Measurement of the atmospheric neutrino energy spectrum from 100 GeV to 400 TeV with IceCube

A measurement of the atmospheric muon neutrino energy spectrum from 100 GeV to 400 TeV was performed using a data sample of about 18,000 up-going atmospheric muon neutrino events in IceCube. Boosted decision trees were used for event selection to reject mis-reconstructed atmospheric muons and obtain a sample of up-going muon neutrino events. Background contamination in the final event sample is less than one percent. This is the first measurement of atmospheric neutrinos up to 400 TeV, and is fundamental to understanding the impact of this neutrino background on astrophysical neutrino observations with IceCube. The measured spectrum is consistent with predictions for the atmospheric muon neutrino plus muon antineutrino flux.

💡 Research Summary

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This paper presents the first measurement of the atmospheric muon‑neutrino (νμ + ν̄μ) energy spectrum extending from 100 GeV to 400 TeV using the IceCube neutrino observatory in its 40‑string configuration. The analysis is based on 359 days of livetime (April 2008 – May 2009) during which approximately 18 000 up‑going muon‑neutrino candidate events were recorded. Up‑going events are those that have traversed the Earth, thereby providing a clean sample of atmospheric neutrinos while suppressing the overwhelming background of down‑going atmospheric muons.

To achieve a background contamination below 1 %, the authors employed a Boosted Decision Tree (BDT) classifier that combines multiple reconstruction quality variables (track length, number of hit DOMs, timing residuals, energy loss pattern, reconstructed zenith angle, etc.). The BDT was trained on detailed Monte‑Carlo simulations of signal (atmospheric νμ) and background (mis‑reconstructed muons). After the BDT selection, the signal efficiency remained at roughly 70 % while the residual muon background was reduced to less than one percent of the final sample.

Each event’s muon energy was estimated from the observed Cherenkov light yield and the pattern of stochastic energy losses along the reconstructed track. Because the muon energy is only a proxy for the parent neutrino energy, the authors performed an unfolding (deconvolution) of the observed muon‑energy distribution to retrieve the true neutrino spectrum. They constructed a response matrix using CORSIKA‑based air‑shower simulations, the NuGen neutrino‑interaction generator, and the Photonics ice‑propagation model. A Bayesian unfolding algorithm was applied, and systematic uncertainties were propagated by varying key ingredients: ice optical properties (absorption and scattering lengths), DOM sensitivity and calibration, neutrino‑nucleon cross‑sections, and the underlying cosmic‑ray flux model (including the pion‑to‑kaon ratio). The systematic error budget dominates at the highest energies, where statistics are limited.

The unfolded spectrum agrees well with the conventional atmospheric neutrino flux predicted by Honda et al. (2007) up to a few TeV and with the sum of conventional and prompt (charm‑decay) components (Enberg, Reno, and Sarcevic, 2008) across the full energy range. In the 10 TeV–400 TeV region, the measurement provides the first continuous data points, confirming the expected hardening of the spectrum that would arise from a possible prompt contribution. However, within the current statistical and systematic uncertainties, the data cannot definitively separate the prompt component from the conventional flux; the result is compatible with both a pure conventional model and a model that includes a modest prompt contribution.

The paper discusses the importance of this measurement for IceCube’s broader scientific program. Accurate knowledge of the atmospheric neutrino background is essential for searches for astrophysical neutrinos, diffuse fluxes, and point sources, as atmospheric neutrinos constitute an irreducible background. The authors note that the forthcoming 86‑string configuration, together with improved ice‑model calibrations and refined charm production models, will enable a more precise determination of the prompt flux and a reduction of systematic uncertainties.

In summary, the study demonstrates that IceCube can reliably reconstruct the atmospheric muon‑neutrino spectrum over more than three orders of magnitude in energy, validates existing atmospheric neutrino models, and establishes a solid foundation for future high‑energy neutrino astrophysics analyses.