The cosmic ray spectrum extends from GeV energies to near 10^20 eV. At the high end, the spectrum consists of subatomic particles with energies reaching 10 to 50 Joules per particle. The souces of these particles remain unknown. Two possibilites, from the many theories proposed, is that they may be produced in AGNs or they may be the result of decays of very heavy particles produced in the early universe..
Protons produced further than 50 Mpc or so from the Earth should be affected by their passage through the 2.7 deg BB radiation and a characteristic spectral cutoff, called the GZK cutoff is expected. Ground array and atmospheric air fluorescence experiments such as AGASA, Fly's Eye, HiRes and Auger have been accumulating data on these very rare particles for several decades. I will review the status of this field and discuss the puzzles ( lack of agreement between the energy scales of ground array and air fluorescence experiments ) and mysteries ( hints of alignment of ultra-high energy cosmic rays with BL-lac objects at great distances from the Earth ) that are preocupying researchers in this field.
Coffee and cookies before the presentation at 3:20 pm, and refreshments after the presentation will both be served in Room 128.