Observational constraints on the ultrahigh energy cosmic neutrino flux from the second flight of the ANITA experiment

The Antarctic Impulsive Transient Antenna (ANITA) completed its second Long Duration Balloon flight in January 2009, with 31 days aloft (28.5 live days) over Antarctica. ANITA searches for impulsive coherent radio Cherenkov emission from 200 to 1200 MHz, arising from the Askaryan charge excess in ultrahigh energy neutrino-induced cascades within Antarctic ice. This flight included significant improvements over the first flight in payload sensitivity, efficiency, and flight trajectory. Analysis of in-flight calibration pulses from surface and subsurface locations verifies the expected sensitivity. In a blind analysis, we find 2 surviving events on a background, mostly anthropogenic, of $0.97\pm 0.42$ events. We set the strongest limit to date for ${10}^{18}–{10}^{21}\mathrm{eV}$ cosmic neutrinos, excluding several current cosmogenic neutrino models.

Figure 1

The ANITA-II payload on ascent with the lower eight horn antennas deployed. The payload height is $\sim 8\mathrm{m}$, and each antenna face is 0.95 m across. The inset shows the balloon and payload viewed telescopically at float altitude of 35 km.

Figure 2

The vertical-polarization reconstruction position of all 21.2 M quality events, with the color scale representing the average peak value of the interferometric image for events which fall in that bin. The dark blue region is consistent with pure thermal noise. The green, yellow, and red regions are increasingly nonthermal and are consistent with anthropogenic noise.

Figure 3

Events remaining after unblinding. The Vpol neutrino channel contains two surviving events. Three candidate UHECR events remain in the Hpol channel. Ice depths are from BEDMAP [28].

Figure 4

Top: Waveforms of incident field strength for the two surviving Vpol events. Event 8381355 is shown filtered between 235–287 MHz to remove weak continuous-wave noise. Middle: Corresponding frequency power spectra with an average thermal-noise spectrum shown in blue for reference. Bottom: Corresponding interferometric images showing the pulse direction. The dashed line is the horizon.

Figure 5

ANITA-II limit for 28.5 days live time. The red curve is the expected limit before unblinding, based on seeing a number of candidates equal to the background estimate. The blue curve is the actual limit, based on the two surviving candidates. Other limits are from AMANDA [29], RICE [30], Auger [31], HiRes [32], and a revised limit from ANITA-I [18]. The Berezinsky and Zatsepin (GZK) neutrino model range is determined by a variety of models [10, 19, 20, 21, 23, 33, 34].