Dark Matter – Structure Forming Agent in the Universe
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 27% of Dark Matter (DM), while the fraction of ordinary visible matter is only about 5%. 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, so-called WIMPs, are promising candidates for Dark Matter, since such particles should have formed in the early Universe in the required amount. But the researchers also consider ‘axions’, ‘sterile neutrinos’ or particles only interacting gravitationally. This is linked to possible extensions of the standard model of elementary particle physics. Furthermore, global analyses, model building and interpretation of data from different experiments aims at a global picture and at resolving controversial results.
MPIK is involved in the direct search for WIMPs with the XENON1T experiment in the Gran Sasso Underground Laboratory in Italy which uses ultrapure liquid xenon as the detector medium. The 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 (right-hand side of the cube in the title picture). Presently, XENON1T is the most sensitive such experiment deeply probing the expected parameter regions where WIMPs and other Dark Matter candidates are expected. In parallel, a larger detector, XENONnT, already is under construction. As it will use the same infrastructure, a rapid extension of the sensitivity will be possible.
In addition, the H.E.S.S. telescopes look for high-energy gamma rays, produced by the annihilation of DM particles in the DM halo of the Milky Way. While Dark Matter searches have reached a sensitivity where one could hope to detect WIMP signatures, none of the detectors has so far seen a signal.
Low Level Techniques
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 is the naturally occurring radioactive radon isotope 222Rn. It can be efficiently removed from even large gaseous or liquid samples using the mobile radon extraction unit MoREx. Ultrapure nitrogen, argon and xenon are essential for neutrino and Dark Matter experiments. “Auto-Ema” measures fully automatically the outgassing of radon from solid materials, thus facilitating material selection and tests of methods to suppress the outgassing.
Neutrinos – Particles with Striking Properties
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 notice them as they interact only rarely with matter. Thus, sensitive detectors with excellent shielding against background signals are required to detect them.
A neutron inside a nucleus beta-decays to a proton, an electron and an antineutrino leading to another element. Some atomic nuclei, one of them the germanium isotope 76Ge, are not subject to the single but instead the double-beta decay: two neutrons are decaying at the same time with either two or no neutrino. The GERDA experiment searches for the neutrinoless double-beta decay in pure germanium crystals enriched with 76Ge. Neutrinoless double-beta decay, should it be possible, is an extremely rare event. Until now, no evidence for the decay was found – only that its half-life in 76Ge must be at least 1026 years. Scientists hope to find it some day using a larger amount of germanium and after a long measurement time – eventually not until the successor project LEGEND. Then neutrinos would proven to be their own antiparticles, so-called Majorana particles, making it possible to deduce their mass.
For the rest mass of neutrinos only limits and differences are known to date. Other experiments to determine the neutrino mass rely on the capture of an electron by a proton in a nucleus. Therefore, the knowledge of the exact mass difference between mother and daughter nucleus is required. A group at the MPIK is performing such precision measurements.
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 long sought-after one of the three mixing angles. The two identically designed detectors with liquid gadolinium-containing scintillator at different distances from the reactors are sensitive only to electron antineutrinos, the number of which declines from the near to the far detector due to the oscillations. The results confirm that also this mixing angle has a nonzero value which means that all oscillations take place.
Indeed, many experiments in the vicinity of nuclear power plants detect about 6% less neutrinos than expected. The STEREO detector tries to find out whether sterile, i. e. non-interacting, neutrinos might be responsible for this reactor neutrino anomaly.
The CONUS experiment also uses reactor neutrinos to investigate the coherent neutrino-nucleus scattering – scattering of neutrinos at the nucleus as a whole. Highly pure germanium detectors with very low energy threshold measure the tiny energy transfer due to this scattering process, which, however, is significantly more probable than the interaction of neutrinos with electrons.
GERDA: Are Neutrinos and Antineutrinos identical? (pdf)
Double Chooz: The Third Mixing Angle of the Neutrinos (pdf)
STEREO: Are sterile neutrinos the explanation for the reactor antineutrino anomaly? (pdf)
CONUS: Detecting coherent neutrino-nucleus scattering (pdf)