Atmospheric neutrino oscillation analysis with external constraints in Super-Kamiokande I-IV (1710.09126v2)

Abstract: An analysis of atmospheric neutrino data from all four run periods of \superk optimized for sensitivity to the neutrino mass hierarchy is presented. Confidence intervals for $\Delta m^2_{32}$, $\sin^2 \theta_{23}$, $\sin^2 \theta_{13}$ and $\delta_{CP}$ are presented for normal neutrino mass hierarchy and inverted neutrino mass hierarchy hypotheses based on atmospheric neutrino data alone. Additional constraints from reactor data on $\theta_{13}$ and from published binned T2K data on muon neutrino disappearance and electron neutrino appearance are added to the atmospheric neutrino fit to give enhanced constraints on the above parameters. Over the range of parameters allowed at 90% confidence level, the normal mass hierarchy is favored by between 91.5% and 94.5% based on the combined result.

Atmospheric Neutrino Oscillation Analysis in Super-Kamiokande I-IV

The paper presents a comprehensive analysis of atmospheric neutrino oscillation data collected over four distinct phases (I-IV) of the Super-Kamiokande (Super-K) experiment. Located in Japan, Super-K is a large cylindrical water Cherenkov detector designed to investigate the properties of neutrinos, including their oscillations. The primary objective is to test the three-neutrino mixing model, often described via the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix, which incorporates parameters like mixing angles, mass splittings, and a CP-violating phase.

One of the key challenges addressed is the determination of the neutrino mass hierarchy—whether the third mass eigenstate is heavier than the first two (normal hierarchy) or lighter (inverted hierarchy). The analysis harnesses data from 328 kiloton-years of exposure, focusing on the evidence that might indicate a preference for the normal hierarchy. Significant emphasis is placed on the role of atmospheric neutrinos in detecting matter effects, which manifest because neutrino oscillations are affected by the density profile of the Earth they traverse.

Summary of Results and Methodology

The analysis employs a binned chi-squared method incorporating systematic uncertainties and constraints from external experiments, primarily reactor data and specific results from the T2K experiment. Reactor data provides a constraint on the mixing angle $\theta_{13}$, while T2K supplies measurements useful for refining the understanding of $\theta_{23}$ and $\Delta m^2_{32}$. The paper reports an intriguing result: the normal mass hierarchy consistently shows marginally better fits to the Super-K data compared to the inverted hierarchy. The confidence in favoring the normal hierarchy ranges from approximately 81.9% to 96.7%, contingent on variations within the parameter space.

Detailed Analysis

The atmospheric neutrino oscillation probabilities are affected by $\theta_{13}$, predominantly through resonant enhancement at energies between 2 and 10 GeV. This analysis highlights the importance of carefully distinguishing between neutrino and antineutrino interactions, as spectral shifts in $e$-like events depending on the hierarchy type can signify oscillation patterns tied to matter effects.

The paper methodically discusses the systematic uncertainties, including those related to neutrino flux and interaction cross-sections. Moreover, substantial effort has been invested in ensuring the calibration of the detector’s energy scale and maintaining sample purity, which serves to strengthen the conclusions drawn from the data analyses.

Additional constraints are integrated from reactor experiments and T2K, demonstrating improvements in the hierarchy determination. Although T2K itself shows limited sensitivity to hierarchy, the intricacies of combined analyses suggest enhanced discriminatory power when atmospheric neutrino data are merged with T2K's findings.

Implications and Future Work

The findings bear implications for oscillation parameter refinement and offer tantalizing prospects for further investigation into CP-violating effects. Importantly, future studies are poised to adopt constraints from additional long-baseline experiments like NOvA and MINOS+. These collaborations could further clarify the mass hierarchy and contribute to precise measurements of oscillation parameters, possibly revealing CP violation in the neutrino sector.

In sum, the paper provides a thorough investigation into neutrino oscillations with atmospheric neutrino data, tempered with external constraints. It exhibits significant advancements toward resolving the neutrino mass hierarchy, albeit with a restrained confidence statement, and lays robust groundwork for subsequent inquiries into the complex field of neutrino physics.