Dark matter is the “Carmen Sandiego/Where’s Waldo?” of the physics world. We know it is there but we just can’t seem to find it.
And that's the case again with this latest test. The Large Underground Xenon (LUX) dark matter experiment has completed its final 20-month-long search and, unfortunately, has not found any dark matter particles.
This is a bit disappointing, but even a non-detection is important when it comes to physics. The detectors could only look for potential particles in a certain mass range. So, if dark matter is really made of particles, they must reside beyond that range.
“The discovery of the nature of the elusive dark matter that accounts for more than four-fifths of the mass of the universe is internationally recognised as one of the highest priorities in science, and the LUX experiment is the world-leading experiment in the direct search of it,” explained UCL LUX collaboration scientist Dr Cham Ghag in a statement.
“Though a positive signal would have been welcome, nature was not so kind! Nonetheless, a null result is significant as it changes the landscape of the field by constraining models for what dark matter could be beyond anything that existed previously.”
The existence of dark matter can explain a lot of features of the universe, like how a spiral galaxy disk rotates at the same speed at the center and at the edge. Alternative hypotheses have been put forward but, so far, dark matter is the most likely explanation.
But even within the dark matter paradigm, there are different explanations. Dark matter could be many different types of particles (or in some scenarios even black holes) so even with a non-discovery LUX has effectively removed many potential candidates from the playing field.
The collaboration will continue to investigate the data, providing a solid analysis of the background source that will then be used in the next detector LUX-ZEPLIN (LZ) presently under construction. LZ will be at least 70 times more sensitive than LUX.
“We’re responsible for ensuring the LZ experiment is constructed to unprecedented radio-purity requirements that limit background to extremely low levels in order to expose any signal from WIMPs hiding underneath,” said Dr Jim Dobson also from UCL. “We must also accurately characterise the background pulses that do remain because before we can say we have detected WIMPs, we must know precisely what we expect from everything else. This will be crucial to ensure any future discoveries of dark matter are valid.”