Albert Einstein Center for Fundamental Physics
The group has moved to the University of Freiburg, Germany, in fall 2016. You can find our new pages here.
The search for dark matter
It is know from numerous astronomical and cosmological observations that the vast majority of the energy content of the Universe is dark: so far it is invisible to our detectors. About 70% is the mysterious dark energy, responsible for the accelerated expansion of the cosmos, and about 25% is made of dark matter. This yet unknown form of matter builds large scale structures in the Universe and dominates the dynamics of galaxies and galaxy clusters. Only 5% is made from ordinary matter such as protons, neutrons, and electrons which build atoms, molecules and eventually our "known" world.
We participate in the XENON dark matter project, which aims to directly detect the particle making up the dark matter and which should be present in our cosmic neighborhood in large amounts. Its interaction probability with normal matter, and hence with particle detectors, must be extremely weak, otherwise the dark matter would not be dark. However, there are plenty of models which predict that there should be a very small interaction probability. One promising candidate is the WIMP (weakly interacting massive particle), which arises naturally in many extensions of the standard model of particle physics.
The expected interaction rate in our detector is very low, but backgrounds are large since natural radioactivity is everywhere in the environment and cosmic rays also interact with detectors. Therefore, we need a detector with an ultra-low background. In order to achieve this, the XENON detectors are placed deep underground in the Italian Gran Sasso Laboratory (LNGS, in the Abbruzzian mountains) which reduces the rate of cosmic ray muons by six orders of magnitude.
The XENON instruments are dual-phase time projection chambers (TPC) filled with ultra-pure xenon, which has been liquefied by cooling it to about -95C. When a particle interacts with the xenon, it creates scintillation light and liberates electrons by ionization. Both signals, scintillation and ionization, are detected by a large number of photosensors. They are used to determine the energy deposition, the number of interactions in the detector, their 3-dimensional interaction vertex, and whether the particle looks more signal- or more background-like.
The combination of these features, together with a careful detector design and selection of all construction materials, allows achieving very low radioactive background levels - one of the prime advantages of this detector technology.
The XENON collaboration has installed the next-generation dark matter detector XENON1T in November 2015 and is currently in the commissioning phase. XENON1T is about 35x larger than its predecessor XENON100 (in terms of target mass) and uses a total of 3.5t of liquid xenon. The larger mass, together with a very low background, will allow us to increase its sensitivity to dark matter interactions by a factor 100 compared to XENON100, and by a factor 40 compared to the current best results .
The main responsibilities of the Bern group are:
XENON100, the previous stage of the experiment has achieved its sensitivity goal in 2012 [PRL 109, 131801 (2012), arXiv:1207.5988], but did not find any indication of the dark matter particle yet. It is installed in a massive shield made from water (against neutrons), lead (against gamma radiation), polyethylene (against neutrons), and copper (against gamma radiation from the polyethylene). This shielding, together with the sophisticated detector design made XENON100, the previous stage of the project, one of the world's most sensitive dark matter detectors [Astropart. Phys. 35, 573 (2012), arXiv:1107.2155].
XENON is an international collaboration with about 110 members from Switzerland, the USA, Italy, Portugal, Germany, France, the Netherlands, Israel, and China.
Dark Matter and Dual-phase Liquid Xenon Detectors: