A first search for cosmogenic neutrinos with the ARIANNA Hexagonal Radio Array

Abstract

The ARIANNA experiment seeks to observe the diffuse flux of neutrinos in the ${10}^8''–''{10}^{10}$ GeV energy range using a grid of radio detectors at the surface of the Ross Ice Shelf of Antarctica. The detector measures the coherent Cherenkov radiation produced at radio frequencies, from about 100 MHz–1 GHz, by charged particle showers generated by neutrino interactions in the ice. The ARIANNA Hexagonal Radio Array (HRA) is being constructed as a prototype for the full array. During the 2013–14 austral summer, three HRA stations collected radio data which was wirelessly transmitted off site in nearly real-time. The performance of these stations is described and a simple analysis to search for neutrino signals is presented. The analysis employs a set of three cuts that reject background triggers while preserving 90% of simulated cosmogenic neutrino triggers. No neutrino candidates are found in the data and a model-independent 90% confidence level Neyman upper limit is placed on the all flavor $\nu +\overline{\nu }$ flux in a sliding decade-wide energy bin. The limit reaches a minimum of $1.9\times {10}^{-23}{\mathrm{GeV}}^{-1}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ in the ${10}^{8.5}''–''{10}^{9.5}$ GeV energy bin. Simulations of the performance of the full detector are also described. The sensitivity of the full ARIANNA experiment is presented and compared with current neutrino flux models.

Introduction

While the flux of cosmic rays has been measured to energies greater than ${10}^{10}$ GeV [1], the sources of such high energy particles remain a mystery. No known galactic source could accelerate particles to such energies, and no particular sources of the very highest energy particles, with large rigidities, have been found [2], [3], [4], [5]. Potential sources of such ultra-high energy (UHE) cosmic rays are limited to our local supercluster (within about 50 $\mathrm{Mpc}$) due to their interaction with the cosmic microwave background (CMB) [6], [7]. The mesons produced by this process promptly decay to leptons, leading to a flux of UHE neutrinos [8], [9], [10].

Cosmogenic neutrinos may reveal cosmic accelerators beyond our local supercluster, as the mean free path of neutrinos through the CMB is larger than the visible universe. Such neutrinos would be produced within about 50 $\mathrm{Mpc}$ of the cosmic ray sources and would travel to Earth without deflection by magnetic fields, potentially pointing back to the accelerating objects.

Several large projects (AMANDA [11], IceCube [12], [13], ANITA [14], [15], [16] and RICE [17], [18]) exploit the fact that ice is transparent to Cherenkov radiation (at both optical and radio wavelengths) in order to search for cosmogenic neutrinos. These experiments complement cosmogenic neutrino searches by air shower detectors such as the Pierre Auger Observatory [19], [20] and HiRes [21], [22]. Below energies of ${10}^{10}$ GeV, IceCube currently provides the best constraints on the UHE neutrino flux and in the ${10}^4''–''{10}^6$ GeV range, IceCube has observed an extra-terrestrial diffuse neutrino flux [23].

A new generation of neutrino experiments is emerging with the efforts of ARA [24], [25], GNO [26] and the Antarctic Ross Ice Shelf Antenna Neutrino Array (ARIANNA, described in this paper). These experiments seek to extend the neutrino flux measurements to ultra-high energies by constructing radio Cherenkov detectors that are orders of magnitude larger in effective sensitive volume than current experiments using well-understood and inexpensive technology. Preparation is underway for the next generation of ballon-borne experiments as well [27], with efforts like that of EVA [28]. A large number of models predict cosmogenic neutrino fluxes that are measurable by such experiments with improved sensitivity to neutrinos above ${10}^8$ GeV, particularly in the ${10}^8''–''{10}^{10}$ GeV range. See Section 2.2 for examples of such models.

The ARIANNA and ARA experiments are proposing the construction of arrays of radio detectors in Antarctica that will reach effective volumes $\mathcal{O}$(100) ${\mathrm{km}}^3$. A third radio experiment, GNO, is exploring the construction of a radio neutrino telescope in Greenland. These experiments will measure the radio-frequency (RF) pulse emitted by the charged particle shower resulting from a UHE neutrino interaction in ice via the Askaryan effect [29], [30]. The Askaryan radio pulse has been measured in a variety of dielectric materials using particle accelerators to induce charged particle showers [31], [32].

The ARIANNA collaboration plans to construct a $36\times 36{\mathrm{km}}^2$ array of 1296 independent, autonomous radio detector stations just below the surface of the Ross Ice Shelf. The ice to water interface below the Ross Ice Shelf serves as a mirror for radio waves, allowing the stations to observe neutrinos arriving from the sky above the detector as well as from the horizon. The detector will measure radio frequencies between about $100\mathrm{MHz}''–''1\mathrm{GHz}$. This bandwidth is sensitive to the linear increase in power of the Askaryan pulse with frequency up to $\sim$1 GHz for signals measured on the Cherenkov cone [33].

The ARIANNA site is roughly 100 $\mathrm{km}$ from the McMurdo Antarctic Station, which provides logistical support during construction. Despite the relative closeness of McMurdo, the ARIANNA site is free of anthropogenic RF noise due to its being buffered by Minna Bluff to the north and the Transantarctic Mountains to the west.

Properties of the ice at the ARIANNA site have been measured by transmitting polarized radio pulses into the ice and observing the reflected pulses at multiple locations. These measurements indicate that the ice to water interface is a near perfect mirror. The attenuation length, measured to be between 400 and 500 $\mathrm{m}$ for radio frequencies, is found to be comparable to the average thickness of the Ross Ice Shelf. The ice shelf thickness has been measured to be $576\pm 8\mathrm{m}$ [34] including a firn layer within the upper 60–70 $\mathrm{m}$ [35] (approximately). The firn layer is characterized by a monotonic increase in mass density as a function of depth. A more complete discussion of the ice properties at the ARIANNA site is presented in Ref. [34].

The construction of the ARIANNA Hexagonal Radio Array (HRA) is approved for completion during the 2014–2015 austral summer. This array of seven ARIANNA stations serves as a research and development project for the full ARIANNA array [36]. Each HRA station consists of four log-periodic dipole antennas (LPDAs), a high-speed data acquisition (DAQ) system, wireless communication peripherals and local renewable power generation.

The expected performance of the full ARIANNA telescope is presented in Section 2. The performance of the HRA stations is discussed in Section 3. A first search for neutrino signals in the HRA data is described in Section 4.