Despite strong pressure on the budget for education and research, astroparticle physics in Germany is becoming a strong and autonomous branch of science, as Axel Lindner explains.
On 16-18 September 2003 German astroparticle physicists and ministry representatives met at the University of Karlsruhe to discuss recent scientific advances, future funding and organizational support by the German Ministry of Education and Research (BMBF). The Karlsruhe workshop was the third in a series initiated by BMBF deputy director-general Hermann-Friedrich Wagner to maintain a close contact between scientists and the ministry. These open discussions have allowed each side to understand the other’s needs better, and have led to the very fast and fruitful development of astroparticle physics in Germany.
As in the previous workshops, which took place in 1999 and 2001 at DESY Zeuthen, high-energy and nuclear physicists, astronomers and astrophysicists also joined and participated in lively cross-disciplinary debates. The large increase in the number of participants (rising from 57 and 124 in 1999 and 2001, respectively, to more than 240 in 2003) reflects the growing interest in astroparticle physics.
The workshop began with special lectures for students, and Werner Hofmann of MPI for Nuclear Physics, Heidelberg, gave an entertaining and surprising evening talk on two very different fictional futures of high-energy physics. Scientific achievements, future prospects and new ideas were then presented in sessions dedicated to selected astroparticle-physics topics.
New astronomies
Wolfgang Rhode of Wuppertal and Jürgen Hößl from Erlangen-Nürnberg reported on the ongoing activities related to large-volume neutrino detectors in the Antarctic ice shield and in natural water in Lake Baikal and the Mediterranean sea. Results from AMANDA at the South Pole and BAIKAL are already beginning to constrain theoretical models on dark-matter annihilations, magnetic monopoles and astrophysical high-energy neutrino production. AMANDA has shown the first ever 100 GeV neutrino sky map and thus opened a new window for astrophysics (figure 1). As expected from the detector’s moderate sensitivity, the sky map does not show evidence for extra-terrestrial neutrino sources, but is compatible with the predicted neutrino production by charged cosmic rays in the Earth’s atmosphere. Based on this proof of principle, the significantly larger ICECUBE project is now under way. The first photomultiplier strings for the 1 km3 detector will be deployed during the next Antarctic summer, in 2004/2005. The installation of all the strings will be finished in 2010. The ANTARES collaboration plans to install 12 detector strings in the Mediterranean by 2006 to test the experimental concept and address the first astrophysical questions. The possibility of realizing a km3 detector in the Mediterranean seems, however, to depend not only on technology and engineering, but also on the strong will of the ANTARES, NEMO and NESTOR collaborations to join behind one common proposal.
Götz Heinzelmann of Hamburg gave an introduction on high-energy gamma astronomy, focusing on the results of the imaging air Cherenkov telescopes (IACTs) of the HEGRA (High Energy Gamma Ray Astronomy) detector on La Palma in the Canary Islands. The pioneering work of HEGRA on new experimental techniques and analysis methods has been extremely fruitful. Although the experiment shut down in 2002, surprising results have since been announced: TeV gamma rays from the shell-type supernova remnant Cassiopeia A may indicate that this source accelerates nucleons to relativistic energies. This observation could be a key to the solution of the 90-year-old mystery of the acceleration sites of cosmic rays. The HEGRA collaboration also presented the first TeV gamma source unidentified at other wavelengths (figure 2), thereby giving an insight into a previously unknown area of the relativistic universe.
Many even more exciting results are expected with the new generation of IACTs now coming into operation. These experiments will be an order of magnitude more sensitive and will provide a reduced energy threshold. Werner Hofmann presented first results of the HESS (High Energy Stereoscopic System) detector in Namibia, and Florian Goebel from MPI for Physics, Munich, reported on the status of the MAGIC telescope at La Palma. Measurements of the cosmologically important extragalactic background light, of dark-matter annihilations, and perhaps even of hints on quantum gravitation are to be expected. Roland Diehl and Gottfried Kanbach, both from MPI for Extraterrestrial Physics, Garching, Martin Merck of Würzburg, Hinrich Meyer of Wuppertal and Masahiro Teshima from MPI for Physics, complemented the session with talks on gamma-ray astronomy with satellites, new ideas for IACTs, and the ultra-high-energy cosmic-ray detector, EUSO, which will be based on the International Space Station (ISS).
