The ANITA Anomalies: Two Upgoing Events Nobody Has Explained

The Antarctic Impulsive Transient Antenna — ANITA — was designed to search for the highest-energy neutrinos in the universe. A balloon-borne array of radio antennas circumnavigating the Antarctic continent every two weeks, ANITA listened for the coherent radio emission that ultra-high-energy neutrino interactions in the polar ice would produce via the Askaryan effect. Across four flights between 2006 and 2016, the experiment provided some of the strongest limits on the cosmogenic neutrino flux predicted from ultra-high-energy cosmic-ray interactions with the cosmic microwave background.

But ANITA’s most discussed results were not the limits. They were two anomalous events — one in the 2006-2007 ANITA-I flight and one in the 2013-2014 ANITA-III flight — that appeared as upward-going air showers detected by their radio emission. Both had energies of approximately $5\times 10^{17}$ eV, comfortably within the energy regime where conventional ultra-high-energy neutrino interactions could in principle produce detectable signals. But both arrived at angles substantially below the horizon — 27 and 35 degrees — where conventional ultra-high-energy neutrinos should have been absorbed by passage through the Earth long before reaching the surface.

In the years since, the two events have attracted dozens of theoretical explanations and a corresponding effort to find them in the data of larger detectors. None of the conventional explanations works convincingly; none of the beyond-Standard-Model explanations has been confirmed. The puzzle is still open, and the next generation of radio-detection experiments — PUEO, ARA, RNO-G — will eventually settle whether the two events were a real anomaly or a statistical fluctuation of the unconventional but conventional kind.

This post is about what ANITA saw, why it is so hard to explain, and what the field is doing about it.

The ANITA technique

Cosmic-ray air showers and neutrino-induced ice showers both produce a characteristic radio signal through the geomagnetic-induced motion of the shower’s charged particles. The signal is coherent in the GHz range, with a polarisation that depends on the geometry of the shower. ANITA’s antennas, mounted on a payload at 35 km altitude over Antarctica, listened for these pulses in a quiet radio environment with horizons extending hundreds of kilometres across the ice sheet.

The principal ANITA signal is cosmic-ray air showers detected from above, when downgoing extensive air showers traverse the atmosphere and produce coherent radio emission visible from the ANITA payload. Hundreds of such cosmic-ray events were recorded across the four flights, calibrating the technique and confirming the radio-detection physics.

The principal ANITA target is ultra-high-energy neutrino interactions in the Antarctic ice. The Askaryan effect produces coherent radio emission from the negative charge excess in a dense-medium electromagnetic shower; the resulting pulse escapes the ice upward and is detected by the antennas. ANITA’s limits on the neutrino flux at exa-electron-volt energies were the world’s best for several years.

The unexpected was the third category: upward-going air showers of the same general kind as the cosmic-ray downgoing events, but with the polarisation flipped, indicating an upward-going direction. The two anomalous events fall in this category.

What makes the events anomalous

A cosmic-ray air shower coming from above produces radio emission polarised in a specific direction relative to the geomagnetic field. The same shower travelling upward — from below the horizon, exiting the Earth’s surface — would produce emission polarised in the opposite direction. The polarity is unambiguous in the radio pulses, so distinguishing upgoing from downgoing events is straightforward in principle.

For the two ANITA anomalous events, the polarity unambiguously indicates upward-going showers from below the horizontal — 27 degrees below in ANITA-I and 35 degrees below in ANITA-III. The reconstructed event energies are both in the range of $0.6\times 10^{18}$ eV.

