New Limits on the Ultrahigh Energy Cosmic Neutrino Flux from the ANITA Experiment

We report initial results of the first flight of the Antarctic Impulsive Transient Antenna (ANITA-1) 2006–2007 Long Duration Balloon flight, which searched for evidence of a diffuse flux of cosmic neutrinos above energies of $E_{\nu }\simeq 3\times {10}^{18}\mathrm{eV}$. ANITA-1 flew for 35 days looking for radio impulses due to the Askaryan effect in neutrino-induced electromagnetic showers within the Antarctic ice sheets. We report here on our initial analysis, which was performed as a blind search of the data. No neutrino candidates are seen, with no detected physics background. We set model-independent limits based on this result. Upper limits derived from our analysis rule out the highest cosmogenic neutrino models. In a background horizontal-polarization channel, we also detect six events consistent with radio impulses from ultrahigh energy extensive air showers.

Figure 1

View of the ANITA-1 payload in launch configuration, with photovoltaics at the top and bottom, and antenna clusters between. The side of each square antenna mouth is about 0.9 m, and the payload stands about 8 m tall.

Figure 2

Left: Plot of all reconstructed events, in both horizontal and vertical polarization; major Antarctic stations are indicated on the map. Right: events remaining after cuts to remove anthropogenic interference. Six events remain in the horizontally polarized group, but these are noncandidates for neutrino events, as discussed in the text.

Figure 3

ANITA-1 limits based on no surviving candidates for 17.3 days of live time. Other limits are from AMANDA [25], RICE [26], ANITA-lite [8], Auger [27], HiRes [28], and FORTE [15]. The BZ (GZK) neutrino model range is determined by a variety of models [18, 19, 20, 21, 22, 29, 30].