Charged cosmic rays
Air shower experiments on the energy spectrum and mass composition of charged cosmic rays focus on the so-called knee region around 1015 eV, where the slope of the energy spectrum suddenly changes, and on the highest energies beyond 1019 eV. Peter Biermann from MPI for Radioastronomy, Bonn, summarized the current theoretical models, while Andreas Haungs from the Research Centre Karlsruhe explained the still puzzling experimental situation around the knee. Data from the KASCADE experiment at Karlsruhe hint at a knee position proportional to the charge, Z, of the nuclei in cosmic rays, but uncertainties in the interpretation of the air shower data remain. At present no simulation is able to provide a consistent description of all KASCADE’s data on particle interactions in the atmosphere; more multi-parameter data are required. In recent years KASCADE has been expanded to KASCADE Grande (covering 640,000 m2), which will extend the accessible energy range up to 3 x 1017 eV. KASCADE Grande should be able to identify the knee for iron-like nuclei and hence prove the above-mentioned Z dependence. This would strongly support the assumption of cosmic-ray acceleration in the shells of supernova remnants – as is also implied by the HEGRA Cherenkov telescope data on Cassiopeia A.
Karl-Heinz Kampert of Wuppertal reported on the experimental situation at the highest cosmic-ray energies. Data from the Akeno Giant Air Shower Array (AGASA) in Japan provide strong evidence for the existence of cosmic rays with energies beyond the Greisen-Zatzepin-Kuzmin (GZK) cut-off – the energy threshold of pion production due to the interaction of very energetic protons with the cosmic 2.7 K background radiation. By contrast, data from the HIRES detector in the US are also compatible with the existence of the GZK cut-off. This discrepancy should be resolved by the 3000 km2 AUGER experiment in Argentina. AUGER will provide much larger event statistics at the highest energies and combine the experimental techniques of AGASA and HIRES to minimize systematic uncertainties. The first data for extended air showers have already been taken successfully, while the production of detector components for AUGER will be finished in 2005. Hans Klages from the Research Centre Karlsruhe described possible expansions of AUGER on its southern and northern sites and stressed the sensitivity of AUGER to neutrino-induced horizontal air showers.
Heino Falcke from MPI for Radioastronomy and the University of Nijmegen, reported on a prototype set-up to detect radio emission from air showers. This project was launched after a discussion at the 2001 astroparticle workshop at DESY Zeuthen, and combined data taking with KASCADE has just begun. With a proof of principle at KASCADE, radio detection of air showers would allow the realization of inexpensive and very extended air shower experiments. Rolf Nahnhauer of DESY presented studies on the acoustic detection of neutrino interactions in ice and water, which potentially would also make extended and inexpensive set-ups possible. Manfred Simon of Siegen closed this session with a summary of the status of the PAMELA experiment, which is to be launched in 2004.
Cosmology and dark matter
Hans Böhringer from MPI for Extraterrestrial Physics opened the session on cosmology and dark matter with a talk on the astrophysical evidence for the existence of dark matter. The recent detailed data of the WMAP satellite on cosmic background radiation rule out the as-yet alternative scenarios of modified Newtonian dynamics (MOND) models. Wolfgang Seidel from MPI for Physics then summarized experimental attempts to detect dark-matter particles directly. The positive evidence published by the DAMA collaboration is still controversial. New experiments with event-by-event background suppression through the simultaneous measurement of heat and ionization are beginning to overtake the sensitivity of older large-mass detectors. In the near future an increase in sensitivity by two orders of magnitude will be reached by different experiments. Wim de Boer of Karlsruhe hinted at a surprising concordance: both the not very well understood galactic emission of GeV photons and the measured amount of positrons and antiprotons can be described very well hypothetically if neutralino annihilations are taken into account together with standard astrophysics. However, experimental uncertainties and a lack of knowledge about the astrophysical sources in the galaxy still prevent firmer conclusions on the existence of supersymmetric particles. Next-generation dark-matter experiments, the indirect searches with IACTs, and the AMS-II experiment on board the ISS will help to clarify the situation.
Dieter Hoffmann from the Technical University of Darmstadt presented the successful start-up of the CAST experiment at CERN, which is looking for axions from the Sun. This experiment is an example of the symbiosis of different branches of physics in astroparticle physics: CAST uses an old prototype magnet for the Large Hadron Collider, combined with particle-physics detectors and X-ray techniques from space-born experiments. Finally, Jens Niemeyer of Würzburg presented the current understanding of the most mysterious ingredient of our universe, dark energy.
