Joint neutrino oscillation analysis from the T2K and NOvA experiments

Abstract

The landmark discovery that neutrinos have mass and can change type (or "flavor") as they propagate -- a process called neutrino oscillation -- has opened up a rich array of theoretical and experimental questions being actively pursued today. Neutrino oscillation remains the most powerful experimental tool for addressing many of these questions, including whether neutrinos violate charge-parity (CP) symmetry, which has possible connections to the unexplained preponderance of matter over antimatter in the universe. Oscillation measurements also probe the mass-squared differences between the different neutrino mass states ($Δm^2$), whether there are two light states and a heavier one (normal ordering) or vice versa (inverted ordering), and the structure of neutrino mass and flavor mixing. Here, we carry out the first joint analysis of data sets from NOvA and T2K, the two currently operating long-baseline neutrino oscillation experiments (hundreds of kilometers of neutrino travel distance), taking advantage of our complementary experimental designs and setting new constraints on several neutrino sector parameters. This analysis provides new precision on the $Δm^2_{32}$ mass difference, finding $2.43^{+0.04}_{-0.03}\ \left(-2.48^{+0.03}_{-0.04}\right)\times 10^{-3}~\mathrm{eV}^2$ in the normal (inverted) ordering, as well as a $3σ$ interval on $δ_{\rm CP}$ of $[-1.38π,\ 0.30π]$ $\left([-0.92π,\ -0.04π]\right)$ in the normal (inverted) ordering. The data show no strong preference for either mass ordering, but notably if inverted ordering were assumed true within the three-flavor mixing paradigm, then our results would provide evidence of CP symmetry violation in the lepton sector.

Figures (13)

• Figure 1: The impact of mass ordering and $\delta_{\rm CP}$ on event rates. A "bi-event" plot that illustrates experimental sensitivity to neutrino mass ordering and $\delta_{\rm CP}$, with panels representing the NOvA (a) and T2K (b) cases. Black points with $1\sigma$ Poisson statistical error bars show the total number of $\nu_e$ and $\bar{\nu}_e$ candidates selected in the far detectors. The oval parametric curves trace out predicted numbers of events under the normal (blue) or inverted (orange) mass ordering assumption as the parameter $\delta_{\rm CP}$ varies from $-\pi$ to $\pi$. Four specific $\delta_{\rm CP}$ values are labeled for reference. All other oscillation parameters are kept fixed in this graphic, set to their most probable values from the joint analysis (Extended Data Table \ref{['tab:hpds']}.)
• Figure 1: Correlation study comparison plots. Posterior probability distributions of $\delta_{\rm CP}$ (a), $\sin^2 \theta_{23}$ (b), and $\Delta m_{32}^2$ (c) and 1$\sigma$ credible regions in $\Delta m_{32}^2$-$\sin^2 \theta_{23}$ (d), marginalized over both neutrino mass ordering hypotheses ("Both MO") from fits to pseudo-data simulated with the NuFit-like oscillation parameter values. The fits were run in three configurations while treating the systematic uncertainties with the largest impact on $\sin^2 \theta_{23}$ (visible neutron energy and 2p2h C/O scale) as either 100% correlated (gray), uncorrelated (teal), or 100% anticorrelated (magenta). Overlaid with the corresponding 1$\sigma$ (dark shaded areas, dashed) and 2$\sigma$ (light shaded areas, dash-dotted) credible intervals.
• Figure 2: Experimental measurements of $\vert \Delta m^2_{32}\vert$. The measurements assume the inverted ordering preferred by this analysis. Sources for the results from top to bottom starting with the second line are as follows: T2K:2023smvNOvA:2021nfiMINOS:2020llmIceCubeCollaboration:2024zecabe2024firstSuper-Kamiokande:2023ahcDayaBay:2022ormRENO:2024msrDayaBay:2024hrv. The normal ordering case is available in Extended Data Fig. \ref{['fig:global']}.
• Figure 2: "Nightmare" study comparisons. 1$\sigma$ credible regions in $\Delta m_{32}^2$-$\sin^2 \theta_{23}$ posterior probability distributions marginalized over both neutrino mass ordering hypotheses ("Both MO") from fits to pseudo-data simulated with the NuFit-like oscillation parameter values and a fully symmetric systematic bias to affect (a) $\Delta m_{32}^2$ ("$\Delta m^2$ nightmare") and (b) $\sin^2 \theta_{23}$ ("$\theta_{23}$ nightmare"). The fits were run while treating the NOvA and T2K nightmare parameters as either 100% correlated (gray), uncorrelated (teal), or 100% anticorrelated (magenta).
• Figure 3: Constraints on $\sin^2\theta_{23}$ and $\delta_{\rm{CP}}$. Marginalized posterior probabilities and 1D or 2D Bayesian credible regions of $\sin^2 \theta_{23}$ and $\delta_{\rm CP}$ in the case of the normal (blue, left side) and inverted (orange, right side) neutrino mass ordering with the reactor constraint applied. Shaded areas correspond to $1 \sigma$, $2 \sigma$, and $3 \sigma$ credible regions. 2D panels of $\sin^2 \theta_{23}$ vs $\delta_{\rm CP}$ (a, b) are overlaid with $1 \sigma$ credible regions from the T2K-only (dark red) and NOvA-only (dark blue) data fits assuming normal (a) and inverted ordering (b). 1D panels show the posterior probabilities of $\sin^2 \theta_{23}$ (c) and $\delta_{\rm CP}$ (d) in the normal ordering, and $\delta_{\rm CP}$ (e) and $\sin^2 \theta_{23}$ (f) in the inverted ordering.