Christian Stegmann, director of DESY’s newly established research division for astroparticle physics, describes the ambitious plans ahead in this vibrant field.
What is your definition of astroparticle physics?
There is no general definition, but let me try nevertheless. Astroparticle physics addresses astrophysical questions through particle-physics experimental methods and, vice versa, questions from particle physics are addressed via astronomical methods. This approach has enabled many scientific breakthroughs and opened new windows to the universe in recent years. In Germany, what drives us is the question of the influence of neutrinos and high-energy processes in the development of our universe, and the direct search for dark matter. There are differences to particle physics both in the physics questions and in the approach: we observe high-energy radiation from our cosmos or rare events in underground laboratories. But there are also many similarities between the two fields of research that make a fruitful exchange possible.
What was your path into the astroparticle field?
I grew up in particle physics: I did my PhD on b-physics at the OPAL experiment at CERN’s LEP collider and then worked for a few years on the HERA-B experiment at DESY. I was not only fascinated by particle physics, but also by the international cooperation at CERN and DESY. Particle physics and astroparticle physics overcome borders, and this is a feat that is particularly important again today. Around 20 years ago I switched to ground-based gamma astronomy. I became fascinated in understanding how nature manages to accelerate particles to such enormous energies as we see them in cosmic rays and what role they play in the development of our universe. I experienced very closely how astroparticle physics has developed into an independent field. Seven years ago, I became head of the DESY site in Zeuthen near Berlin. My task is to develop DESY and in particular the Zeuthen site into an international centre for astroparticle physics. The new research division is also a recognition of the work of the people in Zeuthen and an important step for the future.
What are DESY’s strengths in astroparticle research?
Astroparticle physics began in Zeuthen with neutrino astronomy around 20 years ago. It has evolved from humble beginnings, from a small stake in the Lake Baikal experiment to a major role in the km3-sized IceCube array deep in the Antarctic ice. Having entered high-energy gamma-ray astronomy only a few years ago, the Zeuthen location is now a driving force behind the next-generation gamma-ray observatory the Cherenkov Telescope Array (CTA). The campus in Zeuthen will host the CTA Science Data Management Centre and we are participating in almost all currently operating major gamma-ray experiments to prepare for the CTA science harvest. A growing theoretical group supports all experimental activities. The combination of high-energy neutrinos and gamma rays offers unique opportunities to study processes at energies far beyond those reachable by human-made particle accelerators.
Why did DESY establish a dedicated division?
A dedicated research division underlines the importance of astroparticle physics in general and in DESY’s scientific programme in particular, and offers promising opportunities for the future. Astroparticle physics with cosmic messengers has experienced a tremendous development in recent years. The discovery of a large number of gamma-ray sources, the observation of cosmic neutrinos in 2013, the direct detection of gravitational waves in 2015, the observation of the merger of two neutron stars with more than 40 observatories worldwide triggered by its gravitational waves in August 2017, and the simultaneous observation of neutrinos and high-energy gamma radiation from the direction of a blazar the following month are just a few prominent examples. We are on the threshold of a golden age of multi-messenger astronomy, with gamma rays, neutrinos, gravitational waves and cosmic rays together promising completely new insights into the origins and evolution of our universe.
What are the division’s scale and plans?
The next few years will be exciting for us. We have just completed an architectural competition, new buildings will be built and the entire campus will be redesigned in the coming years. We expect well over 350 people to work on the Zeuthen campus, and hosting the CTA data centre will make us a contact point for astroparticle physicists globally. In addition to the growth through CTA, we are expanding our scientific portfolio to include radio detection of high-energy neutrinos and increased activities in astronomical-transient-event follow-up. We are also establishing close cooperation with other partners. Together with the Weizmann Institute in Israel, the University of Potsdam and the Humboldt University in Berlin, we are currently establishing an international doctoral school for multi-messenger astronomy funded by the Helmholtz Association.
How can we realise the full potential of multi-messenger astronomy?
Our potential lies primarily in committed scientists who use their creativity and ideas to take advantage of existing opportunities. For years we have experienced a large number of young people moving into astroparticle physics. We need new, highly sensitive instruments and there is a whole series of outstanding project proposals waiting to be implemented. CTA is being built, the upgrade of the Pierre Auger Observatory is progressing and the first steps for the further upgrade of IceCube have been taken. The funding for the next generation of gravitational-wave experiments, the Einstein Telescope in Europe, is not yet secured. We are currently discussing a possible participation of DESY in gravitational-wave astronomy. Multi-messenger astronomy promises a breathtaking amount of new discoveries. However, the findings will only be possible if, in addition to the instruments, the data are also made available in a form that allows scientists to jointly analyse the information from the various instruments. DESY will play an important role in all these tasks – from the construction of instruments to the training of young scientists. But we will also be involved in the development of the research-data infrastructure required for multi-messenger astronomy.
I was not only fascinated by particle physics, but also by the international cooperation at CERN and DESY
How would you describe the astroparticle physics landscape?
The community in Europe is growing. Not only in terms of the number of scientists, but also the size and variety of experiments. In many areas, European astroparticle physics is in transition from medium-sized experiments to large research infrastructures. CTA is the outstanding example of this. The large number of new scientists and the ideas for new research infrastructures show the great appeal of astroparticle physics as a young and exciting field. The proposed Einstein Telescope will cross the threshold of projects requiring investments of more than one billion Euros, requiring coordination at European and international level. With the Astroparticle Physics European Consortium (APPEC) we have taken a step towards improved coordination. DESY is one of the founding members of APPEC and I have been elected vice-chairman of the APPEC general assembly for the next two years. In this area, too, we can learn something from particle physics and are very pleased that CERN is an associate member of APPEC.
What implication does the update of the European strategy for particle physics have for your field?
European astroparticle physics provides a wide range of input to the European Strategy for particle physics, from concrete proposals for experiments to contributions from national committees for astroparticle physics. The contribution to the construction of the Einstein Telescope deserves special attention, and my personal wish is that CERN will coordinate the Einstein Telescope, as suggested in the contribution. With the LHC, CERN has again demonstrated in an outstanding way that it can successfully implement major research projects. With the first gravitational- wave events, we saw only the first flashes of a completely unknown part of our universe. The Einstein Telescope would revolutionise our new view of the world.
