Neutrinos travel continuously across the entire earth. Without being absorbed or deflected, they traverse billions of light-years from colliding black holes and exploding stars, arriving undamaged and pointing directly back at their source. However, a city-sized detector is needed to capture enough of them to learn anything.
Thus, researchers employed a cubic kilometer of Antarctic ice as the detector and buried antennae 150–200 meters below the South Pole. Gurgen Askaryan, a Soviet physicist, predicted in 1962 that a high-energy particle colliding with ice would cause a series of secondary particles to emit at radio frequencies. The Askaryan Radio Array (ARA) was designed to detect that.
Thirteen inexplicable radio signals from beneath the ice were detected by ARA in 2019. It took years of innovative simulation methods to separate real signals from the nearby Amundsen-Scott South Pole Station’s radio noise. With a 5.1 sigma confidence level, all 13 occurrences in the 2019 data matched the expected features of Askaryan radiation, including arrival direction, frequency, waveform, and polarization. This means that there is only a 1 in 3.5 million chance that all 13 are noise. Askaryan’s 63-year-old prediction had never been verified in ice.
Instead of neutrinos, which appear almost identical in ice but come from steeper angles, the 13 signals were cosmic rays. Making that distinction demonstrated the effectiveness of the detection technique. According to a report published in Physical Review Letters, a complete multi-year data release from all five ARA sites is anticipated shortly, containing up to seven candidate neutrino occurrences.
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