Outstanding Problems in Particle Astrophysics

@inproceedings{Gaisser2005OutstandingPI,
  title={Outstanding Problems in Particle Astrophysics},
  author={Thomas K. Gaisser},
  year={2005}
}
The general features of the cosmic-ray spectrum have been known for a long time. Although the basic approaches to understanding cosmic-ray propagation and acceleration have also been well understood for many years, there are several questions of great interest that motivate the current intense experimental activity in the field. If the energy-dependence of the secondary to primary ratio of galactic cosmic rays is as steep as observed, why is the flux of PeV particles so nearly isotropic? Can… 
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