Observations and implications of the ultrahigh-energy cosmic rays

Abstract

The authors define ''ultrahigh-energy cosmic rays'' (UHECRs) as those cosmic rays with energies above 10 18 eV. It had been anticipated that there would be a cutoff in the energy spectrum of primary cosmic rays around 6ϫ10 19 eV induced by the interaction of the particles with the 2.7-K primordial photons. However, recent experimental data have established that particles exist with energies greatly exceeding this. It follows that the sources of such particles are probably nearby, on a cosmological scale. However, although the trajectories of such energetic particles through the galactic and intergalactic magnetic fields may be nearly rectilinear, no astronomical sources have as yet been identified. This is the enigma of the highest-energy cosmic rays. The paper reviews the history of research in this energy regime and critically assesses the observational results on the energy spectrum, arrival directions, and composition of the primary cosmic rays based on observations made by six experiments. The detection methods currently available are described. Special techniques have been developed as particles of 10 20 eV or higher occur at a rate of only about 1 per km 2 per century. Errors in measurement are given particular attention. The authors also review the theoretical predictions for a number of candidate sources of cosmic rays beyond the predicted cutoff. Finally, the four major projects planned to address the question of the origin of UHECRs are briefly described. CONTENTS

Search for correlated high energy cosmic ray events with CHICOS

“…The rate of these triggers is comparable to expectations based on the previously measured flux [1] and computer simulations of air showers with the AIRES code [10]. In this paper we study double trigger events where two sites separated by more than 1 km both record trigger events within a certain coincidence time.…”

Loop-generated bounds on changes to the graviton dispersion relation

“…In this case bounds may be obtained from our observation of very high energy particles in cosmic rays. Assuming these cosmic rays to be protons -as the best evidence indicates [14] -the constraint (c p − c g )/c p < 10 −15 was obtained in this way [15], strongly restricting the case where the graviton velocity, c g , is smaller than that of the proton, c p .…”

Rigidity dependent knee and cosmic ray induced high energy neutrino fluxes

“…2 The cosmic ray spectrum Figure 1 shows the observed cosmic ray differential spectrum from several experiments [25]. As mentioned above, the total CR spectrum is well described by power laws dN CR /dE CR ∝ E −α CR , with the spectral index changing from α ≃ 2.7 to α ≃ 3 at the so-called knee, occurring at E knee ≃ 3 × 10 15 eV.…”