Even the waste of dark matter hunting can give new clues

23 October 2020

Rejected signals from the underground XENON detector in Italy are the raw material for a new search for dark matter. Nikhef researchers Marieke Postma and Auke-Pieter Colijn will receive an NWO Klein grant to further investigate XENON’s waste.

This was announced this week by science funding agency NWO. The proposal of the two researchers, UvA professor Colijn as an experimenter and Postma as a theorist, was one of the fifteen funded projects.

The XENON1T detector is assembled in a clean room. (Photo courtesy of the XENON Collaboration)

Nikhef is one of the main partners in the international XENON project in Gran Sasso, Italy. It is located one kilometer underground to shield it from ordinary cosmic radiation. In theory, dark matter particles can penetrate so deeply.

Dark matter is considered one of the greatest riddles of physics. In particle theory there is room for hypothetical particles that can exist next to the known particles. At the same time astronomers everywhere see indications for more gravity than could be expected from the visible matter of stars and nebulae. The invisible component is called dark.

Physicists try to find particles that could form that dark matter. The XENON detector is a device to find a certain type of particles of dark matter called WIMPs. The detector consists of a barrel of liquid xenon. When a dark matter particle hits a xenon atom, it emits a flash of light at the collision, plus electrons.

The XENON detector uses light detectors and electrical sensors to look for signals where first a light signal comes out of the dark tank and then an electrical pulse. All events that have only one of the two signals are rejected as not serious observations. However, the raw measurements are stored.

Colijn and Postma think that the discarded events may still contain clues to dark matter. They are going to re-analyze the rejected observations, using new techniques.

So far, no detector in the world has found hard clues for particles of dark matter. However, the experiments have already ruled out many properties of such particles. This also applies to XENON, which is currently being enlarged to an experiment called XENONnT, which will contain a few thousand kilos of liquid xenon. Colijn is the technical coordinator of that upgrade.