Observation of Electron Neutrino Appearance in a Muon Neutrino Beam (1311.4750v2)

Abstract: The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of 7.3$\sigma$ when compared to 4.92 $\pm$ 0.55 expected background events. In the PMNS mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles $\theta_{12}$, $\theta_{23}$, $\theta_{13}$, a mass difference $\Delta m^2_{32}$ and a CP violating phase $\delta_{\mathrm{CP}}$. In this neutrino oscillation scenario, assuming $|\Delta m^2_{32}| = 2.4 \times 10^{-3}$ $\rm eV^2$, $\sin^2 \theta_{23} = 0.5$, and $\Delta m^2_{32} >0$ ($\Delta m^2_{32} <0$), a best-fit value of $\sin^2 2 \theta_{13}$ = $0.140^{+0.038}_{-0.032}$ ($0.170^{+0.045}_{-0.037}$) is obtained at $\delta_{\mathrm{CP}}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of $\theta_{13}$ from reactor experiments, some values of $\delta_{\mathrm{CP}}$ are disfavored at the 90% CL.

• The paper presents direct evidence of electron neutrino appearance by observing 28 events against an expected background of 4.92, achieving a 7.3σ significance.
• The methodology employs a muon neutrino beam from J-PARC and detailed NEUT simulations with ND280 data to minimize systematic uncertainties.
• The results provide a best-fit sin²2θ13 of 0.14 and support ongoing investigations into CP violation and the matter-antimatter asymmetry.

Observation of Electron Neutrino Appearance in a Muon Neutrino Beam

The paper under consideration details significant findings from the T2K experiment, demonstrating electron neutrino appearance in a muon neutrino beam. This observation is pivotal in the field of neutrino physics as it contributes to the precise measurement of neutrino oscillation parameters within the framework of the PMNS matrix.

Overview

The experiment involves a muon neutrino beam generated at the J-PARC facility in Tokai, Japan. The beam is directed towards the Super-Kamiokande detector, located 295 kilometers away. The measured event count comprises 28 electron neutrino occurrences, substantially exceeding the expected background of 4.92 ± 0.55 events for non-oscillation scenarios. This discrepancy yields a 7.3σ level of statistical significance, confirming electron neutrino appearance.

Methodology

The experiment capitalizes on a neutrino beam created from the decay of mesons. The beam's energy spectrum, predominantly around 0.6 GeV, is designed to optimize detection at the Super-Kamiokande detector while minimizing background noise. The neutrino interactions are simulated with the NEUT model, focusing on charged current quasi-elastic and single resonant pion production at the relevant energies.

Several systematic parameters pertaining to cross sections and interaction targets are accounted for, particularly using data collected from the near detector (ND280). This configuration reduces uncertainty in interaction rates and inherent beam contamination.

Results

The findings are presented within the context of the PMNS oscillation model, where neutrino transformation probabilities are functions of the mixing angles θ12, θ23, θ13, and the CP-violating phase δCP. For a normal hierarchy and δCP = 0, the best-fit value of sin²2θ13 was determined to be 0.140⁺⁰.⁰³⁸₋₀.₀₃₂.

Additionally, the results contribute to the ongoing effort to determine the CP-violating phase, with scenarios excluding certain δCP ranges at 90% confidence levels when integrating existing reactor neutrino data.

Implications and Future Directions

The implication of these results extends to both theoretical and practical realms of particle physics. By providing exacting measurements of θ13, this research underpins efforts to explore CP violations in the lepton sector, a currently elusive phenomenon.

Looking forward, continued data acquisition by T2K will enhance parameter precision and further probe lepton sector CP violation, with potential ramifications for understanding the matter-antimatter asymmetry of the Universe.

The collaboration acknowledges institutional and individual support across several nations and funding bodies, underscoring the collaborative nature and global interest within the neutrino research community.