Dark-Matter Experiment

For 75 years, scientists have vainly searched for particles of dark matter, the invisible substance believed to pervade deep space and to glue galaxies together. Next year, in a vat of chilled liquid buried deep in a cave in central Italy, the universe may finally be ready to give up this great secret. An international team of physicists is preparing XENON100, a simple experiment with a huge ambition: to record the moment when a bit of dark matter—known as a weakly interacting massive particle, or WIMP—smacks into the nucleus of an atom of liquid xenon, triggering a flash of light and an electric charge. “We definitely have a chance to see these events,” says Columbia University physicist and XENON team leader Elena Aprile.

According to the latest theories and observations, the universe has about six times as much dark matter as the atomic matter that makes up our ordinary world. But even though uncounted billions of dark-matter particles pass through Earth (and right through you, in fact) every second, they cannot be seen; they have no electric charge and interact so infrequently with atomic matter that the only way we can hope to find them is by laying a clever trap.

Currently there are about 10 teams of scientists devising experiments to discern the elusive moment when a stray WIMP nudges an atom of ordinary matter, but the latest version of XENON will be by far the most sensitive. All the experiments are lodged far below Earth’s surface to shield the detectors from background radiation. The Italian Gran Sasso National Laboratory is located nearly 4,600 feet beneath the top of a mountain, in caverns off a highway tunnel.

XENON100 is a scaled-up version of XENON10, one of Aprile’s earlier dark-matter experiments. It uses liquid xenon, an inert gas at room temperature, to catch WIMPs. The detector is a stainless-steel cylinder surrounded by a protective “castle” made of two kinds of lead and a layer of polyethylene to screen out residual background interference. Inside, 330 pounds of xenon will be chilled to –140 degrees Fahrenheit. Xenon’s attractive property is that it gives off a brief flash of light if a WIMP bumps into the nucleus of one of its atoms. A set of sensors on the cylinder bottom records this signal, while sensors on top detect the minuscule release of electrons liberated by the WIMP. By reading the two signals and measuring the time interval between them, researchers can fix the point of impact within the cylinder in three dimensions.

Dark matter is not necessarily composed of WIMPs—theorists have identified a host of other possible dark-matter particles—but they are the leading candidates because their presence would close a loophole in the reigning theory of particle physics, called the standard model. To address this discrepancy, scientists have proposed that all particles have large-mass counterparts, or superpartners. The neutralino, even with a mass at least 50 times that of a proton, would be the lightest of these. It is a prime WIMP candidate.

If XENON100 uncovers the long-rumored neutralino, it will mark another huge step in science’s grand humbling of humanity. “Copernicus discovered we’re not the center of the universe,” says Yale physicist Daniel McKinsey, a member of the XENON10 team. “If we find dark matter, we will discover that we are not even made of the stuff that composes most of the universe.”


Guy Gugliotta