Study of Small-Scale Anisotropy of Ultrahigh Energy Cosmic Rays Observed   in Stereo by HiRes
  The High Resolution Fly's Eye (HiRes) experiment is an air fluorescence detector which, operating in stereo mode, has a typical angular resolution of 0.6 degrees and is sensitive to cosmic rays with energies above 10^18 eV. HiRes is thus an excellent instrument for the study of the arrival directions of ultrahigh energy cosmic rays. We present the results of a search for anisotropies in the distribution of arrival directions on small scales (<5 degrees) and at the highest energies (>10^19 eV). The search is based on data recorded between 1999 December and 2004 January, with a total of 271 events above 10^19 eV. No small-scale anisotropy is found, and the strongest clustering found in the HiRes stereo data is consistent at the 52% level with the null hypothesis of isotropically distributed arrival directions. 
The High Resolution Fly's Eye Collaboration (HiRes)