A first search for cosmogenic neutrinos with the ARIANNA Hexagonal Radio Array
Introduction
While the flux of cosmic rays has been measured to energies greater than 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 ) 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 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 GeV, IceCube currently provides the best constraints on the UHE neutrino flux and in the 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 GeV, particularly in the 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 (100) . 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 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 . This bandwidth is sensitive to the linear increase in power of the Askaryan pulse with frequency up to 1 GHz for signals measured on the Cherenkov cone [33].
The ARIANNA site is roughly 100 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 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 [34] including a firn layer within the upper 60–70 [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.
Section snippets
The ARIANNA telescope
The ARIANNA experiment plans to measure the cosmogenic neutrino flux using a large surface array of radio receivers. ARIANNA will build upon previous UHE neutrino searches by greatly increasing the size of the detector. This will improve the sensitivity to neutrinos of GeV by a factor of 13 or more, depending on model, relative to the current best limits (see Section 2.2.2). In order to maximize the effective volume of the telescope, each ARIANNA station of the array will be
The Hexagonal Radio Array
The ARIANNA Hexagonal Radio Array (HRA) is being constructed on the Ross Ice Shelf and consists of seven prototype stations arranged as shown in Fig. 12. This small array, begun in 2009 [77], serves as a prototype for the development and study of ARIANNA hardware, data acquisition (DAQ) and radio data analysis. Three stations have been installed at the ARIANNA site, Stations A, C and G, to form the HRA-3 detector. The fourth station, Station D, is of a preliminary design installed during the
The data set
Data taken by the HRA-3 between January 3 and April 9, 2014 has been analyzed to search for neutrino-induced Askaryan signals. The former date corresponds to the departure of the deployment crew from the Ross Ice Shelf. The latter date is chosen to include all data successfully transferred off of Antarctica. The entire data set taken by the HRA-3 and transferred off site is included in the analysis, resulting in a combined livetime for the three stations of 170 days.
The bulk of the livetime
Conclusions
The ARIANNA experiment proposes the use of the Askaryan effect to search for a diffuse flux of neutrinos in the GeV energy range. The experiment will exploit the long attenuation length of ice at radio frequencies by populating the surface of the Ross Ice Shelf of Antarctica with a grid of radio detectors to reach an effective volume on the order of 100 . The ice to water interface at the bottom of the ice shelf acts as a mirror to radio pulses, making ARIANNA sensitive to neutrinos
Acknowledgements
The authors wish to thank the staff of Antarctic Support Contractors, Lockheed, and the entire crew at McMurdo Station for excellent logistical support. The authors also thank Prof. De Flaviis for the use of the Far Field Anechoic Chamber at U.C. Irvine and would like to acknowledge and thank the CReSIS project and the Anechoic Chamber facility management for the use of the world class anechoic chamber at the University of Kansas.
This work was supported by generous funding from the US National
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