Study of the internal structure of the Earth using neutrino oscillations at IceCube DeepCore

Earth’s mass and internal structure have been primarily studied through gravitational and seismic methods. Neutrinos, however, offer an independent way to explore Earth’s interior via matter effects in neutrino oscillations that depend on the electron distribution inside Earth, and hence its matter density. Our study uses atmospheric neutrinos at DeepCore, a densely instrumented sub-detector of the IceCube Neutrino Observatory, to estimate Earth’s mass and layer densities. We also assess how the upcoming IceCube Upgrade, with denser instrumentation, could improve these measurements.

💡 Research Summary

The paper investigates the feasibility of using atmospheric neutrino oscillations measured by the IceCube DeepCore sub‑detector, and the forthcoming IceCube Upgrade, to probe the Earth’s internal structure in a way that is independent of traditional seismic and gravimetric methods. Neutrinos traversing the Earth experience matter‑induced modifications to their flavor evolution (the Mikheyev–Smirnov–Wolfenstein effect and parametric resonances) that depend on the electron density, and therefore on the bulk mass density, along their path. By exploiting the broad range of baselines and energies (3–100 GeV) available from atmospheric neutrinos, the authors aim to extract two key quantities: (1) the total mass of the Earth, and (2) the correlated densities of its principal internal layers.

The analysis uses a Monte‑Carlo data set equivalent to 9.3 years of DeepCore operation. Events are reconstructed with a convolutional neural network that provides estimates of energy, zenith angle, and particle‑identification (track‑like νμ charged‑current versus cascade‑like νe and neutral‑current interactions). The data are binned into 20 logarithmic energy bins, 20 linear zenith bins (covering the upward‑going hemisphere), and three PID categories. Each bin is weighted by atmospheric flux models, oscillation probabilities (including the latest measurements of θ13), neutrino‑nucleon cross sections, detector response, and systematic uncertainties.

Two complementary parameterizations of the Earth’s density profile are employed. For the total‑mass measurement, a 12‑layer Preliminary Reference Earth Model (PREM) is scaled uniformly by a factor α, allowing the neutrino data alone to infer the Earth’s mass without external constraints. For the correlated‑density measurement, a reduced 5‑layer model (inner core, outer core, inner mantle, middle mantle, outer mantle) is used. The core layers share a common scaling factor to preserve their PREM‑defined ratio, while the inner and middle mantle receive independent scaling factors; the outer mantle is fixed because its density is already poorly constrained. Gravitational constraints on total mass and moment of inertia introduce correlations among these scaling factors, effectively reducing the problem to a single free parameter (the core scaling αc).

Sensitivity is evaluated using the Asimov method, i.e., the expected statistical precision assuming the simulated data represent the true underlying model. The results show that, with DeepCore alone, the Earth’s mass can be measured to a relative precision of about 3 % (α ≈ 1 ± 0.03). When external gravimetric constraints are incorporated, the 1σ allowed region for the five‑layer density parameters shrinks by roughly 30 % compared with the unconstrained PREM band, demonstrating that neutrino oscillation data provide complementary information on the internal density distribution.

The authors also project the impact of the IceCube Upgrade, which will add seven densely instrumented strings and lower the energy threshold to ~1 GeV. Adding three years of simulated Upgrade data to the DeepCore exposure yields a modest but noticeable improvement: the mass uncertainty tightens to ≈2 % (α ≈ 1 ± 0.02), and the correlated‑density 1σ bands narrow by about 0.05 in the scaling parameters. This reflects the increased statistics of low‑energy events where matter effects are strongest.

In conclusion, the study demonstrates that neutrino oscillation tomography can independently determine the Earth’s mass and provide meaningful constraints on the densities of its major internal layers. While systematic uncertainties and model dependencies remain challenges, the forthcoming Upgrade will substantially enhance the precision of such measurements, establishing neutrino‑based Earth tomography as a valuable complement to seismic and gravitational techniques.