National Institute for Subatomic Physics

What is dark matter?

In the Universe, only 4% of what we can see exists of matter that we know, built up of atoms. No less than 96% of the Universe however, exists of what we now call dark energy (73%) and dark matter (23%).

Dark matter doesn't produce or reflect light en thus we cannot see it. However, we are sure it exists, judging from amongst other things the movement of stars and solar systems and other astronomical observations. Next to that, we expect to find dark matter all around us, here on Earth. The most likely candidate for dark matter is the WIMP (Weakly Interacting Massive Particle), a heavy particle participating in weak force. Because of this weak force, the WIMP has almost no interaction with other particles, meaning it hardly collides with other particles.

What does the research entail?

The Nikhef researchers, together with their international colleagues, will go in search of this dark matter. Finding certain 'visible' subatomic particles can be an immensely difficult task in itself, let alone 'invisible' particles. Finding them could be done by looking at the reaction of dark matter with the particles that are known to us. The chances of such a reaction are however very small. This is because dark matter, in all likelihood, participates in the weak force and therefore hardly collides with other particles.

That's why the researchers have to increase the chances of these collisions by making the circumstances ideal. This is where the noble gas xenon comes in.

Xenon is a gas with a density that greatly increases the chances of subatomic collisions; as it happens, the mass of the nucleus of xenon is large (atomic mass A=131, which means it has 54 protons and 77 neutrons in its nucleus). The researchers are in the process of designing a cryostat barrel in which they will built a 2.5 ton chamber with liquid xenon. Around this, they will build a water reservoir to make sure that as little outside influences such as background radiation is caught. The installation will be built inside an Italian mountain, at Gran Sasso National Laboratory. This location, a 1000 metres under a mountain makes sure that as few cosmic muons as possible interact with the experiment. The ideal circumstances to let dark matter collide with our 'own' matter!

Among other things, Nikhef is working on the cryostat barrel in the middle of the picture. The barrel makes sure the xenon in the detector remains liquid. The purpose of the XENON1T experiment is to identify dark matter.
Side view of the XENON1T experiment. The yellow tank on the right is an 11 meter diameter water tank in which the detector is positioned. The water in the tank helps keep back radiation from the experiment. On the two floors next to the tank, the cooling installation and the detector reading system are placed.

How will the researchers know they are dealing with dark matter?

When dark matter collides with the nucleus of a xenon atom, a tiny light flash will be seen. This light flash is generated by the recoil that the xenon atom has experienced.

Dark matter will not be made visible in this research, it remains an indirect process in which the researchers will prove the presence of dark matter by its reaction with a particle known to us. The detector can distinguish between the WIMP and possible leftover background radiation. The researchers hope that on the basis of the measurements they are able to prove it is indeed a new subatomic particle, and determine what the mass of the particle is and what its likelihood of interaction with ordinary matter is exactly.

What is Nikhef's contribution to the project?

Nikhef's researchers and technicians will design and build the cryostat barrel and the supporting structure. Next to that, they will design part of the electronics and data-acquisition system. Together with their international colleagues, the Nikhef researchers will actively participate in the physics groups during the analysis phase of the research project. The international group exists of about forty researchers and technicians.

Tijdlijn Xenon 1T

Why do we want to know this and possible applications of the research

Fundamental physics researchers aim to gather basic knowledge of everything around us. There are many things we already know thanks to fundamental research, such as that all matter known to us is built up of atoms, but there are still many unanswered questions. To put all the puzzle pieces together, we need fundamental research, carried out by curious scientists.

Finding out what dark matter exists of is important in trying to complete the chart of particles known to us. It will give us more insight into what the Universe is exactly made of.

Possible applications

In the medical world (diagnosis, PET scans).

Website of the international collaboration