Kael Hanson: The Future of UHE Neutrino Astronomy on Ice
The earth is opaque to UHE neutrinos: the cross section becomes big since it scales with squared energy. This means the signal is only coming from the top hemisphere. For through-going muons one runs out of target on top, so one is mostly looking at the horizon. Through-going muons and taus are favored channels used for GZK flux probe. Effective areas are 5 times larger at 10^18 eV relative to electrons, but one thus gets only 25% of the neutrinos going to muons. Also, muons of HE lose a third of their energy per kilometer, so this energy measurement is degraded.
Vetoing the Cosmic-Ray background with a surface array could regain the coverage for through-going muons but the ice-top coverage is limited. One can think of a array of detector on top of IceCube as a veto.
One can put acoustic detectors in some of the IceCube strings. One gets acoustic emissions from the energy deposited in ice over a short amount of time. The unfortunate story is that the attenuation length is much shorter than what was expected. The promise of cheap detectors for large volume, but now the general feeling is that acoustic in ice has too high a threshold (E>1EeV) to be useful.
One can think of using Askar’yan effect: coherent radiation from excess negative charge developed in a EM shower (compton production of electrons, with very small amount of positron annihilation). A radio pulse is emitted in ice at a Cherenkov angle of about 55 degrees. The signal is broadband up to GHz frequencies in a cone. Askaryan radiation was observed in a SLAC test beam on a ice block (2007). For use in neutrino detection in ice experiments, Snell’s law says that rays incident shallower than 44 degrees from horizontal will be reflected back into the ice, so the geometry of the interaction is important. The solution is to dig into the ice, 200m below surface being a acceptable compromise.
Holes were drilled (6″ diameter) at 200m for a test. Even at 200 meters, the distribution of vertices that one’d get for 10^16 eV energy is shallow, and one would only catch a small part of the signal.
ARA37 will be a huge array of holes next to the South Pole site. The electronics are connected to antennas at 200m depth. Each station is a autonomous GZK neutrino detector. The electronics needs a fast digitizer. >3GHZ sampling of RF transients from all antennas. The status of this experiment is that a testbed was deployed in 2010-2011 and running since then. Ara1 deployed last year. Ara2 and Ara3 are being deployed this year. Deployment of future stations depends on funding.
Arianna is a similar concept. A shallow detector some meters below the surface of the ice. This is located on Moores Bay. The idea is that you can get bounces from the ice-water interface, increasing your acceptance to the signal of high-energy neutrino signals. Right now four stations have been deployed.
Finally, Anita is a ballon with a RF detector launghed from McMurdo station. Flies above the antarctica at 37km, an dviews more than a million cubic kms of ice. They recently removed the horizontal polarization trigger but still saw 5 ultra-high-energy cosmic ray events. Anita III (2013-2014) will continue the endeavour.
In summary, optical is on the verge of discovery of non-GZK neutrino flux. IceCube will not see many events even after many years. Acoustic development is coming to a close due to the unfortunate situation with the attenuation lenght. Radio is the technology of choice for GZK neutrinos.