Telescope Array Project for Extremely High Energy Cosmic Rays

The existence of extremely high energy cosmic rays (EHECRs) with energies exceeding 10 20 eV has been reported by the AGASA experiment.These cosmic rays may be produced in the active astronomical object in the outer galaxy and propagated a long distance toward the Earth.Such high energy "stars" have been searched in the arrival direction of these events but no clear candidates were found.A new experiment, Telescope Array (TA), is planned to unveil the enigmatic origin of the EHECRs.The physics motivation of TA is reviewed and its present status is reported. §1. Nature of extremely high energy cosmic raysThe EHE proton propagating a long distance through the extragalactic space is expected to lose its energy by interacting with the cosmic microwave background (CMB).Following the Lorentz invariance, the CMB is equivalent to a gamma ray of energy ∼150 MeV in the rest frame of 10 20 eV proton.Thus the proton of this energy is expected to result in a pion photo-production, and a significant part of the initial proton energy is carried away by the produced pions.This mechanism of energy loss was first predicted by Greisen, Zatsepin and Kuzmin 1) and is called the GZK mechanism.Two obvious outcomes of the GZK mechanism are;• The energy spectrum of EHECRs arriving from uniformly distributed sources in the universe should exhibit a clear cutoff structure at ∼10 20 eV (= GZK cutoff).• If the cosmic rays above 10 20 eV (= super-GZK events) are detected, most of them should have an origin within 50 Mpc of the Earth.Despite the theoretical prediction by GZK, the energy spectrum of EHECRs measured by AGASA 2) shows no indication of the GZK cutoff 3) (see Fig. 1).The AGASA recorded 11 super-GZK cosmic rays in 11 years of operation.Moreover, in the sample of 59 events above 10 19.6 eV, 13 events were found to form clusters of 2 or 3 events (5 doublets and 1 triplet) indicating they originate from a common "point source" in the sky 4) (see Fig. 1).A correlation with high energy astronomical objects such as active galactic nuclei, radio galaxy lobes and gamma ray bursts have been searched but no obvious candidates were identified within 100 Mpc. 5) In order to circumvent the problems met with the astrophysical models, several particle physics oriented models have been proposed for the origin of EHECRs.They are the decay of super-heavy relic particles in the galactic halo, 6) the interaction of EHE neutrinos with the cosmic neutrino background in the local cluster 7) (Z-burst model), the violation of Lorentz invariance at the extremely high γ factor 8) and others.