of data, which will probe dark matter at a sensitivity 20 times better than current measurements.