Acoustic detection of ultra-high energy neutrinos

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Over the past two years, data have been collected at an array of hydrophones in a search for acoustic signatures of ultra-high energy neutrinos from space. The Study of Acoustic Ultra-high energy Neutrino Detection (SAUND) is a first step towards developing a novel detection method for one of the outstanding problems of both particle physics and astrophysics. The project has produced the first measurement of the ultra-high energy neutrino flux with an acoustic array. It has also provided important technical information that could be used to design a new array dedicated entirely to cosmic ray neutrino detection. In the past decade several groups have developed undersea arrays of instruments to detect subatomic particles from cosmic radiation. Most of these experiments use arrays of photo-multiplier tubes that detect light emitted by the particles as they traverse water at the speed of light. In July 2001, SAUND equipped seven hydrophones at this facility, the Atlantic Undersea Test and Evaluation Center (AUTEC) for neutrino detection. AUTEC features several arrays of bottom-mounted hydrophones spanning 250 km/sup 2/ in the Tongue of the Ocean (TOTO) in the Bahamas. The TOTO is a good environment for underwater acoustics: it is a deep-sea cul-de-sac, attaining depths of 1-2 km within several miles of shore; and it has very little boat traffic. At AUTEC we have been able to run the SAUND system in parasitic mode, turning it on whenever no AUTEC activities are scheduled. In practice this has allowed us to run the neutrino detector with 70% live time. Our detection strategy is to monitor all 7 hydrophones continuously and pass the pressure level of each through a digital filter matched to the theoretical neutrino waveform. The neutrino waveform is predicted to be a simple bi-polar pulse. Research conducted by SAUND will aid in the development of a large-scale acoustic neutrino detector, should one be built. Because the neutrino path is a line source, the acoustic radiation of neutrinos is confined to a disk 100 times wider than it is thick. We have determined a successful method of acoustic source localization for the specific situation of neutrino detection. This is necessary both for background rejection and for reconstructing the neutrino energy. We have studied the effects of sound-ray refraction, a difficult effect to account for and one that was not previously considered for neutrino detection. Finally, we have studied how to optimize array location relative to the sea floor and surface, interphone spacing, and geometric configuration.