Dark Matter – Structure Forming Agent in the Universe

Light detectors (photomultiplier tubes) for XENON1T, left without casing and entrance window.

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 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 ‘axions’, ‘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 experiments XENON100 and from fall 2015 XENON1T in the Gran Sasso Underground Laboratory in Italy which use ultrapure liquid xenon as the detector medium. The detectors are 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. Compared to XENON100, the sensitivity will be much higher with the 10-fold amount of Xe in XENON1T and even more with its extension to XENONnT, which are also 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.

Division Lindner   Division Hofmann

Theoretical Astroparticle Physics and Cosmology (pdf)
The XENON Project - Enlightening the Dark (pdf)
H.E.S.S.: Cosmic Accelerators in the Light of Gamma Rays (pdf)

Low Level Techniques

Schematic drawing of the GIOVE germanium spectrometer in MPIK’s underground laboratory showing the layers of shielding materials.

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 detectors Double Chooz as well as NUCIFER and STEREO. 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.

Division Lindner

Neutrinos – Particles with Striking Properties

The GERDA double beta decay experiment.

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. In the first measurement phase no evidence for the decay was found yielding the world-leading lower limit of its half-life in 76Ge of 2.1 × 1025 years. Should it be once detected, the decay would mean that neutrinos are their own antiparticles – so-called Majorana particles – making it possible to deduce their mass and 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.

For the rest mass of neutrinos to date 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 long sought-after one of the three mixing angles. The two identically designed detectors at different distances from the reactors are sensitive only to electron antineutrinos, the number of which might decline from the near to the far detector due to the oscillations. The first results confirm that also this mixing angle has a nonzero value which means that all oscillations take place.

However, many experiments in the vicinity of nuclear power plants detect about 6% less neutrinos than expected. The NUCIFER and STEREO detectors try to find out whether sterile neutrinos are responsible for this reactor neutrino anomaly.

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 has confirmed the theoretical prediction for all neutrinos formed in the multistage fusion process and provides information about neutrino oscillations. Borexino also detected geoneutrinos from radioactive decay in the Earth’s interior which contributes significantly to geothermal heat.

Division Lindner    Division Hofmann    Division Blaum

GERDA: Are Neutrinos and Antineutrinos identical? (pdf)
Double Chooz: The Third Mixing Angle of the Neutrinos (pdf)
Borexino: Spectroscopy of Solar Neutrinos (pdf)
Nucifer + Stereo: Are sterile neutrinos the explanation for the reactor neutrino anomaly? (pdf)

The Origin of Mass – Physics Beyond the Standard Model

Feynman diagram of a lepton-flavour violating process; in the background the ATLAS detector at the LHC of CERN.

The Standard Model of elementary particle physics successfully describes the behaviour of all known elementary particles (and corresponding antiparticles): each 6 quarks (two of them form protons and neutrons) and leptons (among them electrons and neutrinos). In addition, there are gauge bosons (among them photons and gluons) mediating the particle’s interactions, and the Higgs boson. Its discovery on 2012 opened a number of fundamental questions that are addressed by theoreticians at the MPIK.

Both Dark Matter and the proof of neutrino masses as well as some theoretical deficiencies require an extension of the Standard Model of elementary particle physics which seems to be valid only up to a certain energy, from which on so-called new physics comes into play. Supersymmetry and Grand Unified Theory are studied as promising extensions of the Standard Model in connection to present and future particle physics experiments, and cosmology.

A lot of theoretical work is done at MPIK on the origin of neutrino masses and mixings via basic and phenomenological studies. The so-called seesaw mechanism is a way to explain the smallness of neutrino masses based on the presence of new heavy particles, which are in fact predicted by many theories beyond the Standard Model. Neutrino masses and Dark Matter may have a common origin. The overall aim is a deeper understanding of the fundamental laws of nature.

Division Lindner    Group Rodejohann    Group Goertz

Theoretical Elementary Particle Physics beyond the Standard Model (pdf)



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