Reactor electron antineutrino disappearance in the Double Chooz experiment

Abstract: The Double Chooz experiment has observed 8,249 candidate electron antineutrino events in 227.93 live days with 33.71 GW-ton-years (reactor power x detector mass x livetime) exposure using a 10.3 cubic meter fiducial volume detector located at 1050 m from the reactor cores of the Chooz nuclear power plant in France. The expectation in case of theta13 = 0 is 8,937 events. The deficit is interpreted as evidence of electron antineutrino disappearance. From a rate plus spectral shape analysis we find sin^2 2{\theta}13 = 0.109 \pm 0.030(stat) \pm 0.025(syst). The data exclude the no-oscillation hypothesis at 99.8% CL (2.9{\sigma}).

• The paper presents a robust measurement of the mixing angle θ13 by detecting a significant antineutrino deficit versus no-oscillation expectations.
• It employs inverse beta decay detection with rigorous calibration and systematic uncertainty assessments over 227.93 live days.
• The findings, validated at 2.9σ significance, enhance our understanding of neutrino oscillations and inform future reactor neutrino research.

An Overview of the Double Chooz Experiment on Reactor Antineutrino Disappearance

This paper presents an analysis of reactor antineutrino disappearance observed by the Double Chooz experiment. The analysis, based on data from a 10.3 m$^3$ fiducial volume detector located 1050 meters from the Chooz nuclear power plant in France, aims to measure the neutrino mixing angle $\theta_{13}$. The document summarizes the methodology and findings over 227.93 live days, with a total exposure of 33.71 GW-ton-years, during which 8,249 electron antineutrino candidates were detected compared to the expected 8,937 in a scenario of no oscillations involving $\theta_{13} = 0$.

The results indicate a deficit in antineutrino events, inferring a non-zero value of $\theta_{13}$. The disappearance phenomenon is analyzed using a combination of rate and spectral shape information, resulting in $$\theta_{13} = 0.109 \pm 0.030 \text{(stat)} \pm 0.025 \text{(syst)}$$. The no-oscillation hypothesis is ruled out with 99.8% confidence level (2.9$\sigma$).

Experimental and Analytical Framework

The Double Chooz experiment employs a reactor neutrino detection methodology based on inverse beta decay (IBD) reactions. Antineutrinos from nuclear reactors interact with protons in the detector, resulting in positrons and neutrons. The analysis is sensitive to neutrino oscillations, indicated by changes in both the rate and the energy spectrum of detected neutrinos compared to expectations. This approach necessitates accurate knowledge of the detector's energy response and calibration, as well as precise reactor flux modeling.

Calibration procedures involved varied sources and cross-checks between different methods, giving confidence in the systematic uncertainty management. The experiment enhances understanding by segmenting data based on reactor operational periods - one or both reactors on - allowing the separation of signal and background contributions.

Background and Systematic Uncertainties

Key backgrounds include cosmogenic isotopes (^9Li and ^8He) and fast neutrons, which mimic IBD events. Accidental coincidences, stemming from unrelated environmental radioactivity-induced triggers, were accurately estimated by time-window analyses. The paper computes systematic uncertainties through Monte Carlo simulations and cross-validates these with available calibrations and comparisons to prior established datasets, such as the Bugey4 normalization.

The detector's efficiency was verified using $^{252}$Cf neutron source calibration. Spill-in and spill-out effects due to neutron capture within or escaping from the detector's fiducial volume were corrected, recognizing minimal discrepancies between data and Monte Carlo predictions.

Implications and Future Directions

The Double Chooz experiment’s findings contribute significantly to the parameter space knowledge concerning neutrino oscillations, particularly $\theta_{13}$. The results support the non-zero determination of this mixing angle, aligning with contributions from other contemporary experiments, thereby enhancing the global understanding of neutrino mixing and potential CP violation in the lepton sector.

The quantitative results pave the way for further explorations in reactor neutrino physics, particularly with the aim of resolving additional parameters such as the mass hierarchy problem and providing constraints on anomalous sterile neutrino mixing. Future iterations and enhancements in the Double Chooz setup or methodology could refine these parameters further along the theoretical and phenomenological trajectories in neutrino physics.

In essence, the Double Chooz findings reinforce the validity and efficiency of spectral shape analyses combined with rate measurements in dissecting neutrino mixing phenomena, and invite ongoing investigation in this fertile area of fundamental physics.