Based on cosmological observations like the revolution of stars in galaxies, gravitational lensing at galaxy clusters or the cosmic microwave background, it is thought that the Universe consists to about 23% of Dark Matter, while the proportion of ordinary visible matter is only about 4%. The remainder is the mysterious Dark Energy which is responsible for the acceleration observed in the expansion of the universe.

From a theoretical point of view, weakly interacting massive particles, WIMPs, are the most promising candidates for Dark Matter, since such particles should have formed in the early Universe in the required amount. But the researchers also consider “sterile neutrinos” or particles only interacting gravitationally. This is linked to possible extensions of the Standard Model of elementary particle physics. Furthermore, a global analysis and interpretation of data from various experiments aims to reconcile the to some extent controversial results.

MPIK is involved in the direct search for WIMPs with the XENON100 and the future XENON1T experiments in the Gran Sasso Underground Laboratory in Italy which use ultrapure liquid xenon as the detector medium. The XENON100 detector is capable of measuring in a correlated manner both the scintillation light and the ionization charge emerging from the rare interactions of WIMPs and Xe atoms. The sensitivity will be much higher with the 10-fold amount of Xe in the XENON1T detector under development, which will also be equipped with an improved shielding.

In addition, the H.E.S.S. telescopes look for high-energy gamma rays, possibly produced by the annihilation of Dark Matter particles in the DM halo of the Milky Way.

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Theoretical Astroparticle Physics and Cosmology (pdf)

Low-level techniques are naturally coupled with the experiments looking for rare events, where identification and reduction of the background plays a key role. At the MPIK, there is a long tradition and a lot of expertise in that field. The Institute’s low-level underground laboratory provides shielding against cosmic rays and thus offers ideal conditions for detector development for low-background experiments. Highly sensitive gamma-ray spectrometers and proportional counters serve to check the radiopurity of materials and are the heart of assay techniques for very low concentrations of radioisotopes such as ⁸⁵Kr.

One of the most interfering contaminants, the naturally occurring radioactive radon isotope ²²²Rn, can be efficiently removed from even large gas or liquid samples using the mobile radon extraction unit MoREx. Ultrapure nitrogen, argon and xenon are essential for neutrino and Dark Matter detectors as well as for double-beta decay experiments.

MPIK scientists have developed the liquid gadolinium-containing scintillator for the neutrino detector Double Chooz. In a specially designed test rig, they characterize the photomultipliers that register the scintillation light produced by rare interactions of neutrinos or Dark Matter particles with the atoms of detector liquids.

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Neutrinos are electrically neutral elementary particles of tiny mass which occur as three different types, so-called flavours. Besides photons, they are the most abundant particles in the Universe, but we don’t recognize them as they interact only rarely with matter. Thus, large and sensitive detectors with excellent shielding against background signals are required to detect them.

In the Gran Sasso Underground Laboratory in Italy, the GERDA experiment searches for the neutrinoless double beta decay in pure germanium crystals enriched with the isotope ⁷⁶Ge. Neutrinoless double-beta decay, should it be possible, is an extremely rare event. If detected, this would mean that neutrinos are their own antiparticles – so-called Majorana particles – implying considerable theoretical consequences. Candidate isotopes for the reverse process, neutrinoless double electron capture, are scrutinized by precisely determining their mass and that of their daughter nuclides.

GERDA will in addition provide information about the rest mass of neutrinos, for which until today only upper limits and differences are known. Another approach to determine neutrino masses is the extremely precise measurement of the mass difference between ³H (tritium) and ³He together with the KATRIN experiment.

The periodic changeover between the three neutrino flavours electron, muon and tauon neutrino (“neutrino oscillations”) is described by so-called mixing angles. The Double Chooz experiment uses electron antineutrinos from a nuclear power plant in France to measure the yet unknown one of the three mixing angles. The two identically designed detectors at different distances from the reactors are sensitive only for electron antineutrinos, the number of which might decline from the near to the far detector due to the oscillations. The software GLoBES has been developed and is applied as a simulation tool for future high-precision neutrino oscillation experiments.

Since 2007 the Borexino experiment is investigating low-energy neutrinos from the Sun and the Earth. The real-time view into the core of the Sun helps to understand the fusion processes in stars and provides valuable information about neutrino oscillations. Borexino also detected geoneutrinos from radioactive decay in the Earth’s interior which contributes significantly to the geothermal heat.

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GERDA: Are Neutrinos and Antineutrinos identical? (pdf)
Double Chooz: Search for the Third Mixing Angle of the Neutrinos (pdf)
Borexino: Spectroscopy of Solar Neutrinos (pdf)