From April 2008 to December 2013, the Helmholtz-University Young Investigators Group Development of a Next Generation Hybrid Detector Concept for the Pierre Auger Observatory (VH-NG-413) worked at the Karlsruhe Institute of Technology under the leadership of Tim Huege as principal investigator. The following gives a short summary of the group's scope:

Members of the Helmholtz-University Young Investigators Group at the LOPES experiment in KarlsruheEven almost 100 years after the discovery of cosmic rays, fundamental questions regarding their nature and origin are still unanswered. This especially applies to the ultra-high energy cosmic rays (UHECRs) above ~10^19 eV, which should directly point back to their sources, yet are extremely difficult to observe due to their scarcity (down to less than one particle per km^2 and century). The Pierre Auger Observatory, currently covering a collecting area of ~3000 km^2 in the southern hemisphere, measures the energy spectrum and elemental composition of UHECRs. With the planned addition of the complementary site in the northern hemisphere, the next challenge will be to locate and study the UHECR sources using full-sky particle astronomy.

So far, UHECRs are measured with particle detectors and optical fluorescence telescopes. The two techniques complement each other in a powerful way in so-called "hybrid" observations. A novel detection technique, the registration of pulsed radio emission emanating from cosmic ray air showers, has recently demonstrated its great potential. In particular the successes of the pioneering LOPES experiment, situated at Campus North of the Karlsruhe Institute of Technology, have triggered intense research activities in the field. As the radio signal is sensitive to the longitudinal development of an air shower, the information contained in it is complementary to that gathered by particle detectors, making it well-suited for hybrid measurements. In addition, radio detection works 24 hours a day, while the fluorescence technique is limited to only ~10% duty cycle. A combination of particle detectors and radio antennas can thus increase hybrid statistics by a factor of ten. At the same time, the radio technique promises to instrument large effective areas at moderate cost, and ideally suits the goal of localising point sources due to its supreme angular resolution.

Several groups involved in the Pierre Auger Observatory have recently begun research and development efforts to adapt the radio technique to the large scales necessary for application to ultra-high energy cosmic rays. The Helmholtz Association has commited considerable resources to this effort in the form of a dedicated Helmholtz-University Young Investigators Group, thereby making important contributions to the long-term goal of identifying the sources of the highest-energy cosmic rays.

The investigators group develops a Next Generation Hybrid Detector Concept for the Pierre Auger Observatory based on the radio technique, bringing the current and future efforts for full-sky particle astronomy a significant step forward. The group addresses both technological and scientific challenges posed by a large scale application of the radio technique in hybrid measurements with particle detectors. Technological challenges include the development and optimisation of a radio detector design that can operate unattended for extended periods of time with reliable absolute calibration, the development of suitable trigger strategies for the independent operation of a radio detector array, and the overall optimisation of array parameters for a large scale cost-effective application. Scientific goals of the project are the development of detailed analysis procedures relating hybrid surface and radio detector measurements to the cosmic ray parameters of interest, the study of systematics in the direction reconstruction using a hybrid radio and surface detector, and advanced modelling of the underlying radio emission physics with Monte Carlo simulations.