Terrestrial Matter Effects on Reactor Antineutrino Oscillations: Constant vs. Fluctuated Density Profiles

TL;DR

This paper analyzes how realistic terrestrial matter density along JUNO’s reactor baselines affects reactor antineutrino oscillations. It builds a detailed piecewise-constant crustal density model for the Taishan–JUNO and Yangjiang–JUNO paths, calibrated by South China geology, and compares constant-density with fluctuated-density profiles under conservative ±10% density and length variations using a Cayley–Hamilton semi-analytical method for exact oscillation propagation. The results show that density variations induce ~10^-3 level differences in the electron antineutrino survival probability, with the mass-ordering discriminant receiving at most ~10^-4 corrections, implying MO determination remains robust; the effects scale with runtime as experimental precision improves, becoming relevant only at per-mille levels. Localized anomalous-density structures produce only negligible changes, suggesting that crustal tomography with reactor neutrinos would require larger detectors. Overall, the work informs high-precision reactor neutrino analyses and motivates future avenues for crustal tomography and the integration of subtle matter-effects corrections into precision oscillation fits.

Abstract

The JUNO Collaboration has recently released its first reactor antineutrino oscillation result, achieving unprecedented precision in the measurement of $Δm^2_{21}$ and $\sin^2θ_{12}$. We emphasize that the accurate determination and modeling of the terrestrial matter density profile are fundamental for extracting the oscillation parameters and probing the neutrino mass ordering. This paper presents a realistic piecewise-constant model for the shallow crustal density profile along the baselines from Taishan and Yangjiang to the experimental hall, based on geological and petrophysical information. The uncertainty in the density profiles arises from variations in the density and length of each segment, both of which are conservatively estimated to be 10%. A careful comparison of constant and fluctuated density profiles is provided and the implications for the precision measurement of oscillation parameters are discussed. Finally, we also discuss the prospect of shallow crust tomography in future reactor neutrino experiments.

Figures (8)

• Figure 1: Topographic (left) and geological (right) maps of the terrain and crustal structure traversed by the reactor antineutrino oscillation baselines from the Taishan (TS) and Yangjiang (YJ) nuclear power plants to the JUNO detector. The white lines indicate the TS--JUNO and YJ--JUNO baselines.
• Figure 2: Geological sampling along the TS–JUNO baseline. The upper panel shows the surface elevation as a function of distance, and the lower panel shows the corresponding matter density of surface rocks.
• Figure 3: Geological sampling along the YJ–JUNO baseline. The upper panel shows the surface elevation as a function of distance, and the lower panel shows the corresponding matter density of surface rocks.
• Figure 4: Absolute difference between the $\bar{\nu}_e$ survival probabilities in matter and in vacuum, $|P^{\rm Matter}_{ee}-P^{\rm Vac}_{ee}|$, as a function of the antineutrino energy $E_\nu$ for the TS--JUNO (left) and YJ--JUNO (right) baselines. Solid and dashed curves correspond to normal (NMO) and inverted (IMO) mass ordering, respectively. The survival probabilities are calculated using the current best-fit values of the oscillation parameters Esteban:2024eli and the piecewise-constant matter-density profiles obtained from the geological sampling along the baselines (see Figs. \ref{['fig:TStoJUNO']} and \ref{['fig:YJtoJUNO']} and Tables \ref{['tab:segments-TS']} and \ref{['tab:segments-YJ']}).
• Figure 5: Variable matter effects on the $\bar{\nu}_e \to \bar{\nu}_e$ survival probability along the TS--JUNO (upper row) and YJ--JUNO (lower row) baselines for normal (NMO, left column) and inverted (IMO, right column) mass ordering. Shown is the absolute difference between the probabilities computed with the benchmark constant-density profile, whose baseline-averaged densities $\rho^{\rm Average}_{\rm TS}$ and $\rho^{\rm Average}_{\rm YJ}$ are defined in Eqs. \ref{['eq: rho_A_TS']} and \ref{['eq: rho_A_YJ']}, and with variable matter density, $|P^{\rm C}_{ee} - P^{\rm Vari}_{ee}|$. The black and gray curves correspond to Scenario A, where all rock densities are coherently rescaled by $\pm 10\%$, while the colored curves show five representative realizations of Scenario B with independent $\pm 10\%$ fluctuations in each rock segment.