| This thesis investigates the origins of ultrahigh energy cosmic rays by searching for evidence of small-scale anisotropy in their arrival directions from space. We begin with a short review of what is known about cosmic rays and specifically about the propagation of charged particles through galactic and extragalactic space. We next describe the experimental techniques of detecting these particles by measuring the extensive air showers they produce in the atmosphere, with special focus on the High Resolution Fly's Eye (HiRes) experiment. In stereo operation the HiRes detector observes cosmic ray arrival directions with unprecedented angular resolution, on the order of 0.5° at all energies above 1018 eV, opening a new window in particle astronomy. Before analyzing the data produced by this experiment, we examine previously claimed evidence for clustering of cosmic rays seen by the Akeno Giant Air Shower Array (AGASA) experiment, and identify potential biases in the analysis which call the evidence into question. An unbiased method is then developed to search for the same kind of clustering signal and is applied to the HiRes data. We next develop an unbinned maximum likelihood ratio test for point-source searches, a technique that is optimal for data combined from experiments with different angular resolution, and we apply this to the combined set of HiRes and AGASA data. Finally, we investigate a series of recent claims that ultrahigh energy cosmic rays, including those observed by HiRes, are correlated with the positions of bright BL Lacertae objects (a class of active galaxies), and propose tests of these claims that require independent data. We conclude that at present no statistically significant evidence exists for small-scale anisotropy. |
