Seasonal Variations of the Atmospheric Muon Neutrino Spectrum measured with IceCube

TL;DR

This work measures energy-dependent seasonal variations in the atmospheric muon-neutrino flux using $11.3$ years of IceCube data, applying a spectrum-unfolding approach to extract the true energy distribution in the range $125~\mathrm{GeV}$–$10~\mathrm{TeV}$ for $90^\circ$–$110^\circ$ zenith. The analysis compares unfolded seasonal spectra to predictions from MCEq with multiple atmospheric parameterizations (e.g., NRLMSISE-00, AIRS, ERA5) and to the daemonflux model, finding that the seasonal amplitude grows with energy and that summer is enhanced while winter is suppressed relative to the annual average; the amplitude reaches about $+3.9\%$ in summer and $-4.5\%$ in winter near $10~\mathrm{TeV}$. Although the unfolded flux normalization can exceed model predictions by up to ~30%, the relative seasonal variations are robust because systematics largely cancel in the ratio. The results validate the use of atmospheric neutrinos to probe Antarctic atmospheric dynamics and kaon/pion production, and set the stage for improved constraints with future IceCube upgrades. Finally, the study demonstrates a data-driven unfolding workflow with careful treatment of statistical and systematic uncertainties, including validation on pseudo-data.

Abstract

This study presents an energy-dependent analysis of seasonal variations in the atmospheric muon neutrino spectrum, using 11.3 years of data from the IceCube Neutrino Observatory. By leveraging a novel spectral unfolding method, we explore the energy range from 125 GeV to 10 TeV for zenith angles between 90° to 110°, corresponding to the Antarctic atmosphere. Our findings reveal that the seasonal variation amplitude decreases with energy reaching ($-4.6 \pm 1.1$)\% during Austral winter and increases ($+3.9 \pm 1.2$)\% during Austral summer relative to the annual average at 10TeV. While the unfolded flux exceeds the model predictions by up to 30\%, the differential measurement of seasonal variations remains unaffected. The measured seasonal variations of the muon neutrino spectrum are consistent with theoretical predictions using the MCEq code and the NRLMSISE-00 atmospheric model.

Figures (14)

• Figure 1: Illustration of the experimental setup. The upper figure depicts the atmospheric region, the Antarctic atmosphere, from which the neutrinos are analyzed in this paper, and the IceCube detector at the South Pole. The illustration at the bottom shows the definition of relevant physical quantities and the zenith region to be analyzed in IceCube coordinates $\theta$. The path of the neutrino along the line of sight is given by $l$, the vertical height of the atmosphere by $h$. The spanned zenith region in the bottom figure is not to scale.
• Figure 2: Neutrino production profiles for January 1st (solid) and July 1st (dashed) for the neutrino sample from the IceCube Neutrino Observatory, integrated over the zenith range from 90110° calculated using MCEq with the NRLMSISE-00 atmospheric model. The profiles in this figure are displayed for the first, fifth and highest energy bin.
• Figure 3: Upper panel: Neutrino spectrum calculated with MCEq, showing contributions from parent particles for two seasons, January 1st and July 1st. The seasonal differences in the total spectrum increase with energy above the critical energy of the respective parent particle. Lower panel: The fraction of the total flux, where the prompt flux component does not show seasonal differences because of its critical energy exceeding the displayed energy range.
• Figure 4: The calculated ratio of the seasonal muon neutrino flux to the annual average, using MCEq with NRLMSISE-00 as the atmospheric density model, for the zenith range from 90110° during Austral summer and winter. The total flux, including the prompt component, is shown with dashed lines, while solid lines represent the conventional flux. The strength of seasonal variation is expected to decrease as prompt neutrinos dominate at energies above several 100TeV.
• Figure 5: Expected total neutrino rate variation per day of year relative to the annual average, calculated from the NRLMSISE-00 model for the zenith range from 90110°. The solid line shows the rate derived for the lowest energy bin ($2.1 \leq \log(E/GeV) \leq 2.29$), while dashed lines show the rate for the highest energy bin ($3.81 \leq \log(E/GeV) \leq 4.0$). The rate variation is depicted for the total neutrino flux in black, while the contribution from kaons is shown in blue, and from pions in red. The seasonal variation in the first bin is dominated by pions. The contribution from kaons is compatible with the annual average as the energies are below the critical energy for kaons. The variations in the highest energy bin are mainly driven by kaons.