The invisible universe

XENON is an international physics experiment that uses an underground detector in Italy to attempt to detect and study dark matter particles.

The XENON detector in Gran Sasso, Italy, is a vessel containing a volume of deep-cooled liquid xenon gas, which is observed using photomultiplier tubes. The detector is located in a cave complex beneath the Gran Sasso mountain, which shields it from cosmic radiation.

XENON consists of a vessel containing 8.6 tonnes of cryogenically cooled liquid xenon. As part of the experiment, a facility for purifying and cooling the xenon has been built, along with a data processing centre.

The xenon measurement volume is enclosed within a large water reservoir to further block background radiation from the rock. In theory, dark matter particles can still penetrate to the underground detector. If they strike a xenon nucleus, a characteristic light signal is expected to be produced, which the experiment can detect. Such signals are, by definition, rare.

Around 150 scientists from 15 countries are involved in the XENON experiment. Nikhef plays a leading role in it.

A serious candidate for dark matter are so-called WIMPs (Weakly Interacting Massive Particles), a hypothetical heavy particle that interacts very weakly with other particles.

WIMPs have never been observed in particle accelerators, but may well be present in vast numbers throughout the universe. A WIMP almost never collides with other matter. The XENON detector is entirely designed to detect extremely rare WIMP collisions.

A smaller, earlier version of XENON at Gran Sasso found no evidence of WIMPs, but was able to set limits on the possible mass of the particles. The larger current XENON detector has tightened those limits, but again has not detected any concrete particles. The measurements have, however, already ruled out various theoretical models for dark matter.

XENON is the flagship experiment of Nikhef’s Dark Matter Group, which comprises around twenty staff members, technicians and PhD students. Researchers and technicians at Nikhef designed the cooling vessel for XENON and the support structure to which the vessel is attached. They have also developed the algorithms to determine which signals from the detector should be retained. Furthermore, they have written the software to prepare the electronic signals for further analysis.

For both the earlier XENON1T and the current XENONnT detector, the Dutch team was closely involved in the analysis of the initial measurement data.

In addition to analyses, the group at the Nikhef lab in Amsterdam also conducts experimental research into measurement techniques using a small test version of the detector, known as XAMS. They are also conducting experiments to identify persistent sources of background signals in the detector, for example due to contamination with solvents in the liquid xenon gas used. The aim is to further suppress the background in XENON and future dark matter detectors.

The group maintains close international contacts with initiatives for new generations of dark matter detectors, such as LUX in the US.

The University of Amsterdam is closely involved in the Nikhef research program Dark Matter – XENON.

Learn more about the international XENON experiment: https://xenonexperiment.org/