The scientific potential for detecting cosmic neutrinos with energies at or above the GZK cut-off — the energy limit for cosmic rays due to interactions with the cosmic microwave background — has been extensively discussed in the literature. The recent observation of a very high-energy event by the KM3NeT telescope has further fueled this discussion, as it may represent the first detection of a GZK neutrino. However, despite this remarkable result, the extremely low expected flux of ultra-high-energy neutrinos (Eν > 10^18 eV) clearly indicates that their detection will require a telescope with an effective volume exceeding 100 km^3.
Acoustic detection offers a promising approach for observing these ultra-high energy cosmic neutrinos. The sound waves generated by their energy deposition in the deep sea can propagate over many kilometers with little attenuation, making it feasible to instrument a vast volume of water for neutrino detection. Achieving this requires the development of acoustic detection technologies capable of supporting a large-scale, deep-sea sensor network.
Fiber-optic hydrophone technology stands out as a promising candidate for such a network, combining the necessary sensitivity to detect the faint acoustic signals from neutrino interactions with the potential for cost-effective large-scale deployment. TNO is actively developing this fiber-optic hydrophone technology to meet the sensitivity and operational requirements of deep-sea environments.
In this presentation, I will report on the progress of the hydrophone development and outline the plans toward a future acoustic neutrino telescope — opening a window to explore the universe beyond the GZK horizon.