Measurements of neutrino oscillation in appearance and disappearance channels by the T2K experiment with 6.6E20 protons on target (1502.01550v2)

Abstract: We report on measurements of neutrino oscillation using data from the T2K long-baseline neutrino experiment collected between 2010 and 2013. In an analysis of muon neutrino disappearance alone, we find the following estimates and 68% confidence intervals for the two possible mass hierarchies: Normal Hierarchy: $\sin^2\theta_{23}=0.514^{+0.055}_{-0.056}$ and $\Delta m^2_{32}=(2.51\pm0.10)\times 10^{-3}$ eV$^2$/c$^4$ Inverted Hierarchy: $\sin^2\theta_{23}=0.511\pm0.055$ and $\Delta m^2_{13}=(2.48\pm0.10)\times 10^{-3}$ eV$^2$/c$^4$ The analysis accounts for multi-nucleon mechanisms in neutrino interactions which were found to introduce negligible bias. We describe our first analyses that combine measurements of muon neutrino disappearance and electron neutrino appearance to estimate four oscillation parameters and the mass hierarchy. Frequentist and Bayesian intervals are presented for combinations of these parameters, with and without including recent reactor measurements. At 90% confidence level and including reactor measurements, we exclude the region: $\delta_{CP}=[0.15,0.83]\pi$ for normal hierarchy and $\delta_{CP}=[-0.08,1.09]\pi$ for inverted hierarchy. The T2K and reactor data weakly favor the normal hierarchy with a Bayes Factor of 2.2. The most probable values and 68% 1D credible intervals for the other oscillation parameters, when reactor data are included, are: $\sin^2\theta_{23}=0.528^{+0.055}_{-0.038}$ and $|\Delta m^2_{32}|=(2.51\pm0.11)\times 10^{-3}$ eV$^2$/c$^4$.

• The paper presents precise measurements of oscillation parameters (sin²θ₂₃ and Δm²₃₂) using both frequentist and Bayesian methods.
• It leverages advanced beamline techniques and dual detector setups to reduce systematic uncertainties in neutrino interaction analyses.
• The results indicate a slight preference for the normal mass hierarchy, setting the stage for future CP violation investigations.

An Overview of the T2K Experiment and Neutrino Oscillation Measurements

The presented paper details the comprehensive measurements of neutrino oscillation parameters carried out by the T2K (Tokai to Kamioka) experiment, leveraging a dataset corresponding to $6.6\times10^{20}$ protons on target. The primary focus is on neutrino oscillations in both appearance and disappearance channels, exploring the parameter space with both frequentist and Bayesian approaches. This document provides a meticulous overview of the experimental setup, analysis methodology, and implications of the findings.

The T2K experiment uniquely benefits from a long-baseline neutrino beam produced at the J-PARC accelerator and directed toward the Super-Kamiokande (SK) detector, 295 km away. The analysis presented addresses two pivotal oscillation parameters, $\sin^2\theta_{23}$ and $\Delta m^2_{32}$, for both the normal and inverted mass hierarchies. The dataset consists of observed muon neutrino disappearance and electron neutrino appearance events, both crucial for testing the three-flavor neutrino oscillation model and searching for CP violation effects.

Experimental and Analytical Setup

The T2K experiment utilizes an advanced neutrino beamline, with finely tuned components to maximize neutrino flux towards SK. This setup, complemented by extensive near detector measurements from ND280, is vital in reducing the systematic uncertainties associated with the beam flux and initial neutrino interaction modeling.

Key to this analysis is the stringent systematic error control across various domains, including beam flux predictions derived from the hadron production model and external data from the NA61/SHINE experiment, uncertainties in neutrino interaction cross sections validated against external datasets such as those from MiniBooNE, and precise SK detector modeling. The latter incorporates sophisticated simulations of photonic and hadronic interactions within the detector medium to ensure accurate modeling of the complex neutrino interactions expected in the T2K energy regime (~600 MeV).

Results and Implications

The T2K analyses yield robust estimates for the oscillation parameters. Assuming the normal hierarchy, the point estimates obtained were $\sin^2\theta_{23} = 0.514$ and $\Delta m^2_{32} = 2.51\times10^{-3}\ \mathrm{eV}^2$, with comparable values for the inverted hierarchy. The inclusion of electron neutrino samples, alongside constraints from reactor experiment results, offered insights into the additional parameter $\sin^2\theta_{13}$ and the CP-violating phase $\delta_{CP}$.

Intriguingly, the T2K data, when juxtaposed with reactor data, suggest the normal hierarchy may be more favored, with a Bayes Factor of 2.2 weakly supporting this preference. However, the statistical evidence remains insufficient to definitively discriminate between hierarchies at the conclusive level.

Future Directions and Theoretical Considerations

The results underscore the necessity and efficacy of combining various sources of data and methodological approaches to constrain the oscillation parameter space effectively. Looking forward, T2K stands ready to enhance sensitivity through continued data collection and anti-neutrino mode running, potentially unraveling the mysteries of CP violation in the leptonic sector and providing inputs vital for the establishment of benchmarks for theories of neutrino mass generation.

Further inquiry, including the exploration of multi-nucleon interaction effects and continued refinement of systematic controls, is anticipated to tighten the existing constraints, moving closer to a comprehensive understanding of neutrino oscillations and their role within the Standard Model and beyond. The methodologies set forth in this work will greatly inform future neutrino oscillation projects, such as Hyper-Kamiokande and DUNE, designed to probe further into these fundamental phenomena.