[en] Being an intense and pure source of low energy electron antineutrinos, nuclear reactors are one of the most powerful tools to investigate neutrino oscillations. The Double Chooz experiment aims for a precise determination of the neutrino mixing angle θ ₁₃ using a two detector configuration with a liquid scintillator target volume read by photomultipliers. The antineutrino detection efficiency systematic uncertainty is the dominant component in the normalization uncertainty affecting the final precision on the θ ₁₃ measurement.The collected data from the near detector since January 2015 will profit from improved detection systematic uncertainties thanks to the cancellation of correlated contributions between both detectors. (paper)

[en] The STEREO experiment measures the electron antineutrino spectrum emitted in a research reactor using the inverse beta decay reaction on H nuclei in a gadolinium loaded liquid scintillator. The detection is based on a signal coincidence of a prompt positron and a delayed neutron capture event. The simulated response of the neutron capture on gadolinium is crucial for the comparison with data, in particular in the case of the detection efficiency. Among all stable isotopes, ${}^{155}$Gd and ${}^{157}$Gd have the highest cross sections for thermal neutron capture. The excited nuclei after the neutron capture emit gamma rays with a total energy of about 8MeV. The complex level schemes of ${}^{156}$Gd and ${}^{158}$Gd are a challenge for the modeling and prediction of the deexcitation spectrum, especially for compact detectors where gamma rays can escape the active volume. With a new description of the Gd (n,$\mathrm{\gamma <!--\; ??\; -->}$) cascades obtained using the FIFRELIN code, the agreement between simulation and measurements with a neutron calibration source was significantly improved in the STEREO experiment. A database of ten millions of deexcitation cascades for each isotope has been generated and is now available for the user.

[en] We present a search for signatures of neutrino mixing of electron anti-neutrinos with additional hypothetical sterile neutrino flavors using the Double Chooz experiment. The search is based on data from 5 years of operation of Double Chooz, including 2 years in the two-detector configuration. The analysis is based on a profile likelihood, i.e. comparing the data to the model prediction of disappearance in a data-to-data comparison of the two respective detectors. The analysis is optimized for a model of three active and one sterile neutrino. It is sensitive in the typical mass range 5 × 10${}^{-<!--\; ???\; -->3}$ eV${}^2$$\lesssim <!--\; ???\; -->$ Δm${}_{41}^2$$\lesssim <!--\; ???\; -->$ 3 × 10${}^{-<!--\; ???\; -->1}$ eV${}^2$ for mixing angles down to sin${}^2$ 2θ${}_{14}$$\gtrsim <!--\; ???\; -->$ 0.02. No significant disappearance additionally to the conventional disappearance related to θ${}_{13}$ is observed and correspondingly exclusion bounds on the sterile mixing parameter θ${}_{14}$ as a function of Δm${}_{41}^2$ are obtained.

[en] The Double Chooz collaboration presents a measurement of the neutrino mixing angle θ_1_3 using reactor ν̄_e observed via the inverse beta decay reaction in which the neutron is captured on hydrogen. This measurement is based on 462.72 live days data, approximately twice as much data as in the previous such analysis, collected with a detector positioned at an average distance of 1050 m from two reactor cores. Several novel techniques have been developed to achieve significant reductions of the backgrounds and systematic uncertainties. Accidental coincidences, the dominant background in this analysis, are suppressed by more than an order of magnitude with respect to our previous publication by a multi-variate analysis. These improvements demonstrate the capability of precise measurement of reactor ν̄_e without gadolinium loading. Spectral distortions from the ν̄_e reactor flux predictions previously reported with the neutron capture on gadolinium events are confirmed in the independent data sample presented here. A value of sin"2 2θ_1_3=0.095_−_0_._0_3_9"+"0"."0"3"8(stat+syst) is obtained from a fit to the observed event rate as a function of the reactor power, a method insensitive to the energy spectrum shape. A simultaneous fit of the hydrogen capture events and of the gadolinium capture events yields a measurement of sin"2 2θ_1_3=0.088±0.033(stat+syst).

