From GeV to EeV: A Data-Driven View of Cosmic Rays

How do high-energy particles traveling across the universe reach Earth, and what are they made of? In this episode, we explore the Global Spline Fit (GSF), a comprehensive framework that unifies cosmic-ray flux and mass composition measurements across eleven orders of magnitude in energy (from 1\text{ GeV} to beyond 10^{11}\text{ GeV}).

Key Topics Covered

• Bridging Direct & Indirect Detection: How researchers combined direct elemental identification from space and balloon instruments (e.g., AMS-02, CALET, DAMPE) with ground-based extensive air-shower arrays (e.g., LHAASO, Pierre Auger Observatory, IceCube, Telescope Array).
• Purely Data-Driven Modeling: Unlike traditional models that impose fixed power-law physics assumptions, the GSF employs cubic basis splines, allowing spectral features like the "knee" and "ankle" to emerge naturally from observational data.
• Energy-Scale Cross-Calibration: Algorithmically cross-calibrating systematic energy-scale uncertainties across experiments proves that global cosmic-ray data is consistent when scales are aligned.
• Mass Group Decomposition: The flux is categorized into four primary mass groups (H^, He^, O^, and Fe^), while accounting for sub-leading elements and specific isotopes like deuterium.
• Impact on Atmospheric Lepton & Neutrino Fluxes: The updated nucleon flux differs by 20% to 50% from legacy parametrizations (such as H3a, H4a, and GST). This significantly alters background predictions for giant neutrino telescopes like IceCube, KM3NeT, and Baikal-GVD.
• Open-Source Science: The release of the `globalsplinefit` Python package allows scientists worldwide to propagate these unified flux uncertainties into downstream research.

Article Reference

Fedynitch, A., Fujisue, K., Dembinski, H., & Engel, R. (2026). Global Spline Fit: A unified data-driven view of the cosmic-ray spectrum and mass composition from GeV to the highest energies. arXiv:2609.32649v1 [astro-ph.HE].

Acknowledements: Podcast prepared with Google/Gemini Notebook. Illustration credits: A. Fedynitch et al.