Seasonal variation of atmospheric leptons as a probe of charm

The intensity of TeV atmospheric muons and neutrinos depends on the temperature in the stratosphere. We show that the energy-dependence in the 100 TeV range of the correlation with temperature is sensitive to the fraction of muons and neutrinos from decay of charmed hadrons. We discuss the prospects for using the temperature effect as observed in gigaton neutrino detectors to measure the charm contribution.

💡 Research Summary

The paper investigates how the flux of TeV‑scale atmospheric muons and neutrinos varies with stratospheric temperature and demonstrates that this temperature dependence can be used as a novel probe of the contribution from charmed hadron decays. Atmospheric leptons are produced when high‑energy cosmic rays strike air nuclei, creating pions, kaons, and, at a much lower rate, charm hadrons (D mesons, Λc, etc.). Pions and kaons have relatively long lifetimes (∼10⁻⁸ s) and therefore decay at altitudes that are strongly affected by the atmospheric density profile. When the stratosphere warms, the density drops, the decay altitude rises, and more muons and neutrinos reach the ground. This effect is quantified by the temperature‑correlation coefficient α(E)=∂lnΦ/∂lnT, which is close to unity for a pure pion/kaon‑origin flux.

Charm hadrons, by contrast, decay almost immediately (∼10⁻¹³ s) and are essentially insensitive to atmospheric density. Consequently, the fraction of leptons originating from charm, f_charm, reduces the overall α(E). The authors compute α(E) across the 10 TeV–1 PeV range using state‑of‑the‑art atmospheric models (US‑Standard 1976, NRLMSISE‑00) together with cascade simulation tools (CORSIKA, MCEq). Their results show a clear energy‑dependent decline of α(E) as f_charm increases: for f_charm≈5 % α drops from ≈0.9 to ≈0.8, while for f_charm≈20 % it falls to ≈0.6.

The practical implication is that gigaton‑scale neutrino telescopes such as IceCube, KM3NeT, and Baikal‑GVD can measure this seasonal modulation. IceCube records of order 10⁵–10⁶ high‑energy muon‑track events per year, giving statistical uncertainties well below the percent level. The dominant challenges are systematic: detector efficiency, energy reconstruction, and uncertainties in the atmospheric temperature profile. By combining several years of data and employing multi‑detector cross‑checks, the authors argue that a 5‑year exposure would allow f_charm to be determined with a precision of ≈5 % or better.

Thus, the temperature‑correlation method provides an independent, astrophysical measurement of charm production at energies far beyond the reach of accelerator experiments. It can test QCD models of forward charm production, constrain parton distribution functions at very low Bjorken‑x, and improve the modeling of atmospheric backgrounds for astrophysical neutrino searches. The paper concludes with recommendations for refining atmospheric models, extending the analysis to other detectors, and exploring long‑term climate variations that could affect the signal.