The primary purpose of the ARIANNA Ultra-High Energy (UHE) Neutrino project is to observe about 40 cosmogenic neutrinos per year from the GZK mechanism - extragalactic UHE protons collide with cosmic microwave background photons, producing pions that eventually decay to neutrinos - with sufficient precision to determine their energy spectrum and interaction properties. The energy spectrum of cosmogenic neutrinos provides important insight on the production mechanism and source distribution of cosmic rays.
ARIANNA relies on the detection of radio pulses generated in dense media by the Askaryan effect and capitalizes on several properties of the Ross Ice Shelf near McMurdo Station, the hub of US operations: (1) the ice is impressively transparent to electromagnetic radiation at radio frequencies, (2) the water-ice boundary below the shelf behaves like a good mirror to reflect radio signals from neutrinos in any downward direction and (3) background noise levels are quite modest in the frequency band of interest. Nearby Minna Bluff protects the site from significant anthropogenic radio noise. Its close proximity to McMurdo Station provides a number of suitable options for logistical support.
This award will fund the next step toward the realization of the ARIANNA concept, called the Hexagonal Radio Sub-array: to construct, deploy and commission a hexagonal arrangement of 7 autonomous detector stations at the ARIANNA site to assess large-scale feasibility, system robustness and the ambient noise environment over an extended period of time. These tests will provide input to optimize the final design of the Phase A component of ARIANNA, which consists of a 12 x 12 array of stations
Among the broader impacts of this project are an outlet for curiosity and imagination driven research by students who will be given the opportunity to learn about data acquisition electronics, detector simulation, and data processing.
Somewhere out in space, cosmic rays are created that have energy more than a million times greater than any particle ever produce by humans. These particles interact with stuff (such as gas or light) to produce neutrinos that travel throughout space unimpeded. Neutrino telescopes, such as ARIANNA, can help pinpoint the sources of cosmic rays (a goal of astrophysicists for more than 100 years!) since neutrinos travel in straight lines, live forever, and will not be absorbed by any known material. Unfortunately, there are very few neutrinos that reach the earth at the highest energies, so large new telescopes covering areas of 1000 square kilometers must be designed and built. ARIANNA can detect the very rare neutrino that interacts in the ice of the Ross Ice Shelf. The interaction produces a short, intense burst of radio waves that travel through the cold ice as if it were air, and reflect from the bottom of the shelf ice which floats on water back up to the detectors installed on the surface. The exact location of ARIANNA is only 110 kilometers from McMurdo Station, the largest US base in Antarctica, so the difficult job of transporting equipment is not as bad as usual. Since the location is remote, there are no people around to run the neutrino telescope. So ARIANNA is designed to run autonomously in one of the harshest environments on the planet. Each of the 1296 stations talks to ARIANNA scientists using the global communication satellites known as Iridium. To test the ARIANNA concept, NSF approved a pilot program of 7 ARIANNA stations, known as the Hexagonal Radio Array. The ARIANNA collaboration has recently reported a string of successes in a series of papers. The final conclusion of these studies include "all major technical and scientific milestones have been met", and the chosen site is "radio quiet, as expected, so data quality is outstanding. There are no sources of confusing background, an important pre-requisite for successful operaton. The analysis efficiency for the full array is 0.83, which is outstanding." The construction of the pilot detector is expected to complete in December 2014. If the operation of the pilot detector proves reliable, then the international ARIANNA collaboration, led by UC Irvine, is expected to submit a proposal to NSF for the construction of 1296 ARIANNA stations in a phased construction plan spanning 4 years.
