ASPACE-Q
The Astrophysics, Space Exploration and Quantum Computing Group
ASPACE-Q
The Astrophysics, Space Exploration and Quantum Computing Group
by Alice Păun, PhD student
ANITA (ANtarctic Impulsive Transient Antenna) is an instrument mounted on a hot-air balloon, optimized to detect polarized radio emissions originating from the electromagnetic component of cosmic ray cascades formed in the atmosphere. ANITA can detect these emissions either reflected off the surface of glaciers or directly, without reflection, from descending events near the horizon. These two types of signals can be easily distinguished due to the phase inversion that occurs when the wave is reflected. Two of the experiment’s flights, ANITA-I and ANITA-III, detected two unusual upward-going events (27.4° and 35° below the horizon) that behave like direct signals, even though they reach the detector at angles typical of reflected signals.
Starting from a 2016 article by the ANITA Collaboration, the observation of these unusual events and the vague explanations regarding their origins led to highly creative but incorrect interpretations by journalists, causing confusion among the general public. Most media reports claimed that researchers had uncovered evidence of an anti-Universe, a parallel universe identical to ours, where time flows backward, and the Big Bang marks its end, not its beginning, as in our universe.
Following this discovery, the scientific community proposed various scenarios, both within the Standard Model (SM) and beyond (BSM), to interpret these high-energy upward-going signals. One of the most promising SM strategies involves neutrinos passing “tangentially” through the Earth (traversing a shorter distance through the planet before exiting), producing electrically charged leptons that lose minimal energy before initiating the particle cascades observed by ANITA. The particle most likely fitting this model is the tau neutrino (ντ), which produces τ leptons through interactions with nucleons. Given the high energies of the two observed events, focusing on tangential neutrinos is explained by the fact that as neutrino energy increases, so does their interaction cross-section (probability of interaction with matter). For energies above 10⁸ GeV, the Earth becomes opaque to neutrinos [Ref1]. While neutrinos are often dubbed the “ghost particles” of the universe due to their near-zero mass and weak interactions, allowing them to travel vast distances unimpeded, at very high energies, their probability of interacting increases, making them more likely to reveal themselves.
Researchers also explored more daring scenarios involving beyond-the-standard-model (BSM) interpretations of these two atypical signals. Proposed models focusing on BSM particles fall into three main categories:
1. Astrophysical neutrinos interact within the Earth and are converted into BSM particles that propagate through matter and eventually reconvert into SM particles capable of initiating electromagnetic cascades like those observed by ANITA. (https://arxiv.org/abs/1809.09615)
2. Dark Matter accumulated inside the Earth decays, generating BSM particles that convert into SM particles near the glacier surface, inducing upward-going electromagnetic cascades.
3. Sterile neutrinos, potentially arising from Dark Matter decay, travel long distances through the Earth, producing τ leptons via interactions with nucleons near the Earth’s surface, which then generate electromagnetic cascades. (https://arxiv.org/abs/1803.11554 , https://doi.org/10.1103/PhysRevD.99.095014)
Up to now, researchers have not provided definitive evidence for any scenario that explains these two high-energy upward-going events within the constraints of the Standard Model. Although none of the proposed BSM scenarios have been confirmed, such a discovery could have significant implications for fundamental physics, potentially bringing us closer to understanding Dark Matter, which, together with Dark Energy, constitutes approximately 95% of the universe, leaving only 5% as ordinary matter. Thus, while no solid explanation for these anomalies exists yet, the purported evidence of a parallel universe remains far from a valid solution to this puzzle.
