George W.-S. Hou (National Taiwan University)
NuTel, Ashra-1, and NTA
The Earth-skimming tau neutrino technique detects extensive air showers produced by tau lepton decays in the atmosphere, which enjoys a large target mass. The tau leptons, produced by VHE tau neutrinos that interact with the Earth matter they traverse, emerge out of a mountain or the ground facing the detector. This method has detection sensitivity in the PeV-EeV region, and can be used to search for neutrinos originating from hadron acceleration in astronomical objects. Additional advantages are perfect shielding of cosmic ray secondaries, precision arrival direction determination, and negligible background from atmospheric neutrinos.

The Neutrino Telescope (NuTel) and the All-Sky High Resolution Air-shower detector (Ashra) projects were independently developed to detect tau neutrinos with the Earth-skimming method. NuTel aims at observing Cherenkov radiation from such tau-neutrino originated air showers for the energy range of 1 to 1000 PeV, with possible sources such as AGNs, GRBs, the Galactic Center (GC), etc. The optical and electronic systems of NuTel were designed, built and tested at 2100 m above sea level on Mei-Fong Mountain in Taiwan, and is ready for physics observation. Aiming for realizing very high energy "multi-particle astronomy", the Ashra detector has been developed since 2002 with 1 arcmin pixel resolution and all-sky coverage. The principal demonstration phase, Ashra-1, has been running at the Mauna Loa site at 3300m above sea level on Hawaii Island since 2008. Ashra-1 succeeded in the first search for PeV-EeV tau neutrinos originating from a GRB as the commissioning run, and has achieved best instantaneous sensitivity since January 2012 after trigger upgrade.

Based on these achievements, we start to form a new collaboration for concrete design and planning for the next generation large Neutrino Telescope Array (NTA). The conceptual layout for the NTA observatory considers three site stations for a 25km-grid triangle watching the total air mass surrounded by the mountains of Mauna Loa, Mauna Kea, and Hualalai. A single site station at the center of the triangle has full-sky coverage. This configuration allows tremendous sensitivity (equivalent to > 100 giga ton water) with Cherenkov-fluorescence stereoscopic observation for PeV-EeV neutrinos in essentially background-free conditions. With the demonstrated fine imaging of Earth-skimming tau showers, we aim for clear discovery and identification of astronomical tau neutrino sources.