Progress in ultra high energy neutrino experiments using radio techniques

Studying the source of Ultra High Energy Cosmic Ray (UHECR) can provide important clues on the understanding of UHE particle physics, astrophysics, and other extremely energetic phenomena in the universe. However, charged CR particles are deflected by magnetic fields and can not point back to the source. Furthermore, UHECR charged particles above the Greisen-Zatsepin-Kuzmin (GZK) cutoff (about 5 × 10¹⁹ eV) suffer severe energy loss due to the interaction with the Cosmic Microwave Background Radiation (CMBR). Consequently almost all the information carried by CR particles about their origin is lost. Neutrinos, which are neutral particles and have extremely weak interactions with other materials can arrive at the earth without deflection and absorption. Therefore UHE neutrinos can be traced back to the place where they are produced. Due to their weak interaction and ultra high energies (thus extremely low flux) the detection of UHE neutrinos requires a large collecting area and massive amounts of material. Cherenkov detection at radio frequency, which has long attenuation lengths and can travel freely in natural dense medium (ice, rock and salt et al), can fulfill the detection requirement. Many UHE neutrino experiments are being performed by radio techniques using natural ice, lunar, and salt as detection mediums. These experiments have obtained much data about radio production, propagation and detection, and the upper limit of UHE neutrino flux.