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Register a new userYields and production rates of cosmogenic $^9$Li and $^8$He measured with the Double Chooz near and far detectors
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The yields and production rates of the radioisotopes $^9$Li and $^8$He created by cosmic muon spallation on $^{12}$C, have been measured by the two detectors of the Double Chooz experiment. The identical detectors are located at separate sites and depths, which means they are subject to different muon spectra. The near (far) detector has an overburden of $sim$120 m.w.e. ($sim$300 m.w.e.) corresponding to a mean muon energy of $32.1pm2.0,mathrm{GeV}$ ($63.7pm5.5,mathrm{GeV}$). Comparing the data to a detailed simulation of the $^9$Li and $^8$He decays, the contribution of the $^8$He radioisotope at both detectors is found to be compatible with zero. The observed $^9$Li yields in the near and far detectors are $5.51pm0.51$ and $7.90pm0.51$, respectively, in units of $10^{-8}mu ^{-1} mathrm{g^{-1} cm^{2} }$. The shallow overburdens of the near and far detectors give a unique insight when combined with measurements by KamLAND and Borexino to give the first multi--experiment, data driven relationship between the $^9$Li yield and the mean muon energy according to the power law $Y = Y_0( <E_{mu} >/ 1,mathrm{GeV})^{overline{alpha}}$, giving $overline{alpha}=0.72pm0.06$ and $Y_0=(0.43pm0.11)times 10^{-8}mu ^{-1} mathrm{g^{-1} cm^{2}}$. This relationship gives future liquid scintillator based experiments the ability to predict their cosmogenic $^9$Li background rates.
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A study on cosmic muons has been performed for the two identical near and far neutrino detectors of the Double Chooz experiment, placed at $sim$120 and $sim$300 m.w.e. underground respectively, including the corresponding simulations using the MUSIC simulation package. This characterization has allowed to measure the muon flux reaching both detectors to be (3.64 $pm$ 0.04) $times$ 10$^{-4}$ cm$^{-2}$s$^{-1}$ for the near detector and (7.00 $pm$ 0.05) $times$ 10$^{-5}$ cm$^{-2}$s$^{-1}$ 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 $alpha_{T}$ = 0.212 $pm$ 0.024 and 0.355 $pm$ 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.
A $theta_{13}$ oscillation analysis based on the observed antineutrino rates at the Double Chooz far and near detectors for different reactor power conditions is presented. This approach provides a so far unique simultaneous determination of $theta_{13}$ and the total background rates without relying on any assumptions on the specific background contributions. The analysis comprises 865 days of data collected in both detectors with at least one reactor in operation. The oscillation results are enhanced by the use of 24.06 days (12.74 days) of reactor-off data in the far (near) detector. The analysis considers the ue interactions up to a visible energy of 8.5 MeV, using the events at higher energies to build a cosmogenic background model considering fast-neutrons interactions and $^{9}$Li decays. The background-model-independent determination of the mixing angle yields sin$^2(2theta_{13})=0.094pm0.017$, being the best-fit total background rates fully consistent with the cosmogenic background model. A second oscillation analysis is also performed constraining the total background rates to the cosmogenic background estimates. While the central value is not significantly modified due to the consistency between the reactor-off data and the background estimates, the addition of the background model reduces the uncertainty on $theta_{13}$ to 0.015. Along with the oscillation results, the normalization of the anti-neutrino rate is measured with a precision of 0.86%, reducing the 1.43% uncertainty associated to the expectation.
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 cdot 10^{-3} $ eV$^2 lesssim Delta m^2_{41} lesssim 3cdot 10^{-1} $ eV$^2$ for mixing angles down to $sin^2 2theta_{14} gtrsim 0.02$. No significant disappearance additionally to the conventional disappearance related to $theta_{13} $ is observed and correspondingly exclusion bounds on the sterile mixing parameter $theta_{14} $ as function of $ Delta m^2_{41} $ are obtained.
Neutrons produced by cosmic ray muons are an important background for underground experiments studying neutrino oscillations, neutrinoless double beta decay, dark matter, and other rare-event signals. A measurement of the neutron yield in the three different experimental halls of the Daya Bay Reactor Neutrino Experiment at varying depth is reported. The neutron yield in Daya Bays liquid scintillator is measured to be $Y_n=(10.26pm 0.86)times 10^{-5}$, $(10.22pm 0.87)times 10^{-5}$, and $(17.03pm 1.22)times 10^{-5}~mu^{-1}~$g$^{-1}~$cm$^2$ at depths of 250, 265, and 860 meters-water-equivalent. These results are compared to other measurements and the simulated neutron yield in Fluka and Geant4. A global fit including the Daya Bay measurements yields a power law coefficient of $0.77 pm 0.03$ for the dependence of the neutron yield on muon energy.
The Double Chooz experiment presents improved measurements of the neutrino mixing angle $theta_{13}$ using the data collected in 467.90 live days from a detector positioned at an average distance of 1050 m from two reactor cores at the Chooz nuclear power plant. Several novel techniques have been developed to achieve significant reductions of the backgrounds and systematic uncertainties with respect to previous publications, whereas the efficiency of the $bar u_{e}$ signal has increased. The value of $theta_{13}$ is measured to be $sin^{2}2theta_{13} = 0.090 ^{+0.032}_{-0.029}$ from a fit to the observed energy spectrum. Deviations from the reactor $bar u_{e}$ prediction observed above a prompt signal energy of 4 MeV and possible explanations are also reported. A consistent value of $theta_{13}$ is obtained from a fit to the observed rate as a function of the reactor power independently of the spectrum shape and background estimation, demonstrating the robustness of the $theta_{13}$ measurement despite the observed distortion.
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