We investigate the possibility of searching for ultra high energy (>~10^18eV) neutrinos in cosmic rays using acoustic techniques in the ocean. This method would provide information complementary to other techniques. The filtering effect of the atmosphere provides clear identification of neutrinos since for detection it is necessary that the shower should fully develop in water. This techique is also attractive because it can be implemented with limited resources using an already existing very large size hydrophone array of the US Navy. We present calculations of the pressure pulse produced by the neutrino-induced shower. A matched filter provides the optimal detection in the ocean background Gaussian noise. We estimate the theoretical sensitivity of this detection algorithm.
This technique was implemented during the summer of 2001, when an experiment was set up by an undergradute student at the AUTEC hydrophone array in Bahamas. We use a computer with a data acquisition card connected to a subset of seven hydrophones that make up a hexagonal pattern. The primary purpose is to analyze the feasibility of a bigger experiment. To study the temporal pattern of the hydrophone system response we used lightbulb implosion calibration, the results of which are compared to theoretical estimates. Among the results of routine data acquisition are data on the background gaussian noise spectra and coherent noise. The Gaussian noise has the expected spectrum and range of variability. The coherent noise is due to animals and possibly ships. There is also some coherent background that originates in the hydrophone electrical system and the data acquisition system. We present the statistics, such as frequency and pulse length distributions, of various types of observed coherent noise, along with the examples of their waveforms. Since the presence of coherent noise is not accounted for by the original detection algorithm, the sensitivity of the experiment is reduced by it, and a second trigger is needed which gets rid of this coherent noise. At the present time, data acquisition at AUTEC (Bahamas) is continuing, and the second trigger will be implemented during the second field trip in the winter of 2001-2002.
More information can be found at http://hep.stanford.edu/~nleht/