The Antarctic Impulsive Transient Antenna (ANITA) experiment has just published their results from their second flight. Like ARIANNA, ANITA searches for radio waves from extra-terrestrial neutrino interactions. However, instead of embedding their antennae in the ice, the antennae are mounted on a long-duration high-altitude balloon. These balloons are launched from McMurdo Station, where the prevailing winds push them in a circle around Antarctica; the data they report is from a 31 day flight in December, 2008, at an altitude of about 35 km (113,000 feet). The picture shows ANITAs 32 antennae, which look for radio waves from neutrino interactions in the ice out to the horizon (up to 650 km away).After a series of cuts to remove background events, they are left with two events, against an estimated background of 1 +/- 0.4 events - no statistically significant signal is observed. They set a limit on the possible flux of extra-terrestrial neutrinos which is considerably more restrictive than previous studies. Their limit is just starting to constrain models of GZK neutrino production. Clearly, a more sensitive experiment is needed.
ANITAs ability to observe a large chunk of Antarctica in one shot is a big plus. However, balloons have a limited observation time. And, the up-to-650 km separation between the source and the detector inevitably means that ANITA can only see the most energetic neutrino interactions. By placing our antennae directly in the Antarctic ice, ARIANNA will be able to see a wider range of neutrino energies, covering most of the GZK energy range.
