[Paper Review] Improved Measurement of Electron Antineutrino Disappearance at Daya Bay (Proceeding to NuFact12)

X. Qian|arXiv (Cornell University)|Nov 2, 2012

Neutrino Physics Research8 references3 citations

TL;DR

This paper presents an improved measurement of electron antineutrino disappearance at the Daya Bay reactor neutrino experiment using 139 days of data, achieving a 5.2σ significance for non-zero θ₁₃. The analysis yields sin²2θ₁₃ = 0.089 ± 0.011 at 68% confidence level, confirming the non-zero value of the mixing angle with enhanced precision through a ratio method using multiple detectors at different baselines.

ABSTRACT

The Daya Bay experiment was designed to be the largest and the deepest underground among the many current-generation reactor antineutrino experiments. With functionally identical detectors deployed at multiple baselines, the experiment aims to achieve the most precise measurement of $\sin^2 2θ_{13}$. The antineutrino rates measured in the two near experimental halls are used to predict the rate at the far experimental hall (average distance of 1648 m from the reactors), assuming there is no neutrino oscillation. The ratio of the measured over the predicted far-hall antineutrino rate is then used to constrain the $\sin^2 2θ_{13}$. The relative systematic uncertainty on this ratio is expected to be 0.2$\sim$0.4%. In this talk, we present an improved measurement of the electron antineutrino disappearance at Daya Bay. With data of 139 days, the deficit of the antineutrino rate in the far experimental hall was measured to be 0.056 $\pm$ 0.007 (stat.) $\pm$ 0.003 (sys.). In the standard three-neutrino framework, the $\sin^2 2 θ_{13}$ was determined to be 0.089 $\pm$ 0.011 at the 68% confidence level in a rate-only analysis.

Motivation & Objective

• To improve the precision of the measurement of the neutrino mixing angle sin²2θ₁₃ using reactor antineutrino disappearance.
• To reduce systematic uncertainties in the measurement by employing multiple functionally identical detectors at different baselines.
• To confirm the non-zero value of θ₁₃ with higher significance using an increased data sample of 139 days.
• To enhance sensitivity to the neutrino mixing parameters through a ratio-based analysis of antineutrino rates at near and far halls.
• To lay the groundwork for future high-precision measurements of Δm²ₑₑ and the neutrino mass hierarchy using the high-statistics IBD sample.

Proposed method

• Utilizes the inverse beta decay (IBD) reaction: ν̄ₑ + p → e⁺ + n, detected via prompt and delayed signals in liquid scintillator detectors.
• Employs a ratio method comparing antineutrino rates in two near-hall detectors (predicting the far-hall rate) to suppress systematic uncertainties.
• Measures the antineutrino rate deficit in the far hall (1648 m baseline) relative to the predicted rate assuming no oscillation.
• Applies a three-neutrino oscillation framework with the standard PMNS matrix, using the oscillation probability formula: P(ν̄ₑ → ν̄ₑ) = 1 - sin²2θ₁₃ cos²θ₁₂ sin²Δ₃₁ - sin²2θ₁₃ sin²θ₁₂ sin²Δ₃₂ - cos⁴θ₁₃ sin²2θ₁₂ sin²Δ₂₁.
• Performs a rate-only analysis with full propagation of uncertainties from backgrounds, detectors, reactor fluxes, and oscillation parameters.
• Uses GPS and Total Station surveys to determine baseline distances with 1.8 cm precision, minimizing distance-related systematic errors.

Experimental results

Research questions

• RQ1 What is the improved precision of the sin²2θ₁₃ measurement using 139 days of Daya Bay data?
• RQ2 How does the measured antineutrino rate deficit in the far hall compare to the predicted rate without oscillation?
• RQ3 To what extent does the data disfavor the null hypothesis of sin²2θ₁₃ = 0?
• RQ4 What is the projected sensitivity of the Daya Bay experiment to sin²2θ₁₃ with full 8-detector operation?
• RQ5 Can the high-statistics IBD sample enable precise measurements of the antineutrino energy spectrum and Δm²ₑₑ?

Key findings

• The antineutrino rate deficit in the far experimental hall was measured as 0.056 ± 0.007 (statistical) ± 0.003 (systematic).
• The sin²2θ₁₃ was determined to be 0.089 ± 0.011 at the 68% confidence level in a rate-only analysis.
• The non-zero value of sin²2θ₁₃ is disfavored at the 7.7 standard deviation level, confirming the earlier 2012 result with higher significance.
• The total uncertainty in the measurement is still dominated by statistical uncertainty, which is about twice the systematic uncertainty.
• With the full 8-detector configuration, Daya Bay is projected to achieve a 5% measurement of sin²2θ₁₃ within approximately 3 years of data-taking.
• The high-statistics IBD sample from Daya Bay enables precise measurement of the antineutrino energy spectrum and provides sensitivity to the effective squared-mass difference Δm²ₑₑ.