Experimental direct dark matter detection: XENON

Faculty members Colijn, Decowski and Pollmann are looking for experimental signatures of dark matter. Detecting ‘visible’ subatomic particles can already be very challenging, so how does one find an ‘invisible’ kind of particles such as dark matter? This may possible by looking for the signature of a collision between dark matter and known particles. However, the likelihood of such an interaction is very small, which means that dark matter does not collide with other particles often.

The amount of dark matter passing earth is fixed, so to increase the chances of observing a collision, one must provide a large amount of known particles for it to interact with. The known particles used by these researchers come in the form of xenon. Xenon nuclei, at an atomic mass of about 130, provide a large target area. Furthermore, by using xenon in its very dense liquid form, one can fit a lot of xenon into a relatively small space. The latest detector, XENONnT, is filled with 8.5 tonnes of liquid xenon at a temperature of about -100°C.

Xenon is used because when other particles collide with a xenon nucleus, tiny amounts of light and electric charge are released. These are registered with high-precision sensors. A specific signature in the emission of light and charge could demonstrate the presence of dark matter. Due to the small chance of collision between dark matter and xenon, this is not a simple measurement. Normal particles, for example from natural radioactivity and cosmic rays, collide with the xenon atoms much more often. To block these, the detector is in an underground laboratory, underneath a mountain, at the Gran Sasso National Laboratory. This location, with 1400 meters of rock overhead, ensures that atmospheric muons are filtered to a large extent. Other sources of natural radioactivity are controlled by carefully selecting detector materials, and by surrounding the detector with several layers of shielding.

The predecessor of XENONnT, XENON1T, took data between 2016 and 2019, providing world-leading dark matter results. The researchers hope to use the measurement data from XENONnT to prove the existence of dark matter and to determine two properties of the dark matter particle: what mass does it have and what is the likelihood of it interacting with ‘ordinary’ matter? But the experiment is also sensitive to other rare interactions, such as collisions involving neutrinos.

In the future, an even bigger detector using approximately 40 tonnes of xenon will provide an even more precise measurement of the possible interaction of dark matter with xenon atoms. The researchers are currently doing R&D to make this detector, called DARWIN, possible.