A low-threshold ultrahigh-energy neutrino search with the Askaryan Radio Array

TL;DR

This paper reports on a low-threshold ultrahigh-energy neutrino search using the Askaryan Radio Array, achieving improved sensitivity and setting new flux limits, with implications for future radio-detection experiments.

Contribution

It demonstrates the effectiveness of reduced trigger thresholds in UHE neutrino detection and introduces the Phased Array triggering technique for future experiments.

Findings

Detected one neutrino candidate in 208.7 days of data.

Set a new upper limit on the UHE neutrino flux.

Showed that trigger-level sensitivity improvements enhance analysis capabilities.

Abstract

In the pursuit of the measurement of the still-elusive ultrahigh-energy (UHE) neutrino flux at energies of order EeV, detectors using the in-ice Askaryan radio technique have increasingly targeted lower trigger thresholds. This has led to improved trigger-level sensitivity to UHE neutrinos. Working with data collected by the Askaryan Radio Array (ARA), we search for neutrino candidates at the lowest threshold achieved to date, leading to improved analysis-level sensitivities. A neutrino search on a data set with 208.7~days of livetime from the reduced-threshold fifth ARA station is performed, achieving a 68\% analysis efficiency over all energies on a simulated mixed-composition neutrino flux with an expected background of $0.10_{-0.04}^{+0.06}$ events passing the analysis. We observe one event passing our analysis and proceed to set a neutrino flux limit using a Feldman-Cousins construction. We show that the improved trigger-level sensitivity can be carried through an analysis, motivating the Phased Array triggering technique for use in future radio-detection experiments. We also include a projection using all available data from this detector. Finally, we find that future analyses will benefit from studies of events near the surface to fully understand the background expected for a large-scale detector.