Establishing Earth's Matter Effect in Atmospheric Neutrino Oscillations at IceCube DeepCore

The discovery of the non-zero value of $θ_{13}$ has opened an exciting opportunity to probe the Earth’s matter effects in three-flavor oscillations of atmospheric neutrinos. These matter effects depend on both neutrino energy and the electron density distributions encountered during their propagation through Earth. In this contribution, we present preliminary sensitivities from the DeepCore detector, a densely instrumented sub-array of the IceCube neutrino observatory at the South Pole, demonstrating its ability to observe these matter effects in atmospheric neutrino oscillations. Using simulated data equivalent to 9.3 years of observations at IceCube DeepCore, we show the sensitivity of the DeepCore to reject the vacuum oscillation hypothesis and align with the Preliminary Reference Earth Model. Additionally, we present the expected improvement in sensitivity for rejecting the vacuum oscillations using the upcoming IceCube Upgrade, a low-energy extension of the IceCube detector.

💡 Research Summary

The paper investigates the capability of the IceCube DeepCore sub‑array to detect Earth‑matter effects in atmospheric neutrino oscillations now that the mixing angle θ₁₃ has been measured to be non‑zero. Atmospheric neutrinos, produced by cosmic‑ray interactions in the atmosphere, travel baselines ranging from a few tens of kilometres to the full Earth diameter and span energies from a few MeV up to hundreds of TeV. In the energy window of roughly 5 – 15 GeV, coherent forward scattering on electrons induces the Mikheyev‑Smirnov‑Wolfenstein (MSW) resonance, while neutrinos crossing the core‑mantle boundary experience a sharp density jump that gives rise to a parametric (oscillation‑length) resonance. Both resonances depend on the electron density profile of the Earth, which is described by the Preliminary Reference Earth Model (PREM).

The authors use a Monte‑Carlo (MC) simulation corresponding to 9.3 years of DeepCore data. The simulated sample has an excellent signal‑to‑background ratio (background < 1 %) because atmospheric muons and detector noise are heavily suppressed. Event reconstruction employs a convolutional neural network (CNN) that provides estimates of the neutrino energy, reconstructed zenith angle (cos θₙ), and particle‑identification (PID) class (track‑like ν_μ charged‑current versus cascade‑like ν_e CC and neutral‑current interactions). The events are binned in three dimensions (energy, cos θₙ, PID) and re‑weighted according to either the PREM electron‑density profile or a vacuum profile (no matter effect). Systematic uncertainties—atmospheric flux, cross‑sections, detector response, and oscillation parameters—are treated as nuisance parameters in a likelihood fit.

The analysis adopts the Asimov‑dataset approach to evaluate the statistical power to reject the “vacuum oscillation” hypothesis in favor of the “matter‑affected” hypothesis. The test is performed for both normal ordering (NO) and inverted ordering (IO) of the neutrino mass spectrum, while fixing δ_CP = 0, θ₁₃, θ₁₂, and Δm²₂₁ at their precisely measured values. Δm²₃₁ and θ₂₃ are left free together with the nuisance parameters. Sensitivity is presented as a function of the true value of sin²θ₂₃, the least‑well‑determined mixing angle apart from δ_CP.

Results show that, assuming a true θ₂₃ ≈ 47.5° (sin²θ₂₃ ≈ 0.58), DeepCore alone would reject the vacuum hypothesis at 1.57 σ for NO and 1.10 σ for IO. Although these significances are modest, they demonstrate that DeepCore can already glimpse Earth‑matter effects. The authors then project the impact of the forthcoming IceCube Upgrade, which will add seven new strings with a lower energy threshold and improved systematic control. Combining 12 years of standard IceCube (IC86) data with 3 years of Upgrade data (IC93) yields an expected sensitivity roughly three times higher, potentially reaching >3 σ for NO.

In conclusion, the study quantifies the present sensitivity of IceCube DeepCore to matter‑induced modifications of atmospheric neutrino oscillations and highlights the substantial gain expected from the Upgrade. The work provides a roadmap for using atmospheric neutrinos as a probe of the Earth’s interior electron density profile, opening a novel avenue in neutrino geophysics once real data are analyzed.