Today, the team behind one of the most sensitive dark matter detectors announced its full experimental run had failed to turn up any of the particles it was looking for. The LUX detector (Large Underground Xenon) is designed to pick up signs of weakly interacting massive particles, or WIMPs, when they engage in one of their rare interactions with normal matter. The null result doesn’t rule out the existence of dark matter, but it limits its potential properties.
As their name implies, WIMPs don’t interact with normal matter often, but they should on occasion bump into an atom, imparting energy to it. LUX provides a tempting target in the form of 370kg 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 any stray charges knocked 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 decays. To handle the former, the detector is located nearly 1.5km below the surface in South Dakota’s Homestake Mine. It’s also partly shielded from the radioactive decays of the surrounding rock by an enormous tank of ultra-pure water. Even so, the scientists behind it had to spend time carefully characterizing the background noise. The success of that effort meant that LUX ended up four times more sensitive than it was originally designed to be.
Despite the heightened sensitivity, 20 months of data have turned up no clear indications of WIMPs hitting the xenon. This failure to turn up 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 provides limits, LUX isn’t able to rule them out entirely.
Any further limits will have to await the successor, (I am not making this up) LUX-ZEPLIN. Built in the same facility that housed LUX, LUX-ZEPLIN will have 30 times the xenon as its predecessor.