A Decade of Antarctic Radio Data Tightens the Search for Extreme Neutrinos

The most energetic cosmic rays arrive from beyond Earth, but magnetic fields bend the paths of charged particles, making their origins difficult to trace. Neutrinos offer a different kind of messenger: they carry no electric charge, so magnetic fields do not deflect them. If cosmic rays interact with matter or radiation, those interactions can also produce neutrinos. Finding such neutrinos could therefore help researchers investigate extreme cosmic environments and the origins of ultrahigh-energy cosmic rays.

The challenge is that neutrinos rarely interact with anything. The Askaryan Radio Array (ARA), embedded in Antarctic ice near the South Pole, searches for the brief radio pulse that can arise when a neutrino interaction creates a particle shower in the ice. The shower develops a slight excess of negative charge, producing a coherent radio signal known as Askaryan emission. Antennas buried in the ice can register the pulse.

Looking for a rare pulse

The collaboration analyzed data from all five ARA stations, including the phased-array detector at one station. The data span January 2013 to December 2023. After data-quality selections, the analysis used 25.9 station-years of data, accumulated over 10.6 years of array-wide livetime. A station-year means one station operating for a year; array-wide livetime counts periods when at least part of the array was taking data.

To estimate how the detector responds to neutrinos, the team used simulations. These modeled neutrinos and their secondary particles traveling through Earth, interacting in or near the ice, and producing signals in the antennas. The analysis also accounted for changes in detector conditions over time, including different operating configurations at the stations.

The researchers then processed recorded waveforms and looked for events with features expected of a neutrino signal: a short, strong pulse appearing coherently across antennas, with arrival times consistent with a source in the ice. They used reconstruction and signal-shape measures to distinguish such events from thermal noise, electronics effects, calibration pulses, cosmic-ray events, and human-made radio interference. Cuts were developed using a randomly selected 10% of the data; the other 90% was kept blinded until the selection and background estimates were finalized.

No candidates, and a stronger limit

After unblinding the full dataset, one event initially passed the selection. Further investigation found a simultaneous, anomalous signal in a nearby detector system. The reconstructed radio signal was consistent with emission from within the detector station, rather than a distant neutrino interaction. The team added a selection to remove events coincident with this kind of detector emission. No events remained.

That outcome is consistent with the estimated background: about 0.13 events, with statistical and systematic uncertainties. It is not evidence that ultrahigh-energy neutrinos do not exist. Instead, with no candidates observed, the researchers calculated an upper limit: a boundary on the diffuse neutrino flux compatible with the data at 90% confidence. They report that this is the most stringent limit in the energy range from 50 to 1,000 exaelectronvolts (EeV), where one EeV is a billion billion electronvolts.

The analysis is not equally efficient for every possible neutrino signal. Its efficiency depends on the assumed flux spectrum and energy, as well as on how likely a neutrino is to trigger the detector and survive the selection cuts. The paper reports 28% efficiency for a flux corresponding to current experimental upper limits; its detailed efficiency results also vary with energy and with the flux model. That distinction matters: the limit is not a direct count of all neutrinos passing Earth, but an inference based on modeled detector acceptance and exposure.

What remains uncertain

The team estimates that systematic uncertainties on the detector’s acceptance—the effective exposure to neutrinos—are substantial. They include uncertainty in how radio waves travel through and are absorbed by ice, the modeled Askaryan signal, and the neutrino interaction cross-section. At 1 EeV, the reported acceptance uncertainty is +40% and −29%; at the highest energies, uncertainty in the neutrino cross-section is especially important. Background estimates also have systematic uncertainty, reported as +0.05 and −0.06 events.

The result is a constraint, not a discovery or identification of a cosmic-ray source. It also tests particular flux models only under their stated assumptions: the paper says one trans-GZK flux model is excluded at 90% confidence, while other models remain unexcluded by this analysis. Larger arrays, more exposure, and improvements to background rejection could extend the search. In particular, the collaboration identifies surface-background rejection as a major opportunity to improve efficiency.

This article is based on an arXiv preprint describing the search methodology and results. A preprint is a research paper shared publicly before, or independently of, formal journal peer review; its claims and methods may be revised. The paper also refers to an accompanying results letter. The reported limits should therefore be understood in the context of the analysis and its uncertainties, rather than as a final answer about the origin of cosmic rays.