There is some disappointing news for the particle physics/astrophysics/cosmology research community this week as the Large Underground Xenon experiment has completed its data run and found no clear signals of any new particles. Due to its long run and its sensitivity, many had hoped for a significant dark matter detection, but it simply isn't there.

As many of my readers already know, our best models of the Universe include a mysterious form of matter known as dark matter. In fact it is believed that approximately one quarter of all energy in the Universe is in the form of dark matter. And aside from the average density and the lack of electromagnetic interactions, there is almost nothing known about this substance.

Because dark matter is not expected to interact very strongly with any known forms of matter, it is very difficult to detect. The most precise method is to create it in high energy particle accelerators such as the Large Hadron Collider, and then look for missing energy and momentum, but this requires a lot of power and expense and still produces a very small signal. The alternative is to build a very large tank or crystal of a pure liquid or solid substance, and let the dark matter in our solar system collide with it.

Since there is believed to be a relatively dense cloud of dark matter surrounding us, and since these experiments use a large quantity of detector material, the hope is that there will be a very large number of dark matter collisions in the detector. Using a number of different methods, these collisions are recorded by the experimentalists. Unfortunately the vast majority of these collisions are due to ordinary effects and signify nothing. And so the people analysing the data have to carefully remove every collision that might possibly be due to background effects, and then see what is left over.

The LUX experiment was built in 2009, and collected data on dark matter collisions from the summer of 2012 until May 2016. By the time it completed its run, it was using 368kg of pure xenon, and became the most sensitive experiment of its kind. 

This week they announced their results, and unfortunately every collision detected was excluded as a (probable) background effect. Some of these collisions could be due to dark matter, but because there is doubt they cannot be considered a clear signal of a new particle. It is also disappointing as two earlier experiments, CoGent and CDMS-II had found weak signals of relatively light dark matter particles, but the LUX results have proven those to be false. 

So what does this mean for dark matter research?

The annoying thing about research into dark matter is that so little is known about it that there are thousands of different potential dark matter particles. And each of these has a number of variables, such as mass or decay rates, that can be adjusted to vary the properties of the dark matter particles. As such the LUX experiment does not really exclude any models outright, but rather restricts the values of the parameters. 

And so the next step will be to build even more sensitive detectors. Already the team behind LUX is beginning development of a new 7-ton detector that will be able to explore further, while other teams around the world are currently collecting data on dark matter models that LUX did not cover. And of course the Large Hadron Collider is continuing to probe higher energy particle collisions, and may in the near future announce their own discovery of dark matter.

For now all we can do is wait for the next experiment to announce results, and continue to speculate on this most mysterious particle.