“We hope to detect the largest proportion of the matter in space”

A conversation with Manfred Lindner on the latest results from the Xenon100 detector

April 27, 2011

The underground laboratory at Gran Sasso in Italy is the home of the Xenon100 experiment, which is being conducted as an international collaboration that includes the Heidelberg-based Max Planck Institute for Nuclear Physics to detect the mysterious particles directly. The researchers recently published the evaluations of one hundred days of measurement time. The result: although there is no significant signal for dark matter as yet, the world’s best limits for the masses and interaction strengths of the WIMPs have been obtained, and already noticeably reach into the predicted range.

For many decades there have been mounting indications that the stuff of which all stars, planets and even we humans are composed accounts for just under five percent of the total matter existing in the universe. Dark matter contributes 23 percent, the lion’s share of 72 percent being provided by dark energy. Theoretical and experimental indications point towards it consisting of yet unknown elementary particles which are present everywhere in the universe. Physicists call them WIMPs (Weakly Interacting Massive Particles), because they have as good as no interaction with normal matter. This is why the particles have so far evaded any direct detection and only make themselves felt in astronomical observations by their gravitational force.

The latest results of the Xenon100 detector have implications for the theory of particle physics and the experiments at the LHC accelerator at CERN in Geneva. Which conclusions can be drawn? We spoke with Manfred Lindner, Head of Xenon100 at the Max Planck Institute for Nuclear Physics.

Can you describe how Xenon100 works in a few words?

The detector consists of a tank which is filled with 162 kilograms of extremely pure liquid xenon. If a WIMP collides with an atom here, a flash of light is triggered and electric charges are generated at the same time. If such a double event is registered, it is an indication for the existence of a dark matter particle whose mass could also be determined. An enormous technical challenge is to protect the detector from interfering radiation – caused mainly by natural radioactivity. To this end, the xenon was purified to a very high degree and the detector shielded from external influences with several layers of different materials.

You registered three events in your measurement data from the first half of last year. Could these be WIMPs?

The three events agree with our prediction of the interfering events to be expected statistically in a hundred days. We therefore say that the result provides only limits for WIMPs. At the moment, the detector is operating with much higher purity so we can’t wait to see what the analysis of these data will provide later in the year.

What can one conclude from this null effect?

We have greatly narrowed down the range in which WIMPs can exist. Theoretical predictions set a most probable mass range for these particles which is in the order of 100 gigaelectronvolts, roughly corresponding to the mass of a xenon nucleus. With Xenon100 we ventured precisely into this mass range and narrowed it down enormously. The permissible range for the WIMPs is thus becoming smaller and smaller.


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