A 1000km3 Ultra-High Energy Neutrino Acoustic Detector

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In recent years scientists have expanded the horizon of astronomy by observing the Universe first with photons of increasingly smaller wavelengths and then by resorting to particles other than photons. The latest additions to our toolkit are neutrinos, neutral particles with a tiny mass, more than 500,000 times lighter than electrons. While the small mass of neutrinos make them already rather "exotic", it is now possible that ultra-high-energy neutrinos raining down to Earth from the outer space will help us understanding some catastrophic events in the universe. We do know that particles (possibly protons) of kinetic energies 1018 eV and above do occur in cosmic radiation and, in fact, some scientists claim to have observed cosmic rays with energies above 1020 eV. This is a tremendous energy for a single elementary particle, equivalent to the kinetic energy of a medium-sized book falling to the floor from a desk! As a comparison our most powerful particle accelerator can only accelerate protons to 1012 eV. We do not fully understand what kind of process can produce such high energy particles but, if also neutrinos of such high energies were to be produced we would have an important clue. The signals from such energetic particles would be dramatic enough to be recorded in conventional particle detector however the rate at which such energetic neutrinos would rain on Earth is expected to be so low that thousands of cubic kilometers of active material would be needed to have the chance of detecting some. A suitable detector cannot be built with nuts and bolts but it has to take advantage of an existing body of material! Ocean water is very common on the Earth surface and our group, with NSF support, is installing a system that will demonstrate the possibility of detecting ultra-high-energy neutrinos in cosmic radiation by the acoustic noise they are expected to produce when they interact and stop in sea water. Indeed their tremendous kinetic energy is expected to be converted into heat that would make the sea-water expand, producing a peculiar sound. The sound will be detected, in our initial study, by an array of hydrophones (underwater microphones) located off the coast of Florida and used by the US Navy for naval exercises. We have an agreement with the Navy that will allow us to install a special data acquisition system that will allow us to find the tiny pulses characteristic of the neutrino interaction in the large sea background caused by human activities and different sort of marine creatures. This project was initiated almost exclusively with undergraduate students. The initial feasibility study was done by Stanford Undergraduate Shaffique Adam, now at Cornell. The first data taking at the Navy array and the relative data analysis was performed by Stanford undergraduate Justin Vandenbroucke who is now working on the NSF Amanda/Ice Cube detectors in Antarctica as a Berkeley graduate student. A Stanford graduate student, Naoko Kurahashi, is leading the installation of the present system, while another Stanford undergraduate, Jason Kerwin, is building parts of a calibration device that we hope to use in the study.