Further topics
Christian Weinheimer of Bonn began the session on neutrino masses and low-energy neutrino astronomy with an overview on the limits on neutrino masses derived from astrophysical observations. Although new astrophysical data are already quite sensitive to neutrino properties, firm model-independent measurements of the neutrino mass require analysis of the endpoint of the energy spectrum in beta decays. Here the KATRIN experiment at Karlsruhe, which should start up in 2007, will improve the sensitivity of the current experiments by an order of magnitude, to 0.2 eV. Supplementary experiments will look for neutrinoless double-beta decays. Stefan Schönert from MPI for Nuclear Physics reported on a proposal for a corresponding initiative to realize a large-scale 76Ge underground detector, and Thierry Lasserre of CEA proposed new reactor neutrino experiments to determine mixing parameters. Franz von Feilitzsch from the Technical University of Munich presented the status of low-energy neutrino astronomy, focusing on the Gallium Neutrino Observatory and the somewhat unlucky BOREXINO experiment, which was at the origin of the environmental issues at the Gran Sasso underground laboratory. However, BOREXINO is still important as the ultimate test of astrophysical models of the Sun.
Lothar Oberauer, also from the Technical University of Munich, described a feasibility study for a 30 kilotonne liquid-scintillator underground detector and its fundamental impact on geophysics, astrophysics, neutrino physics and proton-decay searches. Such an experiment on Low Energy Neutrino Astrophysics (LENA) would also be sensitive enough to detect relic supernova neutrinos, and hence provide data on the history of star formation. A large European initiative will be necessary to realize LENA. Hans Thomas Janka from MPI for Extraterrestrial Physics complemented the session with a presentation on supernova neutrinos.
Karsten Danzmann from MPI for Gravitational Physics, Hannover/ Golm, gave a summary on the status of laser interferometer experiments to detect gravitational waves. Four first-generation experiments – GEO600 near Hannover, LIGO at two sites in the US, TAMA close to Tokyo and VIRGO near Pisa – have begun to or will take data within the next year. Their observation programmes are coordinated to increase the probability of correlated detections. Second-generation experiments are already planned (LIGO with the GEO600 technique) and the path to even more ambitious experiments seems to lie straight ahead, pointing to the ultimate challenge of observing primordial gravitational waves. The Big Bang Observatory could be realized around 2020.
Franz Käppeler from the Research Centre Karlsruhe stressed the need for the central topics of nuclear astrophysics of precise measurements of cross-sections, as well as of the lifetimes and masses of neutron-rich isotopes. Experimental uncertainties on these quantities currently limit our understanding of the energy production in stars and nuclear synthesis.
A strong future
Looking ahead, Hermann-Friedrich Wagner announced that from 2005 the funding of astroparticle physics in German universities will change from the present start-up scenario to a three-year periodic scheme, as applied, for example, in high-energy, nuclear and astrophysics. This decision means that astroparticle physicists in Germany can now make plans on a solid funding basis. In addition, the German Helmholtz association of large research centres now supports so-called “virtual institutes”. These networks will strengthen the co-operation of university groups and research centres.
The maturity astroparticle physics has now reached and the self-confidence of its proponents was also visible at the workshop with the establishment of the KAT (Komitee für Astroteilchenphysik) committee. KAT will be an elected committee responsible for expressing the opinions and needs of astroparticle physicists in Germany, and will be a negotiation partner for similar groups in other branches of physics, funding agencies and international organizations.
On the European scene APPEC, the Astroparticle Physics European Coordination has grown through new members Belgium, Greece, Spain and Poland. APPEC, as Thomas Berghöfer and Christian Spiering of DESY reported, is continuing its peer reviews of major astroparticle-physics activities in Europe. The ILIAS (Integrating Large Infrastructures for Astroparticle Science) proposal, triggered by APPEC as a joint proposal, was recently funded by a €7.5 million grant within the EU’s 6th Framework Programme.
This third workshop on the status and perspectives of astroparticle physics in Germany has revealed a research field that has left its teenage years. Astroparticle physics is now much better anchored in Germany than it was three years ago. Fortunately, however, both the growing importance of astroparticle physics for fundamental physics and astrophysics, as well as the personal enthusiasm of the physicists in this field of research, have retained their youthful sprightliness.
