Status of neutrino oscillations 2018: first hint for normal mass ordering and improved CP sensitivity (1708.01186v2)

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$\nu$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 $\Delta\chi^2 = 1.6 \, (3.2)$ for normal (inverted) ordering. We also obtain a strong preference for values of the CP phase $\delta$ in the range $[\pi,2\pi]$, excluding values close to $\pi/2$ at more than 4$\sigma$. 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$\sigma$. We discuss in detail the origin of the mass ordering, CP violation and octant sensitivities, analyzing the interplay among the different neutrino data samples.

• The paper presents a global fit of neutrino oscillation data that hints at a normal mass ordering with over 3σ confidence.
• The analysis uses combined neutrino and antineutrino T2K data to enhance CP violation sensitivity, excluding CP phase values near π/2 with more than 4σ confidence.
• The study achieves improved precision in determining θ₁₃ by incorporating reactor data from Daya Bay and RENO, strengthening overall parameter measurements.

Insights from the 2018 Status of Neutrino Oscillations Paper

The paper under review provides a comprehensive analysis of the status of neutrino oscillation research as of 2018, utilizing a global fit of neutrino oscillation parameters within the three-neutrino framework. This analysis integrates a broad spectrum of new data from various experiments including T2K, NOνA, Daya Bay, RENO, Double Chooz, IceCube DeepCore, ANTARES, and Super-Kamiokande.

Key Findings

The paper highlights several significant findings from the new global fit:

1. Mass Ordering Preference: The analysis provides hints favoring normal mass ordering (NO) over inverted ordering (IO) with more than 3σ confidence. This result marks the first time such a preference has been demonstrated with substantial statistical significance.
2. CP Violation Sensitivity: The T2K experiment's combination of neutrino and antineutrino data enhances the sensitivity to the CP-violating phase δ. It provides a strong preference for CP phase values in the range [π, 2π], excluding values close to π/2 with more than 4σ confidence.
3. Precision Parameter Determination: The paper notes improvements in the precision determination of θ₁₃, primarily influenced by Daya Bay and RENO data. These reactor experiments have significantly contributed to narrowing down the reactor mixing angle's value, enhancing the overall precision of the fit.
4. The θ₂₃ Octant Problem: While the paper observes a slight preference for θ₂₃ in the upper octant, particularly for NO, the data do not yet provide robust discrimination of the θ₂₃ octant.

Implications

These results have profound implications for both the theoretical understanding and practical investigation of neutrino properties. The preference for NO impacts theoretical models of neutrino mass generation, which might need adjustments to account for this finding. The strong CP violation sensitivity lays ground for further investigation into leptonic CP violation, which could help explain the matter-antimatter asymmetry in the universe. Additionally, the precise measurement of oscillation parameters contributes to more accurate background predictions for experiments searching for new physics phenomena, such as neutrinoless double-beta decay.

Future Directions

The research presented sets the stage for future studies exploring the unresolved challenges in the neutrino oscillation sector. To conclusively resolve the mass ordering and CP violation issues, further data from upcoming experiments with greater sensitivities and different baselines are essential. The tracking of θ₂₃'s octant, another key aim in the neutrino community, remains on the horizon. Future experimental setups designed for long-baseline neutrino oscillation studies, alongside advancements in detector technology, are critical for achieving these goals.

Conclusion

Overall, the paper successfully integrates recent data from multiple sources to provide updated insights into the neutrino oscillation picture. This multifaceted approach not only brings the field closer to answering crucial open questions but also highlights areas where additional research and data are needed. Continued exploration in this domain promises to elucidate some of the most fundamental questions concerning the role and nature of neutrinos in the universe.