Status of neutrino oscillations 2018: first hint for normal mass ordering and improved CP sensitivity

TL;DR

This paper presents a comprehensive global fit of neutrino oscillations within the three-neutrino framework using new long-baseline, reactor, atmospheric, and solar data. It uncovers a first strong hint for normal mass ordering and improved CP-sensitivity, with the CP phase $\delta$ favored in the upper half of its range and $\delta = \pi/2$ strongly disfavored. The analysis also suggests a preference for the atmospheric mixing angle $\theta_{23}$ in the upper octant for normal ordering, while octant determination remains fragile. The authors discuss potential beyond-Standard-Model effects such as NSI and non-unitarity that could affect sensitivities and emphasize the role of forthcoming data for robust resolution of mass ordering and CP violation.

Abstract

We present a new global fit of neutrino oscillation parameters within the simplest three-neutrino picture, including new data which appeared since our previous analysis~\cite{Forero:2014bxa}. In this update we include new long-baseline neutrino data involving the antineutrino channel in T2K, as well as new data in the neutrino channel, data from NO$ν$A, as well as new reactor data, such as the Daya Bay 1230 days electron antineutrino disappearance spectrum data and the 1500 live days prompt spectrum from RENO, as well as new Double Chooz data. We also include atmospheric neutrino data from the IceCube DeepCore and ANTARES neutrino telescopes and from Super-Kamiokande. Finally, we also update our solar oscillation analysis by including the 2055-day day/night spectrum from the fourth phase of the Super-Kamiokande experiment. With the new data we find a preference for the atmospheric angle in the upper octant for both neutrino mass orderings, with maximal mixing allowed at $Δχ^2 = 1.6 \, (3.2)$ for normal (inverted) ordering. We also obtain a strong preference for values of the CP phase $δ$ in the range $[π,2π]$, excluding values close to $π/2$ at more than 4$σ$. More remarkably, our global analysis shows for the first time hints in favour of the normal mass ordering over the inverted one at more than 3$σ$. We discuss in detail the origin of the mass ordering, CP violation and octant sensitivities, analyzing the interplay among the different neutrino data samples.

Figures (8)

• Figure 1: 90 and 99% C.L. (2 d.o.f.) allowed regions at the $\sin^2\theta_{13}$-- $\Delta m^2_{31}$ plane from individual reactor neutrino experiments (dashed and solid lines) and from the combination of the three experiments (coloured regions). The left (right) panels correspond to normal (inverted) mass ordering.
• Figure 2: 90 and 99% C.L. (2 d.o.f.) allowed regions at the $\sin^2\theta_{23}$-- $\Delta m^2_{31}$ plane obtained from the atmospheric neutrino experiments for normal (left) and inverted ordering (right).
• Figure 3: 90 and 99% C.L. (2 d.o.f.) allowed regions at the $\sin^2\theta_{23}$-- $\Delta m^2_{31}$ plane for normal (left) and inverted mass ordering (right) as restricted from the long--baseline experiments.
• Figure 4: Left: 90 and 99% C.L. (2 d.o.f.) regions from T2K (blue lines) and NO$\nu$A (red) data, from the atmospheric Super-K results (green) and from the global fit of all the oscillation experiments (coloured regions). The star indicates the best fit point from our global analysis, found for normal mass ordering, while the black dot indicates the local minimum for inverted mass ordering. Right: $\Delta\chi^2$--profile as a function of the CP phase $\delta$ from T2K, NO$\nu$A and Super-K atmospheric (with the same color code as in the left panel) and from the global fit (magenta). In both cases, the upper (lower) panels correspond to normal (inverted) mass ordering.
• Figure 5: 90 and 99% C.L. (2 d.o.f.) allowed regions in the $\sin^2\theta_{23}$--$\delta$ (left) and $\sin^2\theta_{13}$--$\delta$ (right) planes from long--baseline data only (black lines), long--baseline plus atmospheric (blue), long--baseline plus reactors (cyan) and from the global fit of all experiments (coloured regions). Upper (lower) panels correspond to normal (inverted) mass ordering. The best fit points are indicated by black down--triangles (long--baseline data), blue squares (long--baseline plus atmospheric), cyan up--triangles (long--baseline plus reactors), as well as stars and black dots, following the same convention as in Fig. \ref{['fig:del-T2K-NOvA']}.