[en] This article reports the measurement of the ²³⁵U-induced antineutrino spectrum shape by the Stereo experiment. 43 000 antineutrinos have been detected at about 10 m from the highly enriched core of the ILL reactor during 118 full days equivalent at nominal power. The measured inverse beta decay spectrum is unfolded to provide a pure ²³⁵U spectrum in antineutrino energy. A careful study of the unfolding procedure, including a cross-validation by an independent framework, has shown that no major biases are introduced by the method. A significant local distortion is found with respect to predictions around E _ν ≃ 5.3 MeV. A Gaussian fit of this local excess leads to an amplitude of A = 12.1 ± 3.4% (3.5σ). (paper)

[en] The STEREO experiment is a very short baseline reactor antineutrino experiment aiming at testing the hypothesis of light sterile neutrinos as an explanation of the deficit of the observed neutrino interaction rate with respect to the predicted rate, known as the Reactor Antineutrino Anomaly. The detector center is located 10 m away from the compact, highly ²³⁵U enriched core of the research nuclear reactor of the Institut Laue Langevin in Grenoble, France. This paper describes the STEREO site, the detector components and associated shielding designed to suppress the external sources of background which were characterized on site. It reports the performances in terms of detector response and energy reconstruction.

[en] The STEREO experiment is searching for a non-standard oscillation in the propagation of anti-neutrinos produced by the high flux reactor of the Institut Laue-Langevin which could be the sign for the existence of a sterile state of eV mass and the origin of the reactor anti-neutrino anomaly. In this paper, results from 66 days of reactor turned on and 138 days of reactor turned off are reported excluding large amplitude oscillations. A special focus is put on the data analysis and studies of correlated backgrounds. In particular the origin of the correlated background measured in reactor turned off periods is discussed. This background mainly originates from neutrons produced by cosmic radiation. (authors)

[en] Liquid scintillators are a common choice for neutrino physics experiments, but their capabilities to perform background rejection by scintillation pulse shape discrimination is generally limited in large detectors. This paper describes a novel approach for a pulse shape based event classification developed in the context of the Double Chooz reactor antineutrino experiment. Unlike previous implementations, this method uses the Fourier power spectra of the scintillation pulse shapes to obtain event-wise information. A classification variable built from spectral information was able to achieve an unprecedented performance, despite the lack of optimization at the detector design level. Several examples of event classification are provided, ranging from differentiation between the detector volumes and an efficient rejection of instrumental light noise, to some sensitivity to the particle type, such as stopping muons, ortho-positronium formation, alpha particles as well as electrons and positrons. In combination with other techniques the method is expected to allow for a versatile and more efficient background rejection in the future, especially if detector optimization is taken into account at the design level.

[en] A study on cosmic muons has been performed for the two identical near and far neutrino detectors of the Double Chooz experiment, placed at ∼120 and ∼300 m.w.e. underground respectively, including the corresponding simulations using the MUSIC simulation package. This characterization has allowed us to measure the muon flux reaching both detectors to be (3.64 ± 0.04) × 10⁻⁴ cm⁻²s⁻¹ for the near detector and (7.00 ± 0.05) × 10⁻⁵ cm⁻²s⁻¹ for the far one. The seasonal modulation of the signal has also been studied observing a positive correlation with the atmospheric temperature, leading to an effective temperature coefficient of α _T = 0.212 ± 0.024 and 0.355 ± 0.019 for the near and far detectors respectively. These measurements, in good agreement with expectations based on theoretical models, represent one of the first measurements of this coefficient in shallow depth installations.

[en] The precise modeling of the de-excitation of Gd isotopes is of great interest for experimental studies of neutrinos using Gd-loaded organic liquid scintillators. The FIFRELIN code was recently used within the purposes of the STEREO experiment for the modeling of the Gd de-excitation after neutron capture in order to achieve a good control of the detection efficiency. In this work, we report on the recent additions in the FIFRELIN de-excitation model with the purpose of enhancing further the de-excitation description. Experimental transition intensities from the EGAF database are now included in the FIFRELIN cascades, in order to improve the description of the higher energy part of the spectrum. Furthermore, the angular correlations between $\mathrm{\gamma <!--\; ??\; -->}$ rays are now implemented in FIFRELIN, to account for the relative anisotropies between them. In addition, conversion electrons are now treated more precisely in the whole spectrum range, while the subsequent emission of X rays is also accounted for. The impact of the aforementioned improvements in FIFRELIN is tested by simulating neutron captures in various positions inside the STEREO detector. A repository of up-to-date FIFRELIN simulations of the Gd isotopes is made available for the community, with the possibility of expanding for other isotopes which can be suitable for different applications.