Professor Stefan Westerhoff
Cosmic ray particles were discovered almost one hundred years ago, but still very little is known about the origin of the most energetic particles, above and around 10^18 eV. The existence of particles at these energies, the highest observed in the Universe, continues to challenge our imagination: where do they come from, how are they accelerated, and how can they travel astronomical distances without substantial loss of energy?
We are currently in an exciting new era in cosmic ray physics, with instruments now producing data of unprecedented quality and quantity to tackle the many open questions. Over the last 5 years, the High Resolution Fly's Eye (HiRes) air fluorescence stereo detector has accumulated data characterized by excellent angular resolution. The world's largest detector for cosmic radiation, the Pierre Auger Observatory in Argentina, is nearing completion, and first results have already been published.
In this talk, I will review the current status of cosmic ray physics and summarize recent results from the HiRes and Auger experiments on the energy spectrum, composition, and arrival direction distribution of ultra-high-energy cosmic rays.