Measurement of reactor antineutrino oscillations with 1.46 ktonne-years of data at SNO+

Abstract

The SNO+ Collaboration reports new results on reactor antineutrino oscillations using data acquired from May 2022 through July 2025. The spectral analysis of a flux dominated by nuclear reactors at 240, 350, and 355 kilometers yields the mass-squared difference $Δm^2_{21}=(7.93^{+0.21}_{-0.24})\times 10^{-5}$ eV$^2$. This result is compatible with and approaches the precision of the only other long-baseline reactor antineutrino measurement, by KamLAND. Combining these measurements, along with those from solar neutrino experiments, the global values of the neutrino mixing parameters become: $Δm^2_{21}$ = $(7.63\pm0.17)\times 10^{-5}$ eV$^2$ and $\sin^2{θ_{12}}=0.310\pm0.012$. The analysis of geoneutrinos at SNO+ is also improved, with a measured signal of 49$^{+13}_{-12}$ TNU.

Figures (4)

• Figure 1: Positions of prompt and delayed events selected in the full dataset, within the 5.7 m fiducial volume.
• Figure 2: Energy distribution of prompt events and best-fit predictions. Upper panel: fit with $\Delta m^2_{21}$ and $\sin^2\theta_{12}$ unconstrained. Lower panel: with constraints and the cut on the ($\alpha, n$) classifier, which is applied only below 3.5 MeV.
• Figure 3: Likelihood contours of $\Delta m^2_{21}$ vs. $\sin^2\theta_{12}$. The 1-D $\log$ likelihood curve is also shown separately for each variable, profiling over the other. Contours and 1-D curves for results from KamLAND kamland_on_off and combined solar measurements Super-Kamiokande_solar are shown for comparison, along with the combined fit of SNO+ and the PDG 2025 global results PDG2025.
• Figure 4: Distributions of the ($\alpha, n$) classifier value for prompt events with energy below 3.5 MeV, for the full dataset. Classifier values are shifted so that the cut threshold for each dataset is at 0, denoted with a vertical dashed line.