The topic Matter and Radiation from the Universe is a wide and diverse field of research ranging from the study of high-energy particles from the Universe and the measurement of fundamental neutrino properties to the search for Dark Matter particles. The topic documents the common effort to understand the role of these particles in the evolution of the Universe at different length, time and energy scales. In Germany the topic is part of astroparticle physics, which developed under this label since about the year 2000; the Helmholtz Alliance for Astroparticle Physics has been active since 2011, see here. The previous and still ongoing Helmholtz programme has shaped astroparticle physics in Germany and on an international level, and strengthened the European coordination. The Helmholtz Association is a leading organisation and KIT and DESY are arguably the two most powerful proponents in this research area, at least in Europe. In the next programme period, we will further develop our pivotal role. The research of this topic covers three key aspects of astroparticle physics, i.e., the non-thermal Universe, neutrino physics, and Dark Matter.
The non-thermal Universe
The Earth is bombarded by high-energy particles, so-called cosmic rays, with energies spanning 11 orders of magnitude up to macroscopic energies of more than 1020 eV. Cosmic rays are messengers from the non-thermal Universe, governed by high-energy phenomena. In contrast to what has been expected in the past, high-energy phenomena occur in the life cycle of many cosmic objects. To date the sources of cosmic rays are largely unknown and the role of high-energy phenomena in the evolution of cosmic objects is unclear. The enormous energy and flux range from 1 particle/cm2/s at GeV energies down to 1 particle/km2/century at energies of 1019 eV calls for a variety of different instrumental techniques in order to measure cosmic rays. Cosmic rays are charged particles being deflected in interstellar and intergalactic magnetic fields. Neutral particles, like gamma rays and neutrinos, being produced in the sources of cosmic rays, and ultra-high energy cosmic rays are the only messengers to identify the sources and to study the acceleration processes and propagation of cosmic rays in the Universe. Within this topic we are, as prominent members in world-leading collaborations, in the unique position to cover experimentally the full energy range of cosmic rays above GeV energies in a single programme. We perform gamma-ray astronomy up to energies of 100TeV with the existing instruments H.E.S.S., MAGIC and VERITAS and in the future with the Cherenkov Telescope Array CTA, we measure high-energy neutrinos well beyond PeV energies with the IceCube neutrino observatory, and measure ultra-high energy cosmic rays with the Pierre Auger Observatory. The upcoming programme period will be characterised by an extensive scientific harvest from the Auger Observatory, IceCube and the gamma-ray instruments in a multi-messenger context.
Gamma ray astronomy — towards an open observatory
In recent years, existing gamma-ray instruments have shown breakthroughs not only in the number and variety of detected sources but also in quality of the data. With the existing instruments we are able to measure the morphology of extended sources, and their variability from ms to years, to determine source positions with arc-second precision and to measure energy spectra over three decades. The spectacular results from the current instruments have generated the considerable interest to build the Cherenkov Telescope Array (CTA), which will be a major step forward compared to the current instruments in terms of sensitivity, energy range, and angular resolution. It is the key project of DESY which already constitutes the strongest group in the CTA consortium. In the next funding period DESY will, together with two Max Planck Institutes and partners from Universities, focus on the construction and commissioning of CTA while preparing the scientific harvest of CTA through active participation in the running of gamma-ray instruments.
Neutrino astronomy — a breakthrough
The recent first detection of high-energy cosmic neutrinos in the PeV range by the IceCube neutrino observatory at the South Pole marks the breakthrough of neutrino astronomy. The IceCube collaboration, with DESY as the second largest group, has shown that cosmic high-energy neutrinos exist and that the ice at the South Pole is a unique environment for their precision measurement. The IceCube collaboration is preparing for a long-term programme at the South Pole. The first step of the programme is a low threshold array, called PINGU, with which a measurement of the so-far unknown neutrino mass hierarchy comes within reach. In the long term, an extension of the 1 km3 IceCube detector with a fiducial volume of up to 12 km3 is planned. DESY intends to participate in the preparation, construction and operation of the IceCube extension with substantial investments in instrumentation and manpower.
The highest energy particles from the Universe — heavier than expected?
The Pierre Auger Observatory is the largest ever cosmic-ray detector field. In recent years, the Auger Observatory has measured the cosmic-ray spectrum with unprecedented precision. A clear suppression of the cosmic-ray spectrum at the highest energies was measured. But, together with the surprising result of a mass composition with more heavy elements than expected, the origin of the highest energy cosmic rays remains a mystery. Therefore, the Auger Collaboration is preparing for an instrumental upgrade programme (Auger2023), which addresses the optimisation of particle identification. KIT and the German university partners are committed to making crucial contributions to this endeavour.
Neutrino properties — about to be measured
Neutrinos play a key role for our understanding of the Universe at the largest scales, as truly enormous numbers of neutrinos have been produced in the Big Bang. Due to their non-zero rest mass, these relic neutrinos play a distinct role as Hot Dark Matter in the evolution of large-scale structures. Neutrinos are also by far the lightest elementary fermions in nature and their small rest mass, expected to be in the sub-eV range, is widely regarded as first evidence for physics beyond the SM. The measurement of their unknown absolute mass scale and mass pattern thus allows us to probe novel mass-generating mechanisms in nature. With the KATRIN experiment at KIT we will conduct a worldwide-unique experiment which will measure the absolute neutrino mass scale in a model-independent way in tritium β-decay with unprecedented precision. By improving the neutrino mass sensitivity down to 200 meV/c2 the entire range of quasi-degenerated mass scenarios can be covered. When comparing the KATRIN result with other, model-dependent methods such as the search for neutrinoless double beta decay and cosmological studies, surprises can be anticipated in view of a long list of previous unexpected results in neutrino physics. The experiment is extremely challenging from the technological point of view due to required intensity and stability levels of the tritium source, the required energy resolution and the unprecedented control of systematic effects. The long-term expertise in tritium handling of the Tritium Laboratory Karlsruhe means that KIT is the only place (at least in Europe) where this experiment could be performed. KIT has bundled its efforts in a large multi-disciplinary team which has taken over core responsibilities and leads the commissioning and operation of the experiment, and provides crucial contributions to data analysis, modelling and simulations. The spectrometer has recently been commissioned with excellent initial performance, and the entire experiment will start its long-term measurements within the upcoming programme period to provide a scientific harvest in the form of high-sensitivity sub-eV neutrino-mass results.
Dark Matter searches — the need for sensitivity
The Standard Model of cosmology requires large amounts of non-baryonic Dark Matter, which is not fully understood. We aim to detect such Weakly Interacting Massive Particles (WIMPs) by direct collisions with detector nuclei and indirectly by excess fluxes originating from WIMP annihilation. The EDELWEISS experiment is situated at LSM (The "Laboratoire Souterrain de Modane", LSM, is located in the Frejus tunnel between Italy and France) and uses Germanium bolometers for direct detection. It has increased its sensitivity to WIMP scattering by more than a factor of 20 within the last five years. The future of this technique lies in an upscaled detector array of one tonne of target material, most likely in a joint endeavour with the US-led SuperCDMS collaboration. The opportunity of detecting DM with indirect searches will increase significantly due to an order-of-magnitude boost in sensitivity through the construction of the CTA observatory and through future observations and refinements of analysis techniques in existing observatories like AMS-02, IceCube, MAGIC, H.E.S.S., VERITAS.
Theory and modelling — a glue between experiments
Theoretical work is an important link between the various research activities. We strive to strengthen the in-house analysis and modelling of experimental data and the physics interpretation. As a consequence, professorships and working groups for astroparticle theory have been established at DESY and KIT.