_{1}

^{*}

We suggest that the unusual events observed by the ANITA experiment originate from axion particles traversing the Earth. Under the influence of the geomagnetic field, the axion may oscillate into a photon and vice-versa. To amplify the axion transition into photon, we consider that the phenomenon takes place at resonance, where the effective photon mass is equal to the axion mass. This requirement fixes the axion mass at 44 eV. An axion at this mass scale reproduces the cold dark matter scenario. If our interpretation prevails, with the help of axions we can establish an axion tomography of the Earth.

The Antarctic Impulsive Transient Antenna (ANITA) experiment has observed two air shower events with energy ~500 PeV emerging from the Earth with exit angles ~30˚ above the horizon [_{E} (6371 km).

One might think the neutrino lies at the origin of the ANITA unusual events. The neutrinos interact with the nucleons through the weak charged current, resulting in absorption, and the weak neutral current, which implies a redistribution of the neutrino energy. For a detailed analysis see [

Let us recall that the raison d’être of the axion particle is the strong CP problem. The QCD Lagrangian respects all symmetries (P, C, CP…). At low energies, the non-linear nature of the theory introduces a non-trivial vacuum which violates the CP symmetry. The CP-violating term is parameterized by θ and experimental bounds indicate that θ < 10^{−9}. This is a very small number, and the smallness of this parameter creates what is known as the strong CP problem. An elegant solution has been offered by Peccei-Quinn [_{PQ} symmetry is introduced, the spontaneous breaking of which provides the cancellation of the θ-term. As a byproduct, we obtain the axion field, the Nambu-Goldstone boson of the broken U(1)_{PQ} symmetry. There are extensive reviews covering the theoretical aspects and the experimental searches for the axion [

A general feature of the axion is its two-photon coupling

L a γ γ = − 1 4 g a F μ ν F ˜ μ ν = g a E → ⋅ B → (1)

where a is the axion field, F μ ν ( F ˜ μ ν ) the (dual) electromagnetic field strength tensor and g the photon-axion coupling constant. Accordingly, in the presence of a magnetic field B → , a photon may oscillate into an axion and vice-versa. A prototype experiment in the search for solar axions is the CAST experiment, which set the limit g < 10^{−10} GeV^{−1} [^{2} is (BL)^{2} ≈ 7000 T^{2}m^{2}. Our proposal involves Earth’s magnetic field, a magnetic dipole with a mean value B 0 ≈ 3 × 10 − 5 T on the Earth’s surface. The weakness of the geomagnetic field B is compensated by the larger L value, of the order of Earth’s radius R_{E}. Therefore, in our case, the scale is (BL)^{2} ≈ 36,100 T^{2}m^{2}. This increased value allows a higher accuracy and the exploration of a new range of g and m_{a} (coupling constant and axion mass respectively) [

Consider a travelling photon of energy E and let us define as z-axis the direction of photon’s propagation. The polarization of the photon A → lies then at the x-y plane. The photon is moving in the presence of the geomagnetic field B → . The component of B → parallel to the direction of motion does not induce photon-axion mixing. Following Equation (1), the transverse magnetic field B → Τ couples to A I I , the photon polarization parallel to B → Τ and decouples from A ⊥ , the photon polarization orthogonal to B → Τ . The photon-axion mixing is governed by the following equation:

( E − i ϑ z + M ) ( A I I a ) = 0 (2)

The 2-dimensional matrix M is

M = ( − m γ 2 2 E g B T 2 g B T 2 − m a 2 2 E ) (3)

For a photon, moving in a medium with number density of electrons N_{e}, the effective photon mass m_{γ} is given by

m γ 2 = 4 π α N e m e (4)

Assuming that Earth’s material contains an equal number of protons and neutrons, we obtain the estimate [

N e ~ ρ / ( 2 m N ) (5)

Therefore

m γ 2 = 2 π α m e m N ρ (6)

The density of the Earth as a function of the distance is rather well known and very close to the two-density model description, in which the core and the mantle each have a separate and constant energy [_{2} = 3490 km and whose constant density is 11.0 g/cm^{3}. The mantle, a spherically symmetric shell of constant density 4.4 g/cm^{3}, surrounds the core and extends out to R_{E} = 6371 km. For a nadir angle ~60˚, our particle crosses the Earth at a distance 5517 km far from the center, traversing a distance of 6371 km within the Earth. Moving entirely within the mantle, ρ = 4.4 g/cm^{3} and the effective photon mass is m_{γ} ~44 eV.

Matrix M is diagonalized through the angle Θ with

tan 2 Θ = 2 g B T E m a 2 − m γ 2 (8)

Defining

D = 1 2 E [ ( m a 2 − m γ 2 ) 2 + 4 g 2 B T 2 E 2 ] 1 / 2 (9)

sin 2 Θ = g B T D (10)

we obtain for the probability that an axion converts into a photon after travelling a distance s

P ( a → γ ) = sin 2 2 Θ sin 2 D s 2 (11)

A resonance phenomenon occurs, offering the maximum probability, when

m a = m γ (12)

We gather that the most favorable value for the axion mass is m_{a} ~44 eV. Proceeding along these lines we obtain that at resonance and for values of D s ≪ 1

P ( a → γ ) = 1 4 g 2 B T 2 s 2 (13)

Putting the appropriate numbers (g = 10^{−10} GeV^{−1}, B = 3 × 10^{−5} T, s = R_{E}) we obtain

P ( α → γ ) = 10 − 16 . (14)

Let us summarize our findings. We suggest that highly energetic axions traverse the Earth and they are becoming photons under the influence of the geomagnetic field. These photons create the showers observed by ANITA. The photon polarization is parallel to the geomagnetic field in the Antarctica and therefore we expect a strongly horizontally polarized (H_{pol}) signal. Indeed this is observed by the ANITA experiments [_{a} of the PQ symmetry breaking, similar to how the low energy pion interactions are fixed by the pion decay constant f_{π}. Next to QCD interactions, we should include the electroweak interactions and also the gravitational interactions [

One might wonder what the origin of these energetic axions is. We can imagine that the inverse phenomenon takes place at gigantic extragalactic scale. VHE photons in the presence of magnetic fields at their source suffer conversion into axions, thus avoiding absorption by γγ collisions on the extragalactic background light. Through this mechanism, we obtain a spectrum of “hard” axions [

There are other proposals to address the unusual ANITA events. It has been suggested that an axion pulse is transformed into an electromagnetic pulse in Earth’s ionosphere [

Note Added: Our work was followed by the findings of the XENON1T experiment (https://arxiv.org/pdf/2006.09721.pdf) reporting an excess of events, attributed most probably to axions.

Dimitris Evangelinos of the Physics Department helped enormously in the typesetting of the text.

The author declares no conflicts of interest regarding the publication of this paper.

Nicolaidis, A. (2021) An Axion Interpretation of the ANITA Events. Journal of Modern Physics, 12, 59-64. https://doi.org/10.4236/jmp.2021.122006