Summary: Acoustic Detection of EHE Neutrinos

Neutrino astronomy was initiated primarily to search for TeV to PeV neutrinos from Active Galactic Nuclei, and the optical Cherenkov technique is well suited for this energy range. Interest has grown recently in detecting EeV neutrinos, particularly the “cosmogenic” neutrinos produced during propagation of ultra-high-energy cosmic rays (UHECR) through the microwave background radiation. These neutrinos could be a powerful tool both to resolve the mystery of the UHECR sources and to test fundamental physics at the ∼100 TeV scale. The optical technique is not cost effective at these energies and newer techniques such as radio and acoustic detection are necessary. Accelerator experiments have confirmed the production of both types of signals from high-energy showers in various media, and quantitative measurements have confirmed theoretical descriptions of the signal strength, frequency content and pulse shape. While radio experiments have set the strongest limits so far, the acoustic method could contribute with an entirely independent signal production and detection mechanism and may be more effective at the highest energies. Efforts are underway to develop the acoustic method in various media around the world, with arrays operating in ocean water at the Bahamas, the UK, and the Mediterranean Sea; detectors prepared for deployment in the South Pole ice in the next year; and ideas for future acoustic detectors in salt domes and on Antarctica’s Ross Ice Shelf. Regardless of which method is individually most sensitive, the best configuration may be to co-deploy arrays to combine the techniques and seek coincident detection of individual neutrino events. 1. Acoustic Neutrino Detection in Water, Ice, and Salt Ocean water is the most abundantly available and best understood acoustic medium for neutrino detection studies. The acoustic properties of the oceans have been mapped out in detail by military and marine science researchers. The first detailed search for neutrino-like signals from a large undersea array was completed by the SAUND (Study of Acoustic Ultra-high-energy Neutrino Detection) project [1] using a military array in the Bahamas. Lessons learned in signal simulation, online triggering, and background rejection are now being applied to a larger array at the same location in the SAUND-II project. Acoustic neutrino arrays in water currently have an energy threshold well above the EeV scale. Although site selection, signal processing and background rejection advances, and optimization of an array specifically for neutrinos could all lower the threshold, other media likely feature an inherently higher signal to noise ratio resulting in a lower neutrino energy threshold. Oceanbased arrays may be capable of probing topological defect fluxes and other super-EeV models, but seem not to have sufficient sensitivity at the Greisen-Zatsepin-Kuzmin (GZK) energy range. Underground salt domes extending several km on each side could have long absorption and scattering lengths for both radio and acoustic waves. The cost of drilling into these domes, however, is prohibitive. Drilling into ice is less expensive and may be better suited for radio and acoustic as well as optical neutrino detection. Greenland ice and ice in temperate glaciers is dirty, ruling out the optical Cherenkov technique, and warm, causing it to absorb radio and acoustic waves well. South Pole ice, on the other hand, is clean and cold. To compare media as targets for acoustic neutrino detection, several quantities should be considered: background noise, absorption, scattering, and signal strength (Table 1). In the ocean, background noise is site-dependent but is dominated by weather patterns (wind and rain on the surface). Transient events that constitute a neutrino background include the sounds of crustacean, cetacean, and human activities. Noise levels and transient backgrounds in salt and ice are currently unknown in the relevant (∼1-100 kHz) band. Possible noise sources include seasonal human activity at the station, seasonal wind (note that surface sources are somewhat shielded by the firn, which refracts waves back to the surface), and cracking in the ice bulk or at the bedrock interface due to glacial movement. Seismically (at ∼100 Hz), South Pole is measured to be the quietest place on Earth. Table 1. Acoustic properties of several media. The neutrino-induced pressure signal strength scales with the Gruneisen parameter. Ocean South Pole Ice Salt Temperature T (◦C) 15 -51 30 Sound speed vL (m/s) 1530 3920 4560 Volume expansivity β (10 K) 25.5 12.5 11.6 Heat capacity CP (J/kg/K) 390