A conventional ultra-high-energy neutrino would have to traverse the Earth before producing such a shower. At the 27-degree-below-horizontal angle, the column density of Earth between the entry point and the surface is approximately $10^{10}$ g/cm². The neutrino-nucleon cross-section at exa-electron-volt energies is roughly $10^{-32}$ cm². The mean free path is therefore

${\lambda }_{\nu }=\frac{1}{n_N{\sigma }_{\nu }}\approx 1700\mathrm{km}$

For the column density at 27 degrees below horizontal — approximately 9000 km of Earth-equivalent — the survival probability is

$P_{\mathrm{survive}}\approx \exp (-9000/1700)\approx 5\times 10^{-3}$

A 35-degree angle gives survival probability $\sim 10^{-5}$. Even an enormous neutrino flux at the source would be substantially absorbed at these angles.

Worse, no plausible neutrino source has been identified producing a flux at exa-electron-volt energies large enough to make even a $10^{-3}$-survival event detectable at ANITA’s effective area. The implied flux to deliver two events at the relevant angles is in tension with stringent IceCube limits on the diffuse neutrino flux at the same energies. The conventional neutrino-origin picture, taken seriously, does not work.

The leading conventional alternative: earth-skimming taus

The most commonly invoked conventional explanation is earth-skimming tau neutrinos. The mechanism:

1. A ${\nu }_{\tau }$ enters the Earth at a shallow angle near the horizon.
2. The neutrino undergoes a charged-current interaction with a nucleon, producing a $\tau$ lepton.
3. The $\tau$, with a decay length of roughly 50 m per PeV of energy, exits the Earth before decaying — but only at shallow angles where the column of Earth is thin enough for the $\tau$ to escape.
4. The $\tau$ decays in the atmosphere, producing the air shower whose radio emission ANITA detects.

For this mechanism to work, the angle must be near horizontal — within about 10-15 degrees below horizontal at most, where the column of Earth allows the chain ${\nu }_{\tau }\to \tau \to$ shower to proceed efficiently. At larger angles below horizontal, the column is too thick for the parent ${\nu }_{\tau }$ flux to survive.

Both ANITA anomalous events sit outside the geometrically favoured range for earth-skimming taus. The 27-degree angle in particular requires the parent ${\nu }_{\tau }$ flux to be enormously larger than IceCube’s limits allow, even after accounting for the $\tau$-amplification gain that converts a neutrino into a shower.

Quantitative analyses (Romero-Wolf et al., De Vries et al.) have placed the probability of both ANITA anomalous events being earth-skimming tau-neutrino events at the $10^{-3}$ level or smaller. The conventional explanation, while not formally ruled out, is highly disfavoured.

Beyond-Standard-Model proposals

The puzzle has generated a large theoretical literature of beyond-Standard-Model explanations. A small sample:

Stau leptons in supersymmetry. A long-lived stau with mass in the relevant range could be produced in extragalactic accelerators, propagate through cosmological distances, traverse the Earth at substantially smaller cross-section than a Standard Model particle, and decay in the atmosphere to start an air shower. Several SUSY scenarios with long-lived staus produce signatures qualitatively matching the ANITA events.

Sterile neutrinos with energy-dependent mixing. A sterile neutrino with a small but nonzero mixing into the active sector, with the mixing growing with energy, could effectively appear in the atmosphere from below.

Heavy neutral leptons with modified couplings. HNLs with reduced interactions with matter could traverse Earth-scale columns and decay to shower-producing visible particles.

Modified neutrino-nucleon cross-sections at ultra-high energies. Some beyond-Standard-Model scenarios reduce the effective cross-section at the relevant energies, allowing conventional neutrinos to survive longer Earth crossings.

Dark matter decay products. A heavy decaying dark-matter particle could produce ultra-high-energy secondaries that reach ANITA’s altitude through the atmosphere.

None of these has been confirmed by independent observation, and most face their own constraints from other experiments. The status remains: an unsolved puzzle.

What IceCube has said

IceCube’s much larger effective area and continuous Antarctic-ice exposure provides a powerful independent check. If the ANITA events reflected a real population of upward-going $\sim 10^{18}$-eV events, IceCube should have seen a substantial number of correlated events.

