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

Abstract

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 ν ≃ 3 × 10 18 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 ultra-high energy extensive air showers.In all standard models for ultra-high energy cosmic ray (UHECR) propagation, their range is ultimately limited by the opacity of the cosmic microwave background radiation. The UHECR energy above which this becomes significant is about 6 × 10 19 eV in the current epoch. This cuts off their travel beyond distances of order 50 Mpc as first noted by Greisen [1], and Zatseptin and Kuzmin [2] (GZK). As a result of this absorption, the UHECR energy above this GZK cutoff is ultimately converted to photons, neutrinos, and lower energy hadrons. The resulting neutrinos were first described by Berezinsky and Zatsepin (BZ) [3]. In standard UHECR source models the BZ neutrino fluxes peak at energies about 2 orders of magnitude below the GZK energy. Thus a "guaranteed" flux of neutrinos at energies of E ν = 10 17−20 eV exists. Its detection is one of the clearest ways to reveal the nature and cosmic distribution of the UHECR sources [4], which is one of the longest-standing problems in high energy astrophysics.The ANITA-1 Long Duration Balloon experiment was designed specifically to search for this cosmogenic BZ neutrino flux. ANITA-1 exploits the Askaryan effect, in which strong coherent radio emission arises from electromagnetic showers in any dielectric medium [5]. The effect was first observed in 2000 [6], and has now been clearly confirmed and characterized for ice as the medium, as part of the pre-flight calibration of the ANITA-1 payload [7]. A prior flight of a prototype payload called ANITA-lite in 2003ANITA-lite in -2004 led to validation of the technique and initial neutrino flux limits that ruled out several UHE neutrino models [8].In a previous paper [9], we describe in detail the ANITA-1 instrument, payload, and flight system. Reference [9] also includes details of the instrument performance during the flight, estimates of the overall sensitivity of the instrument to neutrino fluxes, and discussions of possible backgrounds. Because of the complexity of the flight system and methodology, we refer the reader to ref.[9] for more detail when necessary.The ANITA-1 payload (Fig. 1) launched from Williams Field, Antarctica near McMurdo station, on December 15, 2006, and executed m...

Measurement of the Muon Atmospheric Production Depth with the Water Cherenkov Detectors of the Pierre Auger Observatory

“…[38,39,40,41] and neutrinos (right) [52] obtained from the Pierre Auger Observatory. The data are shown together with the current limits from other experiments [53,54,55,56,57,58,59] and some examples of predicted fluxes.…”

“…In figure 1.4 are shown the latest limits on the fluxes of photons [38,39,40,41] and neutrinos [42,43,44,45] including observations from the Pierre Auger Observatory [46], Anita-II [47] and IceCube [48]. Model scenarios for sources of UHECR like the top-down, in which the observed particles are produced by the decay of extremely heavy particles are strongly disfavour by the actual limits.…”

Upward-Pointing Cosmic-Ray-like Events Observed with ANITA

“…Such simulations have been applied to neutrino [4,5] and cosmic ray [2,11] analyses. In this study, we take a simplified approach with the objective of setting an upper bound to the sensitivity of ANITA.…”

“…The competing hypotheses for the τ-lepton air shower candidate are that it is either a downgoing cosmic ray extensive air shower with its radio impulsive emission reflected off the surface of the ice (see [2]) or an anthropogenic background (see [3,4,5]). The τ-lepton decay is favored over the down-going cosmic ray hypothesis due to the polarity of the impulse and the correlation of the polarization vector to the geomagnetic field.…”

Constraints on ultra-high-energy neutrino flux from radio observations of the Moon

“…Therefore, at high frequencies, only cascades directed towards the surface can be detected at Earth. Such cascades only occur at the rim of the Moon, when neutrinos enter from behind (Gorham et al, 2004). At low frequencies, the isotropic radiation is able to escape the Moon for most geometries and the complete visible lunar surface is part of the effective detector volume.…”