Indication of Reactor ${\overline{\nu }}_e$ Disappearance in the Double Chooz Experiment

The Double Chooz experiment presents an indication of reactor electron antineutrino disappearance consistent with neutrino oscillations. An observed-to-predicted ratio of events of $0.944\pm 0.016(\mathrm{stat})\pm 0.040(\mathrm{syst})$ was obtained in 101 days of running at the Chooz nuclear power plant in France, with two $4.25{\mathrm{GW}}_{\mathrm{th}}$ reactors. The results were obtained from a single $10{\mathrm{m}}^3$ fiducial volume detector located 1050 m from the two reactor cores. The reactor antineutrino flux prediction used the Bugey4 flux measurement after correction for differences in core composition. The deficit can be interpreted as an indication of a nonzero value of the still unmeasured neutrino mixing parameter ${sin}^{2}2{\theta }_{13}$. Analyzing both the rate of the prompt positrons and their energy spectrum, we find ${sin}^{2}2{\theta }_{13}=0.086\pm 0.041(\mathrm{stat})\pm 0.030(\mathrm{syst})$, or, at 90% C.L., $0.017<{sin}^{2}2{\theta }_{13}<0.16$.

Figure 1

A cross-sectional view of the Double Chooz detector system.

Figure 2

The peak of the energy of neutron captures on Gd in IBD events (right scale) and its deviation from its average value (left scale) as a function of the elapsed (calendar) day.

Figure 3

Daily number of ${\overline{\nu }}_e$ candidates as a function of the expected number of ${\overline{\nu }}_e$. The dashed line is a fit to the data; the band is the 90% C.L. of this fit. The dotted line is the expectation in the no-oscillation scenario. The triangle indicates the measurement with both reactors off.

Figure 4

Top: Expected prompt energy spectra, including backgrounds, for the no-oscillation case and for the best fit $\mathrm{si}{\mathrm{n}}^{2}2{\theta }_{13}$, superimposed on the measured spectrum. Inset: Stacked histogram of backgrounds. Bottom: Difference between data and the no-oscillation spectrum (data points) and difference between the best fit and no-oscillation expectations (curve).