Atmospheric Radio Signals From Galactic Dark Matter

If the dark matter of our galaxy is composed of nuggets of quarks or antiquarks in a colour superconducting phase there will be a small but non-zero flux of these objects through the Earth’s atmosphere. A nugget of quark matter will deposit only a small fraction of its kinetic energy in the atmosphere and is unlikely to be detectable. If however the impacting object is composed of antiquarks the energy deposited can be quite large and contain a significant charged particle content. These relativistic secondary particles will subsequently be deflected by the earth’s magnetic field resulting in the emission of synchrotron radiation. This work will argue that this radiation should be detectable at radio frequencies and that present and proposed experiments are capable of detecting such a signal.

💡 Research Summary

The paper proposes a novel detection channel for a class of dark‑matter candidates known as quark (or antiquark) nuggets—macroscopic objects composed of dense quark matter in a colour‑superconducting phase. Assuming that a substantial fraction of these nuggets are made of antiquarks, the authors argue that when such an anti‑nugget traverses the Earth’s atmosphere it will annihilate atmospheric nuclei on its surface, releasing a large amount of energy despite the nugget’s tiny geometric cross‑section. Only a small fraction of the nugget’s mass actually annihilates during the transit, but the annihilation products include high‑energy muons and gamma rays, as well as heating of the nugget’s surface to temperatures of order 10 keV.

Two distinct mechanisms generate radio‑frequency emission:

1. Thermal emission from the heated surface. The electrosphere surrounding the quark matter modifies the black‑body spectrum, yielding a power per unit frequency of roughly 10⁻¹⁰ J s⁻¹ MHz⁻¹ for a nugget of radius ~10⁻⁵ cm at T≈10 keV. In the ω < T regime the spectrum is essentially flat with a weak logarithmic dependence, providing a broadband radio signal that scales with the inverse square of the distance to the observer.

2. Geo‑synchrotron emission from relativistic muons that escape the nugget. These muons (β≈0.9–0.99 c) experience a small transverse acceleration in Earth’s magnetic field (B≈5 µT). The resulting synchrotron radiation is beamed along the muon velocity and its intensity depends on the muon production efficiency f_μ (estimated 0.01–0.1 per annihilation) and the number of annihilations encountered, which in turn is set by the atmospheric density profile and the nugget’s physical cross‑section (σ_N≈10⁻¹⁰ cm²). The electric field amplitude follows equations (6)–(8) in the paper, with a frequency dependence governed by an integral over the muon‑observer separation.

The authors estimate the flux of such nuggets using the local dark‑matter density (ρ≈1 GeV cm⁻³) and a typical galactic velocity (v≈200 km s⁻¹). For baryon numbers B≈10²⁵–10³¹ the resulting yearly flux through a square kilometre is of order 10⁻⁴ km⁻² yr⁻¹, comparable to the flux of ultra‑high‑energy cosmic rays near the GZK cutoff. Consequently, existing large‑area radio observatories—ANITA, ARA, LOFAR, and upcoming facilities such as SKA‑Low—have sufficient exposure to detect or constrain these events.

Key uncertainties identified by the authors include: (i) the exact muon multiplicity and energy spectrum per annihilation, (ii) the equilibrium surface temperature at which annihilation heating balances radiative cooling (which determines the saturation altitude h_eq of the annihilation rate), and (iii) the detailed electrosphere suppression factor for thermal emission below the electron mass. The paper emphasizes that while these parameters are currently known only to order‑of‑magnitude, the predicted radio signal is robust enough to be within reach of current instrumentation.

In the concluding sections, the authors suggest that a combined analysis of broadband thermal radio emission and the more directional geo‑synchrotron component could provide a distinctive signature that separates nugget‑induced air showers from conventional cosmic‑ray‑induced events. They advocate for dedicated searches in archival data of existing radio experiments and for the inclusion of specific trigger algorithms in future low‑frequency arrays to capture the short‑duration (microsecond to millisecond) radio bursts expected from anti‑nugget passages.

Overall, the work opens a new observational window on macroscopic dark‑matter candidates, linking high‑energy particle physics, atmospheric physics, and radio astronomy, and it outlines a concrete pathway for experimental verification using both current and planned radio detection facilities.