Indication of Electron Neutrino Appearance from an Accelerator-produced Off-axis Muon Neutrino Beam (1106.2822v2)

Abstract: The T2K experiment observes indications of $\nu_\mu\rightarrow \nu_e$ appearance in data accumulated with $1.43\times10^{20}$ protons on target. Six events pass all selection criteria at the far detector. In a three-flavor neutrino oscillation scenario with $|\Delta m_{23}^2|=2.4\times10^{-3}$ eV$^2$, $\sin^2 2\theta_{23}=1$ and $\sin^2 2\theta_{13}=0$, the expected number of such events is 1.5$\pm$0.3(syst.). Under this hypothesis, the probability to observe six or more candidate events is 7$\times10^{-3}$, equivalent to 2.5$\sigma$ significance. At 90% C.L., the data are consistent with 0.03(0.04)$<\sin^2 2\theta_{13}<$ 0.28(0.34) for $\delta_{\rm CP}=0$ and a normal (inverted) hierarchy.

• The paper reports that six electron neutrino events were observed, achieving a 2.5σ significance against an expected 1.5 ± 0.3 events.
• The study constrains sin²2θ13 between 0.03 and 0.34 (90% C.L.), suggesting a potential non-zero value crucial to neutrino oscillation models.
• The T2K experiment’s off-axis muon neutrino beam setup at 295 km efficiently minimizes high-energy backgrounds, enhancing the precision of oscillation measurements.

Indication of Electron Neutrino Appearance in the T2K Experiment

The paper presents results from the T2K experiment, which provide an indication of electron neutrino ($\nu_e$) appearance from an accelerator-produced off-axis muon neutrino ($\nu_\mu$) beam. This paper extends the understanding of neutrino oscillations, integral to the three-flavor mixing scenario described by the PMNS matrix, involving mixing angles $\theta_{12}$, $\theta_{23}$, and $\theta_{13}$, as well as the CP-violating phase $\delta_{\rm CP}$.

Key Findings

1. Observation and Significance:
   • Data accumulated with $1.43 \times 10^{20}$ protons on target yielded six events passing all selection criteria at the far detector.
   • Under the three-flavor neutrino oscillation scenario, with parameters such as $|\Delta m_{23}^2| = 2.4 \times 10^{-3}$ eV$^2$ and $\sin^2 2\theta_{23} = 1$, the expected number of such events is 1.5 ± 0.3.
   • The probability of observing six or more such events under these conditions is $7 \times 10^{-3}$, achieving a significance of 2.5$\sigma$.
2. Parameter Constraints:
   • At a 90% confidence level (C.L.), the data are consistent with $0.03(0.04) < \sin^2 2\theta_{13} < 0.28(0.34)$ for $\delta_{\rm CP} = 0$ and normal (inverted) hierarchy.
   • The best fit values are 0.11 and 0.14 respectively, indicating a potential non-zero $\theta_{13}$, which has been elusive in previous experimental efforts.

Experimental Setup

The T2K experiment uses a conventional neutrino beam generated at J-PARC, directed 2.5° off-axis to the Super-Kamiokande (SK) detector, 295 km away. This setup produces a narrow-band $\nu_\mu$ beam optimized to maximize oscillation effects at the energy of around 0.6 GeV.

• Neutrino Beam and Detection:
  • The beam flux models are refined using experimental data and tuned for precision.
  • The off-axis beam setup helps suppress high-energy backgrounds, crucial for extracting clean signals of neutrino oscillations.
• Systematic Uncertainties:
  • The T2K setup accounts comprehensively for systematic uncertainties emanating from neutrino flux predictions, cross-section modeling, and selection efficiencies. These are critical to ensuring robust statistical significance.

Implications and Future Directions

• The data supports the indication that $\nu_e$ appearance from $\nu_\mu$ beams is plausible under the established oscillation framework. The significance underlines the importance of $\theta_{13}$ in neutrino physics and suggests a non-zero value.
• These findings necessitate further data to conclusively validate $\nu_e$ appearance and refine measurements of $\theta_{13}$, offering insights into CP violation in the lepton sector.
• The T2K experiment's success could propel subsequent studies focused on determining the CP-violating phase $\delta_{\rm CP}$, pivotal to understanding matter-antimatter asymmetry in the universe.

Moving forward, continued data collection and analysis enhancements will drive more precise understanding, potentially incorporating results from upcoming neutrino experiments globally, fostering a unified framework for neutrino oscillations. This research signifies a vital stride in neutrino physics, aligning with theoretical predictions while presenting a case for further exploration in the CP violation arena.