Dark Matter Evades the World's Most Sensitive Search

At the 11th Identification of Dark Matter Conference, LUX, the Large Underground Xenon experiment, announced the results of the world’s most sensitive search for dark matter. Physicists at LUX have been racing for a dark matter discovery since the collaboration’s inception. Observers in the field were aware that the LUX announcement would include more data with vastly improved performance compared to experiment’s initial run in 2013. Despite all of the anticipation surrounding the announcement, not a single dark matter interaction was observed in the detector.

The LUX experiment is designed to detect Weakly Interacting Massive Particles, or WIMPs, a leading candidate for dark matter particles.  Located in a former mine, deep under South Dakota, LUX consists of liquid xenon surrounded by light sensors and vast amounts of shielding. The shielding is designed to prevent ordinary matter from interacting with the experiment. Dark matter particles however, could easily pass through the shielding, and potentially interact with the liquid xenon. If a dark matter particle managed to collide with a xenon atom, the collision would result in a flash of light recorded by the experiment’s sensors.

Despite any disappointment physicists may have, a null result is still physically interesting. Dark matter is estimated to make up roughly 27% of the observable universe. The properties of dark matter can be inferred from the cosmic microwave background, the large scale structure of the universe, and gravitational lensing, in which gravity bends light around massive objects such as galaxies. Scientists know that dark matter is a particle, and has a very weak interaction with normal matter. What remains unknown is the mass and interaction strength of such a particle.

LUX’s results have set the world’s most stringent limits on the properties of WIMP dark matter. These new constraints inherently shape the landscape of particle and astrophysics by ruling out WIMPs of certain masses and interaction strengths. With the LUX results, some physicists are considering searching for other types of dark matter candidates, while others insist on simply improving the sensitivities of their measurements. The US Department of Energy recently approved a search for lower mass WIMPs than LUX can detect, as well as an experiment devoted to axions, another candidate dark matter particle. A upgrade to LUX, the LUX-ZEPLIN detector is scheduled to go online in 2020, and will be 100 times more sensitive than its predecessor. No one knows exactly what these experiments will find, but there is no question physicists won’t stop until a discovery is made.