2013 Volume 68 Issue 10 Pages 670-674
The results from the Telescope Array experiment after the first five year observation are reviewed. We measured the energy spectrum of ultra-high energy cosmic rays (UHECRs) by two different types of detectors, the surface detector array (SD) and the fluorescence detectors (FDs). The energy spectrum by TA is fully consistent with the result from the HiRes experiment in the energy scale and the shape. We found a steepening in the energy spectrum at 5.4 x 10^<19> eV, which is also consistent with the claim firstly reported by HiRes, and later by Auger. The analysis of longitudinal development of air showers in the atmosphere shows that the UHECR mass composition is dominated by protons around the steepening. This is a supporting evidence of the Greisen-Zatsepin-Kuzmin cutoff, which may be formed as a result of UHECR energy losses via interaction with the cosmic microwave background photons in the inter-galactic space. However, we found no statistically significant anisotropy in the arrival directions of the current TA data, which is expected if UHECRs from nearby sources are preferably observed in the energy region beyond the steepening. The interpretation of the results and the future prospects are discussed.