A major dark matter experiment has taken a swipe with its technological net in the hopes of catching some of the elusive particles that make up the universe's missing mass, and once again that net has come up empty. But in swiping and missing, the Xenon100 experiment has closed in a bit tighter on where dark matter—the invisible stuff theorized to outweigh the ordinary matter in the universe by a factor of five—might be hiding.

Xenon100, a tank of liquid xenon deep underground in Italy, has been designed to identify the rare instances when ambient dark matter particles would recoil off regular matter, a subtle collision that should generate a tiny flash of ultraviolet light and a slight electric charge from ionization effects. The detector's 62 kilograms of cryogenic xenon are shielded from non–dark matter contamination by layers of copper, polyethylene, lead, water—and a thick slab of rock overhead that keeps out almost all cosmic rays.

Even so, the detector gets some deceptive signals, for instance stray neutrons from the occasional radioactive decay of atoms within the experimental apparatus and electrons from radioactive krypton contaminating the liquid xenon itself. And after 100.9 days of collecting data in 2010, Xenon100 recorded only three hits that looked like dark matter, an insignificant amount compared to the expected background of about 1.8 such events. In other words, researchers from the project report, "the observation of 3 events does not constitute evidence for dark matter"; those three hits could easily be background noise. The Xenon100 collaboration, led by Elena Aprile of Columbia University, reported the latest results from the experiment in a paper posted online April 13 at arXiv.org, a physics preprint Web site.

So Xenon100 has not found any evidence for dark matter—at least not yet—but that nondetection helps rule out certain parameters for the stuff, in terms of the putative dark matter particle's mass and how often it bumps into ordinary matter. The kinds of dark matter particles now ruled out at the 90 percent confidence level include those that were candidates to explain the signals recorded by two other detectors, CoGeNT in Minnesota and DAMA in Italy, casting doubt on those signals as genuine dark-matter hits. The data even rule out part of the regime where Europe's Large Hadron Collider (LHC) might look for evidence for dark matter as a so-called supersymmetric particle, a hypothetical heavyweight partner to one of the standard elementary particles.

Even though the most recent swipe at dark matter came up empty, the Xenon100 researchers are already back on the hunt. The detector is now taking more data, they report, with fewer false positives thanks to reduced krypton contamination in the detector.

Photo: Xenon100 collaboration