The detection, a few decades ago, of cosmic rays with energies that can exceed 1020 eV confronted us with some of the most interesting and challenging questions in astrophysics: Where do they come from? How can they be accelerated to such high energies? What do they tell us about these extreme cosmic accelerators? In spite of all these years of experimental and theoretical endeavor, these questions remain unanswered. The measurement of a flux suppression at the highest energies, reminiscent of the "GZK cut-off" produced by the interaction of particles with the cosmic microwave background photons for propagations over intergalactic scales, has appeased the debate concerning the extragalactic provenance of ultrahigh energy cosmic rays (UHECRs). This feature not only suggests that UHECRs would originate outside of our Galaxy, but also that the sources of the highest energy particles should be located within ~100 Mpc distance, in our local Universe. However, the sources remain a mystery and results from the Auger Observatory on the arrival directions and chemical composition of UHECRs make the picture even more puzzling. Indeed, no powerful counterpart sources are observed in the arrival directions of the highest energy particles (where the deflections due to magnetic fields should become negligible), and the measurements of the chemical composition seem to indicate an unexpected trend towards heavy nuclei at the highest energies.
Candidate sources of UHECRs range from the birth of pulsars to explosions related to long-duration gamma-ray bursts or to events in active galactic nuclei.
Working at the interface of cosmology, high energy astrophysics, plasma physics and particle physics, our group is trying to unveil some of this mystery.
Permanent researchers: Kumiko Kotera, Martin Lemoine