Indication for the disappearance of reactor electron antineutrinos in the Double Chooz experiment

Abstract: The Double Chooz Experiment presents an indication of reactor electron antineutrino disappearance consistent with neutrino oscillations. A ratio of 0.944 $\pm$ 0.016 (stat) $\pm$ 0.040 (syst) observed to predicted events was obtained in 101 days of running at the Chooz Nuclear Power Plant in France, with two 4.25 GW$_{th}$ reactors. The results were obtained from a single 10 m$^3$ fiducial volume detector located 1050 m from the two reactor cores. The reactor antineutrino flux prediction used the Bugey4 measurement as an anchor point. The deficit can be interpreted as an indication of a non-zero value of the still unmeasured neutrino mixing parameter \sang. Analyzing both the rate of the prompt positrons and their energy spectrum we find \sang = 0.086 $\pm$ 0.041 (stat) $\pm$ 0.030 (syst), or, at 90% CL, 0.015 $<$ \sang $\ <$ 0.16.

• The paper demonstrates reactor antineutrino disappearance as evidence for neutrino oscillation, establishing a non-zero sin²2θ₁₃ measurement.
• It uses inverse beta decay detection with a single detector 1050 meters from reactors and a rigorous background analysis for accuracy.
• The results exclude the no-oscillation hypothesis at 94.6% confidence, refining neutrino mixing parameters essential for CP violation studies.

Indication for the Disappearance of Reactor Antineutrinos in the Double Chooz Experiment

The Double Chooz experiment addresses a fundamental question in neutrino physics: the potential for electron antineutrinos to oscillate as they travel from nuclear reactors. More specifically, the study seeks to determine the non-zero value of the neutrino mixing parameter, sin²2θ₁₃, which is critical for understanding CP violation in the neutrino sector and impacts the design requirements for long-baseline neutrino oscillation experiments.

Summary of Findings

The experiment was conducted using a single detector positioned approximately 1050 meters away from two nuclear reactors at the Chooz Nuclear Power Plant in France. Over the course of 101 days, an observed-to-predicted ratio of antineutrino events of 0.944 ± 0.016 (statistical) ± 0.040 (systematic) was reported, indicating that reactor antineutrinos are indeed disappearing. This phenomenon is interpreted as evidence for neutrino oscillations, implying a non-zero value for sin²2θ₁₃.

The analysis of both the rate and energy spectrum of detected events yielded sin²2θ₁₃ = 0.086 ± 0.041 (statistical) ± 0.030 (systematic). The no-oscillation hypothesis was excluded at a confidence level of 94.6%, with an allowed range for sin²2θ₁₃ at 90% confidence level of 0.017 < sin²2θ₁₃ < 0.16.

Methodology

The detection strategy relies on the inverse beta decay process, with electron antineutrinos interacting with protons to produce positrons and neutrons. The Double Chooz detector, housed in a shielded environment beneath 300 meters of rock to minimize background noise, uses a liquid scintillator with added gadolinium to enhance neutron capture efficiency. The detector's design includes multiple concentric tanks that are used to differentiate between actual signal events and various background noise sources.

Background and Systematic Studies

To ensure the validity of the findings, a detailed study of potential background noise was conducted, including uncorrelated coincidences, fast neutrons, and cosmogenic isotopes such as ⁹Li. The paper reports careful calibration of the detector using a range of isotopes and an extensive simulation process. Reactor data were corroborated using simulations provided by MURE and DRAGON codes, with systematic errors considered for various parameters including fission rates and energy per fission.

Implications and Future Prospects

The Double Chooz results are significant as they provide a non-zero measurement for sin²2θ₁₃, narrowing down the parameter space for neutrino oscillation research. The collaboration's findings have implications for the theoretical modeling of neutrino masses and mixing, enhancing the understanding of CP violation effects and informing next-generation neutrino experiments.

With ongoing data collection and the addition of a near detector, Double Chooz aims to further refine the measurement of sin²2θ₁₃, enhancing precision and enabling a more comprehensive study of both systematic and statistical uncertainties. Such efforts will contribute substantially to the global endeavor of understanding the nuances of neutrino behavior and their impact on the broader field of particle physics.