Will dark matter ever be found?

"It could be a completely new type of particle"

For years, researchers in the Gran Sasso underground laboratory have been looking for so-called WIMPs with the highly sensitive XENON1T detector. These particles are the most promising candidates for dark matter. The name WIMP stands for “Weakly Interacting Massive Particles”, ie weakly interacting massive particles. After years of measurements and extensive analyzes, the researchers have now identified a surprising signal in the data. In an interview with Welt der Physik, Manfred Lindner from the Max Planck Institute for Nuclear Physics in Heidelberg speaks about possible causes for the previously inexplicable signal.

World of Physics: What was surprising about the signal that you detected with the XENON1T?

Manfred Lindner

Manfred Lindner: Essentially, we measure two things with XENON1T: firstly, triggered xenon atomic nuclei and, secondly, triggered electrons. So far, however, we have not yet been able to find any WIMPs that should make themselves noticeable in our experiment through collisions with the xenon atomic nuclei. In June of this year, however, we measured electrons that were struck, which must have another origin.

What kind of processes are possible for this?

On the one hand, it could be a completely new type of particle or new physical laws. That would be extremely exciting and would raise completely new questions in particle physics. On the other hand, it could also simply be a hitherto not understood, minimal contamination of the measuring apparatus, which now simulates a false signal. The XENON1T detector was so well shielded during the measurements that it was the place with the lowest radiation in the universe. The extreme purity was continuously checked with very sensitive measuring methods. But you also have to see that with this apparatus we have built an incredibly sensitive experiment that reacts a million times more sensitively to rare processes than previous experiments.

What would be the most plausible explanations, should it be about novel particles or new physical laws?

A number of studies have been carried out on this signal. Theorists have already submitted over 150 publications on this. The most important ideas can be roughly divided into three groups. First, the signal could come from neutrinos if these volatile elementary particles have somewhat different properties than we assume today. Neutrinos could have an unusual magnetic moment, for example. Then the many neutrinos that come from the sun would hit some electrons in the detector and trigger such a signal.

What would the second option be?

It could also be that dark matter consists of very light particles - so-called axions or similar particles. Theorists have been studying this possibility intensively for several years. If there were such particles, they should arise in great numbers in the sun. In principle, axions or similar particles could trigger our signal. However, there are still some unanswered questions here. Because these particles would have to have very specific properties in order not to conflict with other observations.

And the third option?

Perhaps it is also a previously unexpected type of dark matter. So far, we have been looking in vain for our experiment for WIMPs that should be as heavy as the xenon atoms in our detector. Because shocks between similarly heavy partners are easier to detect. But if the WIMPs were much lighter, they might just bump into the much lighter electrons.

Detector in the water tank

How do you want to determine the origin of the signal?

The data that we have now analyzed come from the last measurement period of XENON1T, which lasted until 2018. In the meantime, the significantly larger detector XENONnT has been set up and will soon begin with the measurements. It will be much more sensitive again and then give us better information about the origin of this signal.

How did the construction of the new detector look like?

Due to the restrictions due to the corona pandemic, only a certain number of people were allowed to stay in the laboratories and underground. That led to tricky situations. Because certain parts of the equipment must not come into contact with the ambient air. After just a few seconds, the apparatus would be so heavily contaminated with foreign atoms that the sensitivity of the experiment would suffer severe losses. If all the work involved in setting up such a complex experiment can only be spread over a few shoulders, you have to plan and prepare the individual work steps very carefully. But thanks to the great personal commitment of everyone involved, it looks like everything will be finished according to plan.