'Cosmic Rays' from Quark Matter

I describe a dark matter candidate based in qcd physics in which the dark matter is composed of macroscopically large “nuggets” of quark and anti-quark matter. These objects may have a sufficiently massive low number density to avoid constraints from direct detection searches. Though not “baryonic” in the conventional sense quark matter is strongly interacting and will produce a clear signal in ground based detectors. As the prospects of detecting these objects are mainly limited by the detector cross-section large scale cosmic ray detectors are a promising search platform. To this end I describe the basic properties of the air shower induced by the passage of a quark nugget through the earth’s atmosphere. It will be shown that this shower is similar in several important ways to the shower induced by a single ultrahigh energy cosmic ray.

💡 Research Summary

The paper proposes a novel dark‑matter candidate that is fundamentally different from the usual weakly interacting massive particles (WIMPs). It consists of macroscopic “nuggets” of quark and antiquark matter—often called quark nuggets or strangelets—stabilized by QCD dynamics during the early‑Universe phase transition. These objects can have masses ranging from 10⁻⁵ g up to a kilogram and radii from nanometres to millimetres, giving them an extremely low number density. Because of their large mass and low flux, conventional direct‑detection experiments (which are optimized for sub‑GeV to TeV‑scale particles with weak interactions) are essentially blind to them. However, the nuggets are strongly interacting: their surfaces retain a net electric charge (partial charge separation between quarks and antiquarks) that creates a powerful electromagnetic field.

When a nugget traverses the Earth’s atmosphere at relativistic speeds (β≈0.9), it collides with atmospheric molecules at a rate proportional to its geometric cross‑section (πR²) and the atmospheric number density. Each collision ionizes many molecules, producing thousands to tens of thousands of ion pairs per centimetre of path for a micron‑scale nugget. The liberated electrons and positrons are immediately accelerated by the nugget’s surface electric field, emitting intense bremsstrahlung photons. In addition, high‑energy gamma rays and neutrons are generated in nuclear excitations and de‑excitation processes. These secondary photons further produce electron‑positron pairs, amplifying the cascade.

A particularly important process is the “quantum‑evaporation” of charge from the nugget surface: as the surface charge is neutralized, a burst of electron‑positron pairs is emitted. This burst initiates an electromagnetic air shower that closely resembles the shower produced by an ultra‑high‑energy cosmic ray (UHECR). The paper shows that the depth of shower maximum (Xmax) and the lateral distribution of particles can be comparable to those of a 10¹⁹ eV proton‑induced shower, even though the individual particle energies in the nugget‑induced shower are typically in the 100 MeV–GeV range.

Because the shower morphology is similar, large‑area ground‑based cosmic‑ray observatories—such as the Pierre Auger Observatory, Telescope Array, and future radio‑detection arrays—are naturally suited to search for these events. The detection probability scales with the product of the nugget’s cross‑section and the atmospheric column depth traversed. Although the expected flux is extremely low (of order 10⁻⁴ nuggets per year over the whole Earth for the parameter space considered), the enormous exposure of existing detectors (hundreds of km² observed for many years) provides a non‑negligible chance of capturing a candidate.

The authors contrast this approach with traditional direct‑detection limits, emphasizing that the strong interaction and macroscopic size place the nuggets in a regime where existing underground experiments impose only weak constraints. They argue that a dedicated re‑analysis of archival air‑shower data, looking for anomalous events with unusually high muon content, delayed radio signals, or atypical longitudinal development, could already set meaningful bounds on the nugget parameter space.

Finally, the paper outlines future work: more sophisticated Monte‑Carlo simulations of the nugget‑atmosphere interaction, detailed modeling of the surface charge distribution, and systematic searches in the data streams of Auger, TA, and upcoming facilities such as GRAND and POEMMA. If a detection were made, it would not only confirm a dark‑matter component but also provide a unique laboratory for studying QCD matter at extreme densities.