The Antarctic Impulsive Transient Antenna (ANITA) experiment logged peculiar radio wave transmissions, detected by a suite of sensors suspended from NASA balloons traversing the stratospheric expanse above Antarctica. This apparatus is meticulously engineered to identify radio emissions originating from high-energy cosmic ray impacts within the Earth’s atmosphere. The overarching objective of ANITA is to illuminate the nature of remote cosmic phenomena by meticulously examining the signals that traverse the vastness to reach our planet. Instead of observable reflections from the icy terrain, these radio bursts appeared to emanate from beneath the horizon, a geometrical orientation that defies established paradigms in particle physics, potentially indicating the existence of novel particle types or interactions heretofore unknown to the scientific community.
ANITA’s strategic deployment over Antarctica is attributed to the minimal incidence of signal interference in that region; to capture emission phenomena, the aerially deployed radio detection system navigates immense ice fields, observing phenomena termed ice showers. Image credit: Stephanie Wissel / Penn State.
“The radio waves we observed were at exceptionally steep angles, approximately 30 degrees below the ice’s surface,” stated Dr. Stephanie Wissel, a distinguished physicist affiliated with Penn State.
“Our computations indicate that the anomalous signal must have traversed and interacted with thousands of kilometers of terrestrial rock before reaching the detector, a journey that should have rendered the radio wave undetectable due to absorption within the geological strata.”
“This presents an intriguing conundrum, as we presently lack a definitive explanation for the origin of these anomalies, but it is highly probable that they do not originate from neutrinos.”
Typically emanating from energetic sources such as the Sun or cataclysmic cosmic events like supernovas or even the Big Bang, neutrino signals are ubiquitously present.
However, these particles pose significant challenges for detection.
“At any given moment, a billion neutrinos are passing through your thumbnail, yet neutrinos exhibit a very low interaction cross-section,” Dr. Wissel elaborated.
“Consequently, we face a paradoxical situation: if we manage to detect them, it implies they have journeyed immense distances without engaging in any interactions.”
“It’s conceivable that we are detecting a neutrino originating from the very edge of the observable Universe.”
“Upon their detection and subsequent tracing to their cosmic origin, these particles can yield more profound insights into celestial events than even the most potent telescopes, owing to their capacity to travel unimpeded and at speeds approaching the velocity of light, thereby furnishing clues about astronomical occurrences millions of light-years distant.”
“Research collectives worldwide have dedicated substantial efforts to the conceptualization and fabrication of specialized detectors designed to capture even minute quantities of these highly sensitive neutrino signals.”
“Even a singular, faint signal from a neutrino contains an immense wealth of information, underscoring the profound significance of all collected data.”
“We employ radio detection methodologies to endeavor in the construction of exceptionally large-scale neutrino telescopes, facilitating our pursuit of events with a relatively low predicted occurrence rate.”
ANITA stands as one such sophisticated detector, strategically positioned in Antarctica to minimize the potential for external signal interference.
The capture of emitted signals is facilitated by dispatching the balloon-borne radio detector to traverse extensive tracts of ice, where it observes phenomena known as ice showers.
“Our radio antennae are mounted on a balloon that circumnavigates the Antarctic ice at an altitude of 40 kilometers,” Dr. Wissel explained.
“We direct our antennae downwards towards the ice surface, actively searching for neutrinos that interact within the ice, thereby generating radio emissions detectable by our instruments.”
These specific neutrinos, designated as tau neutrinos, undergo interactions within the ice, resulting in the production of a secondary particle known as a tau lepton. This lepton is subsequently ejected from the ice and undergoes decay, a physical process where the particle progressively loses energy as it propagates through space and disintegrates into its constituent particles, generating emissions referred to as air showers.
“If they were perceptible to the unaided eye, air showers might resemble a sparkler’s trail, with luminous particles following its trajectory,” Dr. Wissel remarked.
“Our analytical capabilities enable us to differentiate between these two distinct signal types – ice and air showers – thereby allowing us to ascertain the characteristics of the particle responsible for generating the observed signal.”
“These signals can subsequently be meticulously traced back to their point of origin, analogous to the predictable trajectory of a projectile launched at an angle, which will rebound at a corresponding angle.”
However, the recently identified anomalous findings preclude such straightforward origin tracing, as their angular trajectory is considerably steeper than predicted by current theoretical models.
Through rigorous analysis of data acquired across multiple ANITA operational periods, juxtaposed with sophisticated mathematical models and extensive simulations encompassing both conventional cosmic rays and upward-propagating air showers, the research team successfully mitigated background noise and definitively excluded the possibility of signals originating from other known particle types.
Subsequently, the scientists corroborated their findings by cross-referencing data from complementary observational platforms, including the IceCube Neutrino Observatory and the Pierre Auger Observatory, to ascertain whether any upward-going air showers, exhibiting characteristics similar to those documented by ANITA, had been registered by these independent experimental setups.
The comprehensive analysis revealed that the aforementioned complementary detectors had not recorded any phenomena that could account for ANITA’s observations, leading the researchers to classify the detected signal as anomalous, signifying that the constituent particles responsible for this signal are not neutrinos.
These observed signals do not align with the prevailing framework of particle physics. While numerous hypotheses posit a connection to dark matter, the absence of subsequent confirmatory observations from instruments like IceCube and the Pierre Auger Observatory significantly constrains such possibilities.
“Our team is presently undertaking the design and construction of the next generation of large-scale detector technology,” Dr. Wissel indicated.
“This forthcoming instrument, designated PUEO, will possess increased dimensions and enhanced sensitivity for neutrino signal detection, and it is anticipated to illuminate the precise nature of the anomalous signal.”
“My conjecture is that a peculiar radio propagation phenomenon occurs in proximity to ice and also near the horizon, which I do not fully comprehend at this juncture. Nevertheless, we have thoroughly investigated several such possibilities without success to date.”
“Consequently, this remains a persistent enigma, and I am eagerly anticipating the enhanced sensitivity that will be afforded by the PUEO mission, which will undoubtedly enable us to detect more anomalies and, potentially, ascertain their underlying causes.”
“Furthermore, it is within the realm of possibility that we may achieve the detection of neutrinos, an outcome that, in many respects, would be considerably more exhilarating.”
The researchers’ published paper detailing these findings has been featured in the esteemed journal Physical Review Letters.
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A. Abdul Halim et al. (Pierre Auger Collaboration). 2025. Search for the Anomalous Events Detected by ANITA Using the Pierre Auger Observatory. Phys. Rev. Lett 134, 121003; doi: 10.1103/PhysRevLett.134.121003
This article is derived from a press release disseminated by Penn State.
