Alan Watson (University of Leeds)
This question can now be asked because of two results obtained using data recorded at the Pierre Auger Observatory. It has been established, at the 6-sigma level, that the flux of the highest energy cosmic rays is suppressed at energies beyond 5 x 10^19 eV and that above this energy an anisotropy in the arrival directions of the particles is apparent. The arrival directions appear to be associated with sources within the GZK horizon (z ~ 0.018 or 75 Mpc). From these observations it seems probable that we have observed the long-sought Greisen-Zatsepin-Kuzmin effect, demonstrating that ultra-high energy cosmic rays are of extragalactic origin. It is also probable that these particles are protons, thus offering the possibility of insights into features of particle physics at centre-of-mass energies 30 times greater than will be reached at the LHC. Preliminary conclusions from studies of detailed features of extensive air showers suggest that extrapolations from Tevatron energies may not be what have been anticipated hitherto. Much further work remains to be done.