Several IceCube searches have been performed:

• Stacking search at the ANITA event positions: no significant excess.
• Diffuse search for high-energy upward-going tau-neutrino events: limits inconsistent with the ANITA-event flux interpretation.
• Search for the specific event type of high-energy through-going tracks from below: no anomaly.

The combination has reduced the parameter space for any astrophysical interpretation of the ANITA events to negligible levels. The events, whatever they are, are not part of a substantial population — they are either rare statistical fluctuations or pointlike instances of a specific mechanism.

The next generation

Several upcoming radio-detection experiments will return to the ANITA energy range with much larger exposure and better systematics:

PUEO (Payload for Ultrahigh Energy Observations), the successor to ANITA, will fly with improved trigger and reconstruction systems and a substantially larger effective area at the ANITA event energies. The mission timeline targets first flights in the late 2020s.

RNO-G (Radio Neutrino Observatory in Greenland) is a ground-based radio array deployed in the Greenland ice cap, with an instrumented volume of about 200 km³ planned over its full configuration. The Greenland location provides complementary sky coverage to Antarctic-based experiments.

IceCube-Gen2 radio array will instrument a portion of the South Pole ice with a sparse radio array at scale, with first deployment expected in the late 2020s.

GRAND (Giant Radio Array for Neutrino Detection) is a proposed array of antennas spread across mountainous terrain, designed specifically to detect earth-skimming tau-neutrino-induced air showers. If GRAND is deployed at the design configuration, it will provide independent confirmation or refutation of the conventional tau-neutrino interpretation of ANITA-like signatures.

The combined sensitivity of these experiments over the coming decade should determine whether the ANITA events were a real anomaly representing new physics or whether they were a rare combination of background contamination and reconstruction effects that happened to look like upward-going showers.

Why the puzzle is interesting

The ANITA events sit in a corner of phase space — energy and angle — where almost any conventional explanation requires a substantial fluence that other experiments rule out. The two-orders-of-magnitude tension with IceCube’s limits is real and represents one of the most pointed open anomalies in high-energy astroparticle physics. Most anomalies of this kind eventually resolve into either a systematic effect or a confirmed new-physics signal, and the ANITA puzzle is overdue for one or the other.

If the events turn out to be a misidentification — radio reflections from large internal ice features in Antarctica, or unusual atmospheric conditions during the relevant balloon flights — the resolution will be a useful lesson in radio-detection systematics. If they turn out to be a real signal of beyond-Standard-Model physics at exa-electron-volt energies, they will define the next big direction for high-energy neutrino physics.

For now, the ANITA events remain on the small list of unresolved anomalies that the field watches carefully, alongside the gallium anomaly (BEST), the muon $g-2$ tension, and various flavour-physics anomalies in B-meson decays. Each of these has resisted resolution longer than initially expected, and each requires continued attention from the next generation of dedicated experiments.

Summary

The ANITA balloon-borne radio antenna array detected two anomalous radio pulses consistent with upward-going air showers at energies near $5\times 10^{17}$ eV, arriving at angles 27 and 35 degrees below the horizontal. Conventional ultra-high-energy neutrinos cannot survive the Earth-crossing column at these angles; earth-skimming tau-neutrino interpretations are kinematically disfavoured; and IceCube has ruled out any substantial population of similar events at the inferred flux. The two anomalous events therefore sit at the intersection of “no Standard-Model explanation works” and “no decisive new-physics explanation has been confirmed.” Several beyond-Standard-Model scenarios — long-lived supersymmetric particles, heavy neutral leptons, modified ultra-high-energy cross-sections, dark-matter decays — have been proposed but none has emerged as a clear preferred explanation. The next generation of radio-detection experiments (PUEO, RNO-G, IceCube-Gen2 radio, GRAND) will return to this energy range with much larger effective area over the coming decade and should either confirm a continuing anomaly or resolve the two events as a statistical fluctuation. Either way, the ANITA puzzle remains one of the more pointed open questions at the high-energy frontier of neutrino physics.