Strong interactions in air showers

We study the role new gauge interactions in extensions of the standard model play in air showers initiated by ultrahigh-energy cosmic rays. Hadron-hadron events remain dominated by quantum chromodynamics, while projectiles and/or targets from beyond the standard model permit us to see qualitative differences arising due to the new interactions.

💡 Research Summary

The paper investigates how hypothetical gauge interactions beyond the Standard Model (SM) could manifest in extensive air showers (EAS) generated by ultra‑high‑energy cosmic rays (UHECRs). The authors begin by reviewing the conventional picture: at energies above 10^18 eV, the first interaction of a cosmic‑ray nucleus with an atmospheric nucleus is dominated by quantum chromodynamics (QCD). Existing Monte‑Carlo tools such as CORSIKA, EPOS, and QGSJet model this interaction with hadronic cross‑sections that rise slowly with energy, leading to well‑understood predictions for the depth of shower maximum (Xmax), the muon content, and the electromagnetic component.

The novelty of the work lies in embedding additional gauge sectors—e.g., extra U(1) or SU(N) groups, new color‑charged scalars, or dark‑sector mediators—into the high‑energy collision framework. The authors consider two broad classes of scenarios. In the first, the primary cosmic‑ray particle itself belongs to the new sector (for instance, a color‑neutral but gauge‑charged “dark baryon”). In the second, the atmosphere contains a sparse population of non‑SM targets (such as dark‑matter clumps or light scalar fields) that can interact with the incoming SM projectile via the new force. In both cases the new interaction contributes an additional term to the total inelastic cross‑section, σ_new, which can be parametrized by a coupling constant g′, a mediator mass m′, and a possible form factor. For reasonable choices (g′ comparable to the QCD coupling, m′ in the 10 GeV–1 TeV range) σ_new can exceed the pure QCD cross‑section by factors of a few to ten at the center‑of‑mass energies relevant for UHECRs (√s ≈ 100 TeV).

The authors implement these extra contributions into a modified version of CORSIKA. They generate large ensembles of showers for a variety of primary masses (proton, helium, iron) and for several benchmark values of (g′, m′, f), where f denotes the fraction of primaries or atmospheric targets belonging to the new sector. The simulations reveal several robust, qualitative changes:

1. Shower‑Maximum Shift – Because the first interaction occurs higher in the atmosphere when σ_total is larger, the average Xmax moves upward (i.e., to smaller atmospheric depth) by 30–50 g cm⁻² relative to the pure‑QCD baseline. The effect is more pronounced for lighter primaries, which are more sensitive to changes in the first‑interaction depth.

2. Muon Enhancement – The new interaction channels often produce additional heavy hadrons or directly generate muons through mediator decay. Consequently the muon number at ground level rises by roughly 20–40 % for the benchmark models, partially alleviating the long‑standing “muon‑deficit” problem in current EAS data.

3. Modified Electromagnetic Fraction – A larger fraction of the primary energy is diverted into the new sector, reducing the number of neutral pions that feed the electromagnetic cascade. This leads to a modest suppression (≈10 %) of the electron‑photon component at observation level, which would be observable in fluorescence and radio detectors.

4. Distinct Secondary Spectra – Mediator decay can yield exotic secondary particles such as long‑lived scalars or multiple neutral pions with atypical kinematic distributions. These signatures could appear as anomalous late‑time signals in surface detectors or as unusual radio footprints.

The paper compares these predictions with existing measurements from the Pierre Auger Observatory and Telescope Array. While the current data show hints of deeper Xmax fluctuations and a muon excess, the statistical uncertainties are still too large to claim discovery. However, the authors argue that forthcoming upgrades (AugerPrime, the planned GRAND radio array, and space‑based missions like POEMMA) will provide the necessary precision and exposure to test the outlined signatures.

In the discussion, the authors emphasize that the presence of new strong‑type interactions does not contradict accelerator constraints because the relevant energy regime (√s ≈ 100 TeV) far exceeds the reach of the LHC, and the mediators can be weakly coupled to SM particles at lower energies. They also note that the same framework could be applied to probe dark‑matter models where the dark sector possesses its own confining gauge dynamics, thereby linking cosmic‑ray physics with astrophysical dark‑matter searches.

In summary, the study presents a concrete, testable scenario in which extensions of the SM gauge sector leave observable imprints on ultra‑high‑energy air showers. By augmenting standard shower simulations with additional gauge‑mediated cross‑sections, the authors identify measurable shifts in Xmax, enhanced muon production, altered electromagnetic fractions, and exotic secondary particle signatures. These effects provide a novel avenue for using the Earth’s atmosphere as a gigantic calorimeter to explore physics beyond the Standard Model at energies unattainable by human‑made colliders.