New constraints on oscillation parameters from $ν_e$ appearance and $ν_μ$ disappearance in the NOvA experiment (1806.00096v2)

Abstract: We present updated results from the NOvA experiment for $\nu_\mu\rightarrow\nu_\mu$ and $\nu_\mu\rightarrow\nu_e$ oscillations from an exposure of $8.85\times10^{20}$ protons on target, which represents an increase of 46% compared to our previous publication. The results utilize significant improvements in both the simulations and analysis of the data. A joint fit to the data for $\nu_\mu$ disappearance and $\nu_e$ appearance gives the best fit point as normal mass hierarchy, $\Delta m^2_{32} = 2.44\times 10^{-3}{{\rm eV}^2}/c^4$, $\sin^2\theta_{23} = 0.56$, and $\delta_{CP} = 1.21\pi$. The 68.3% confidence intervals in the normal mass hierarchy are $\Delta m^2_{32} \in [2.37,2.52]\times 10^{-3}{{\rm eV}^2}/c^4$, $\sin^2\theta_{23} \in [0.43,0.51] \cup [0.52,0.60]$, and $\delta_{CP} \in [0,0.12\pi] \cup [0.91\pi,2\pi]$. The inverted mass hierarchy is disfavored at the 95% confidence level for all choices of the other oscillation parameters.

Analysis of Neutrino Oscillation Parameters in the NOvA Experiment

The paper discusses recent advancements in the measurement of neutrino oscillation parameters using the NOvA experiment. This experiment is an integral part of contemporary efforts to refine our understanding of neutrino properties, focusing on the neutrino mass hierarchy, oscillation angles, and CP violation phase. NOvA uses a high-intensity neutrino beam produced at Fermilab and targets two remote detectors over a baseline of 810 km.

Experimental Framework

NOvA employs a two-detector setup: the Near Detector (ND) located approximately 1 km from the neutrino source, and the Far Detector (FD), positioned 810 km downstream. This design allows for precise baseline studies and minimizes systematic uncertainties that might arise from beamline fluctuations. The detectors are constructed predominantly from scintillating materials and utilize optical fibers connected to avalanche photodiodes (APD) for enhanced light collection efficiency, crucial for distinguishing between neutrino-induced signals and cosmic backgrounds.

Methodological Innovations

The paper emphasizes several methodological improvements over previous analyses. Key advancements include:

• Enhanced Simulation and Data Analysis: Improvements in neutrino simulations involve introducing a data-driven neutrino flux model, refined treatment of multi-nucleon interactions, and a more nuanced light propagation model, factoring in Cherenkov radiation.
• Deep Learning Techniques: The utilization of deep-learning classifiers—specifically the Convolutional Visual Network (CVN)—to enhance signal characterization in both the appearance and disappearance channels. CVN substantially improves the event selection by categorizing event types accurately based on complex visual features extracted from the data.
• Energy Resolution Enhancements: By categorizing events into different energy resolution subsets, the analysis optimizes energy reconstruction, significantly impacting the precision of oscillation parameters derived from the data.

Key Findings and Results

The analysis achieved through these advancements indicates several important findings:

• Mass Hierarchy Preference: The analysis disfavors the inverted mass hierarchy at the 95% confidence level, and the data best fit supports the normal mass hierarchy.
• Oscillation Parameters: The best-fit points for the oscillation parameters are identified as Δm²_32 = $2.44 \times 10^{-3} \ \text{eV}^2$ for the normal hierarchy and θ_23 = 0.56, which places the mixing angle in the upper octant.
• CP Violation Phase: Though the sensitivity to the CP violation phase is limited, the analysis places constraints on δ_CP in the range [0, 0.12π] ∪ [0.91π, 2π] at the 68.3% confidence interval.

Implications and Future Directions

The findings from NOvA have significant implications for neutrino physics. Precision in determining the neutrino mass hierarchy is crucial for understanding the structure of the neutrino sector, which has ramifications for CP symmetry violations observed in the lepton sector that could have contributed to the matter-antimatter asymmetry in the universe. The constraints on the oscillation parameters help refine models of neutrino mass and mixing in theoretical physics.

Future iterations of the NOvA experiment, potentially alongside complementary experiments like T2K, are expected to further reduce uncertainties in these parameters. As methodologies and technologies advance, particularly in data analysis techniques such as machine learning, the precision of observations is anticipated to improve. Further data collection and analysis phases could more definitively resolve the remaining questions regarding the CP phenomenon and θ_23 octant ambiguity, offering deeper insights into fundamental particle physics.

In conclusion, the NOvA experiment exemplifies the complexity and rigor of contemporary neutrino research, contributing valuable insights into some of the most profound questions about the universe's fundamental constituents.