WIMPs: Masters of Elusion

Dark matter particles are the Harry Potters of our universe - always hiding underneath a cloak of invisibility. They do not emit light or other forms of electromagnetic radiation and therefore are undetectable to the human eye and telescopes.

That does not stop scientists like Laura Baudis, a professor at the Physik Institut of the University of Zurich, from attempting to track these elusive wizards down. To find a dark matter (DM) particle would confirm a series of cosmological theories that define the structure and behaviour of our universe.

DM particles, the leading theory stipulates, are made up of weakly interacting massive particles, WIMPs for short.  In her talk "Direct WIMP searches: an update" for SUSY 2013 at ICTP, Baudis discussed the various methods, numerous detectors and global collaborations comprising the search for DM particles.

At least five different types of DM detectors currently lie in wait for a WIMP. Some detectors are filled with liquid xenon or argon, while others contain superheated, heavy fluorinated liquid. Supersymmetry, Baudis explained, helps scientists determine what rate to expect WIMP interactions given a certain material and how much sits within the detector. Some rates, she estimated, could be as low as one event per ton of detection material per year.

From little bubbles to charge-coupled devices (CCDs), different detectors have different ways to identify a WIMP interaction. For example, the Chicagoland Observatory for Underground Particle Physics (COUPP) is a type of bubble chamber that will burp a single bubble if a WIMP interacts with one of its iodotrifluoromethane molecules.

In addition to the small reaction rates within detectors, Baudis said another major challenge to detecting WIMPs is that they are embedded in a background of other particles that are millions of times more abundant, such as neutrinos and photons from stars.  Therefore, many detectors lie thousands of feet below the Earth's surface, which acts as a partial net for unwanted particle contamination within the detector.

Scientists are still unsure at what mass and energy they will find a WIMP. Therefore, it's important to be capable of probing a large energy range. "Direct detection experiments have reached unprecedented sensitivity and can probe WIMP with masses from a few GeV to a few TeV," Baudis stated in her presentation. "We have entered the era of ...direct detection ...however, we should be prepared for surprises. "

This story is part of the Focus Feature on SUSY 2013. Go to the Focus Feature webpage for the complete coverage.