On the Evidence for Clustering in the Arrival Directions of AGASA's Ultrahigh Energy Cosmic Rays
Abstract
Previous analyses of cosmic rays above 40 EeV observed by the AGASA experiment have suggested that their arrival directions may be clustered. However, estimates of the chance probability of this clustering signal vary from 10^{-2} to 10^{-6} and beyond. It is essential that the strength of this evidence be well understood in order to compare it with anisotropy studies in other cosmic ray experiments. We apply two methods for extracting a meaningful significance from this data set: one can scan for the cuts which optimize the clustering signal, using simulations to determine the appropriate statistical penalty for the scan; alternatively, one can optimize the cuts with a first set of data, and then apply them to the remaining data directly without statistical penalty. While the former method is more useful in general, in the present case only the latter is an unbiased test of the clustering hypothesis. We find that the AGASA data is consistent at the 20% level with the null hypothesis of isotropically distributed arrival directions.Keywords
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