About ARA
One of the most tantalizing questions in astronomy and astrophysics, namely the origin and the evolution of the cosmic accelerators that produce the highest energy (UHE) cosmic rays, may be best addressed through the observation of UHE cosmogenic neutrinos. Neutrinos travel from their source undeflected by magnetic fields and unimpeded by interactions with the cosmic microwave background. However, uncertainties in their predicted fluxes of make it difficult to design an array with sufficient sensitivity to collect a statistically meaningful sample of events. At high energies (above $10^{15}$ eV), neutrinos could be most efficiently detected in dense, radio frequency (RF) transparent media via the Askaryan effect. The abundant cold ice covering the geographic South Pole, with its exceptional RF clarity, has been host to several pioneering efforts to develop this approach, including RICE and ANITA.
Building on the expertise gained in these efforts, and the infrastructure developed in the construction of the IceCube optical Cherenkov observatory, we are developing an array, known as ARA (The Askaryan Radio Array), and installing it in the deep ice near the geographical South Pole. South Polar ice is, in fact, perhaps the most extensively-studied on the planet, the combination of ice thickness and favorable radiofrequency dielectric characteristics, as well as the excellent scientific infrastructure and the co-location of the IceCube Observatory, makes that site unparalleled for this study. With a fiducial area of an unprecedented 80 km$^2$, ARA's size was chosen to ensure the detection of the flux of neutrinos ``guaranteed'' by the observation of the GZK cutoff by HiRes and the Pierre Auger Observatory. Within 3 years of commencing operation, the full ARA will exceed the sensitivity of any other instrument in the 0.1-10 EeV energy range by an order of magnitude. Because the antennas will be deployed in boreholes extending below the firn layer to 200 m depth, it will have the ability to distinguish surface noise from sources originating in the ice cap, otherwise not possible in the ballon borne approach employed by ANITA. Even under the extreme assumption that UHE cosmic rays are pure iron, ARA will have sufficient sensitivity to establish the presence or absence of the secondary UHE neutrinos produced by the interaction of cosmic rays with the cosmic microwave background. Such an observatory would also provide an unique probe of long baseline high energy neutrino interactions unattainable with any man-made neutrino beam.
The primary goal of the ARA array is to establish the absolute cosmogenic neutrino flux through a modest number of gold plated events. We have therefore adopted a clustered geometry in which a single localized cluster may act as a standalone array, which trades precise angular resolution for increased coverage. This information would frame the performance requirements needed to expand the array in the future to measure a larger number of neutrinos with greater angular precision in order to study their spectrum and origins.