The scientific and technological challenges that arise in addressing the fundamental open questions in the study of matter and radiation from the Universe are formidable indeed and require close inter-topic and cross-topic cooperation.
The performance of current instruments measuring cosmic messengers is still far from fundamental limits imposed by their respective detection principle. Challenges arise from the need for an improved flux sensitivity and background suppression in order to increase the sensitivity of the instruments to detect even fainter fluxes of cosmic messengers. Better energy, charge and angular resolutions are needed for high-precision spectra and imaging of cosmic-ray sources with neutral particles and for the identification of the cosmic-ray composition with charged particles. An improved understanding of atmospheric properties for air showers, and of ice and earth properties for high-energy neutrino measurements is needed to reduce systematic uncertainties. The efficient and failsafe operation of the instruments at remote locations poses an additional challenge. Given the different stages of the different instruments the above mentioned challenges are tackled differently. While the next step in gamma-ray astronomy is an open observatory requiring progress in all areas, the next step in neutrino astronomy is an increase in the flux sensitivity, i.e., a step from the present 1 km3 instrumented volume to an even larger multi-km3 observatory. For the detection of ultra-high cosmic rays the next step is an improved composition measurement and to establish new detection methods like radio as viable techniques.
The model-independent measurement of the neutrino mass with a sensitivity of 200 meV/c2 is challenging due to the fact that the resulting spectral modifications become exceedingly small and confined to a narrow region close to the β-decay endpoint, where statistics is limited. To counterbalance this, an increase in source intensity by two orders of magnitude is necessary. Technologically, the implementation of a high luminosity windowless gaseous tritium source is challenging, both with regard to the required yearly tritium throughput on the kg-scale, and the unprecedented levels of stability and isotopic purity to decrease systematics by one order of magnitude. Likewise, to obtain high precision β-spectroscopy, the energy resolution at the endpoint has to be increased fourfold, entailing a huge upscale of the spectrometer dimensions. Other challenges arise from the decoupling of the enormous source intensity from the spectrometer, which has to be operated with a low intrinsic background rate. Taken altogether, these physics-driven demands make the KATRIN experiment extremely challenging and require the development of many technologies beyond the present state of the art. When addressing the unknown neutrino mass pattern with GeV-scale atmospheric neutrinos, the major challenge in distinguishing different scenarios is closely related to the energy resolution and precision of event reconstruction which could be achieved by the proposed IceCube in-fill array PINGU.
In the search for DM, the key challenge will be to identify its particle origin and properties such as WIMP mass and interaction cross sections. This can only be achieved by cross-topic cooperation activities, where LHC information on the production of supersymmetric particles is combined with direct WIMP detection experiments and indirect searches for DM annihilation, decay or interaction processes in the Universe The challenge in direct detection experiments will be to improve the current sensitivity to WIMP interaction cross sections with matter by more than one order of magnitude, calling for essentially background-free detection methods and target masses in excess of 100 kg. In view of current experimental hints for low mass WIMPs at around 10 GeV from laboratory experiments and for higher mass WIMPs beyond 100 GeV from satellite missions (AMS-02, Fermi satellite), the DM sector may be more complex and even consist of several particle species, calling for different detection techniques and target nuclei. Cryogenic bolometers offer ideal properties for low mass WIMPs in particular due to their low threshold and low intrinsic background. The next-generation cryogenic experiment on the ton scale will require combining efforts on a worldwide level in a suitable underground laboratory. Likewise, the improvement of the sensitivity of indirect searches requires a new ground-based gamma-ray observatory, like CTA, as well as the extension of the IceCube detector together with long-term operation of AMS-02 on the ISS.