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Constraints on the diffuse flux of ultrahigh energy neutrinos from four years of Askaryan Radio Array data in two stations

P. Allison et al. (ARA Collaboration)
Phys. Rev. D 102, 043021 – Published 27 August 2020
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  • INTRODUCTION
  • INSTRUMENT DESCRIPTION
  • DATA ANALYSIS
  • RESULTS
  • SYSTEMATIC UNCERTAINTIES
  • DISCUSSION AND OUTLOOK
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    The Askaryan Radio Array (ARA) is an ultrahigh energy (UHE, >1017  eV) neutrino detector designed to observe neutrinos by searching for the radio waves emitted by the relativistic products of neutrino-nucleon interactions in Antarctic ice. In this paper, we present constraints on the diffuse flux of ultrahigh energy neutrinos between 1016 and 1021  eV resulting from a search for neutrinos in two complementary analyses, both analyzing four years of data (2013–2016) from the two deep stations (A2, A3) operating at that time. We place a 90% CL upper limit on the diffuse all flavor neutrino flux at 1018  eV of EF(E)=5.6×10−16  cm−2 s−1 sr−1. This analysis includes four times the exposure of the previous ARA result and represents approximately 1/5th the exposure expected from operating ARA until the end of 2022.

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    • Received 11 May 2020
    • Accepted 15 July 2020

    DOI:https://doi.org/10.1103/PhysRevD.102.043021

    © 2020 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Cosmic rays & astroparticlesExtrasolar neutrino astronomyParticle astrophysics
    1. Techniques
    Cosmic ray & astroparticle detectorsNeutrino detection
    Gravitation, Cosmology & Astrophysics

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    Issue

    Vol. 102, Iss. 4 — 15 August 2020

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    Images

    • Figure 1
      Figure 1

      A diagram showing how a high-energy neutrino interaction might be observed in an ARA station. The insets show how the Askaryan emission and its polarization would be observed if seen along, and perpendicular to, the shower axis. A more detailed view of an ARA station can be found in Fig. 2.

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    • Figure 2
      Figure 2

      (left) A top-down view of the ARA5 instrument as deployed at the South Pole, with stations color-coded by the year they were deployed. The green stations, A2 and A3, are the focus of the analysis described in this paper. (right) A schematic of the electronics and instrumentation in an ARA station; “FO” is a fiber-optic transmitter.

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    • Figure 3
      Figure 3

      An event display showing the 16 waveforms recorded in A2 for a VPol calibration pulser.

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    • Figure 4
      Figure 4

      Operational fractional live times for A2 (left) and A3 (right) from deployment in February 2013 through the end of the analysis period in 2016; each bin is one month wide. From the four years of deployment, 1141 days from A2, and 1021 days from A3, are good for analysis. This is mostly due to intermittent downtime; quality cuts remove less than 2% of live time.

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    • Figure 5
      Figure 5

      (top) The simulated trigger-level effective area steradian ([AΩ]eff) for A2, averaged across configurations. For comparison, we also show the analysis-level sensitivity of IceCube [36]. (bottom) The percent difference between the A2 and A3 effective areas.

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    • Figure 6
      Figure 6

      An example of the bivariate cut plane, for which the final 2D box cut is made for A3 configuration 3. (left) The plane as observed in 10% “burn-sample” data, showing events clustering at low correlation and low root power ratio. (right) The plane populated with simulated neutrinos at 1018  eV, showing events distributed throughout. Events at low correlation and low root power ratio are cut; events at higher values define the signal region and pass the analysis.

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    • Figure 7
      Figure 7

      The 90% confidence-level upper limit on the all-flavor diffuse flux of neutrinos set by this analysis (thick black line). The limit accounts for uncertainties in the background estimate and systematic uncertainties on the neutrino sensitivity. We also plot the projected trigger-level single-event sensitivity (TL SES) for the five-station ARA5 array by 2022 as a black-dashed curve. Also shown are the latest limits and flux measurements from IceCube [20, 45], Auger [21] (rescaled with decade-wide bins and for all-flavors), ANITA [23] (rescaled with decade-wide bins), and ARIANNA [24]. Shown for comparison are several benchmark cosmogenic neutrino flux models [13, 16, 50].

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    • Figure 8
      Figure 8

      Monte Carlo estimated analysis efficiency as a function of signal-to-noise ratio (left) and neutrino energy (right) for Analysis A. For context, the trigger efficiency of an ARA station has been measured to reach 50% at an SNR of 3.7 [46]. In the left figure, we assume an unbroken power-law spectrum with a spectral index of −2.13 to weight the energies contributing to the efficiency. The efficiency decrease around SNR=14 is due to waveform saturation effects as simulated in arasim.

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    • Figure 9
      Figure 9

      Uncertainties between the central values used in the simulation and upper/lower bounds for each model parameters. Theoretical systematics (shaded regions), such as the Askaryan model and the neutrino-nucleon cross section, are not accounted for when calculating the neutrino limit. Uncertainties associated with the detector and medium (dashed and solid lines) are accounted for in the calculation.

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    • Figure 10
      Figure 10

      Same as Fig. 7, but with an alternative flux scaling.

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    • Figure 11
      Figure 11

      Signal efficiency from Analysis A for each configuration in A2 and A3, respectively. Top row: signal efficiency as a function of neutrino energy for A2 (top left) and A3 (top right). Bottom row: signal efficiency as a function of signal-to-noise-ratio for A2 (bottom left) and A3 (bottom right) assuming the same energy weighting scheme as in Fig. 8.

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