New Sensitivity

Cosmological observations show that the universe is largely made up of dark matter, but what this consists of is still unknown. Physical theories predict that the dark matter particles might be the so-called WIMPs (weakly interacting massive particles). The international XENON collaboration, with key members being from the University of Freiburg’s astroparticle physics group around Prof. Dr. Marc Schumann and Dr. Daniel Coderre, has now set up the strongest limit for the interaction of WIMPs with normal matter.

In simultaneous talks at Gran Sasso National Laboratory (LNGS) in Italy and at CERN European Research Center in Geneva in Switzerland, researchers presented the new results of XENON1T. This is the world’s largest and most sensitive detector searching for WIMPs. Although it is estimated that one billion of these particles fly through every square meter of the earth’s surface every second, they are extremely hard to detect. The extraordinarily large dataset of XENON1T agrees with the values expected for background signals that are caused by natural radioactivity and therefore sets the strongest limit for the interaction of WIMPs with normal matter. These results show that WIMPs – if they really are the dark matter particles – generate such a rare signal that even the largest and most sensitive detector to date cannot measure them.

The XENON1T detector has been taking data at the Gran Sasso National Laboratory in Italy since autumn 2016. It consists of a cylindrical vacuum container a little more than a meter in height and diameter, filled with liquid Xenon at –95 degrees Celsius. In this gas, the interaction of a WIMP with a Xenon atom would be revealed by a weak light signal and the release of electrons, which in turn also generate slightly delayed light signals. Extremely sensitive light sensors register both signals. From this, the scientists can determine the precise location and the energy released for each individual event. The greatest challenge in developing the detector was to reduce the background signals that are caused by natural radioactivity. In addition, the experiment is located 1.5 kilometers below ground, where rock protects it against potential interference from cosmic rays. Next year the researchers will be launching an even larger version of the detector.

The international XENON collaboration involves more than 165 scientists from 27 institutions. The Freiburg experimental astroparticle physics group planned and organized the construction of the detector and developed the data readout system. In addition, Freiburg junior scientist Daniel Coderre headed the analysis to which more than 50 researchers contributed and which has now been published.

Article on Marc Schumann’s research in the University of Freiburg’s online magazine