No abstract

The origin of ultra-high energy cosmic rays (UHECRs) is a long-standing mystery in astrophysics. Two collaborations are running experiments, the Telescope Array in the northern hemisphere and the Pierre Auger Observatory in the southern hemisphere, recording the signals coming from the extensive air showers made by the most energetic particles observed in nature, in search of their origin. We review the issues raised by the experimental results and discuss the possible origins of UHECR.

The origin of ultra-high-energy cosmic rays (UHECRs) is a long-standing puzzle in astrophysics. Currently, two experiments are being done, one at the Pierre Auger Observatory in the southern hemisphere and the other at the Telescope Array in the northern hemisphere, to collect data on the high energy tail of cosmic rays. We review the recent results on the energy spectrum, arrival directions, and mass composition and discuss the issues raised by the experimental results and by the search for the origin of UHECRs.