Today, the team behind one of the most sensitive dark matter detectors announced that their entire experimental run failed to yield any of the particles they were looking for. The LUX (Large Underground Xenon) detector is designed to pick up signs of weakly interacting massive particles, or WIMPs, as they enter one of their rare interactions with normal matter. The null result does not rule out the existence of dark matter, but it does limit its potential properties.
As their name implies, WIMPs don’t often interact with normal matter, but they should occasionally bump into an atom and energize it. LUX offers a tempting target in the form of 370 kg of liquid xenon. The detector is flanked by photodetectors to pick up any stray photons from the interactions, as well as hardware that picks up stray charges on the loose.
The challenge is to determine which signals are caused by dark matter and which are the product of cosmic rays or the natural background of radioactive decay. To tackle the former, the detector is located nearly a mile below the surface at the Homestake Mine in South Dakota. It is also partially shielded from the radioactive decay of the surrounding rock by a huge tank of ultrapure water. Still, the scientists behind it had to spend time carefully characterizing the background noise. The success of that effort meant that LUX became four times more sensitive than originally designed.
Despite the increased sensitivity, 20 months of data have provided no clear evidence that WIMPs hit the xenon. This inability to detect signs of dark matter particles puts limits on their interactions with normal matter, which in turn should limit proposals about the identity of WIMPs based on theoretical ideas. But while it offers limits, LUX cannot completely exclude them.
Any further limits will have to wait for the successor, (I’m not making this up) LUX-ZEPLIN. Built in the same facility that housed LUX, LUX-ZEPLIN will have 30 times the xenon of its predecessor.