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The Double Chooz experiment has determined the value of the neutrino oscillation parameter $theta_{13}$ from an analysis of inverse beta decay interactions with neutron capture on hydrogen. This analysis uses a three times larger fiducial volume than the standard Double Chooz assessment, which is restricted to a region doped with gadolinium (Gd), yielding an exposure of 113.1 GW-ton-years. The data sample used in this analysis is distinct from that of the Gd analysis, and the systematic uncertainties are also largely independent, with some exceptions, such as the reactor neutrino flux prediction. A combined rate- and energy-dependent fit finds $sin^2 2theta_{13}=0.097pm 0.034(stat.) pm 0.034 (syst.)$, excluding the no-oscillation hypothesis at 2.0 sigma. This result is consistent with previous measurements of $sin^2 2theta_{13}$.
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The establishment of the neutrino oscillations phenomenon as a solution to both solar and atmospheric neutrino anomalies had two consequences: a new oscillation mode, labelled $mathbf{theta_{13}}$, and the possibility to observe CP violation, if $mathbf{theta_{13}}$ was sizeable. CP violation implies that neutrino oscillations behave differently for neutrinos and anti-neutrinos -- a rare fundamental phenomenon key for our understanding of the Universe. The experimental demonstration of $mathbf{theta_{13}}$ has aided the completion of a quest lasting half a century. The best $mathbf{theta_{13}}$ knowledge is today inferred from high-precision reactor neutrino disappearance. The Double Chooz (DC) experiment has played a pioneering role in this channel by providing the first positive evidence, in 2011, in combination with the T2K experiment appearance data. The establishment of $mathbf{theta_{13}}$ awaited the Daya Bay experiments observation in 2012; confirmed soon after by the RENO experiment. Todays best knowledge on $mathbf{theta_{13}}$ from reactor experiments is a key input to many neutrino experiments. Here DC reports its first multi-detector $mathbf{theta_{13}}$ measurement exploiting several unprecedented techniques for a major precision improvement.
The Double Chooz collaboration presents a measurement of the neutrino mixing angle $theta_{13}$ using reactor $overline{ u}_{e}$ 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 1050m 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 $overline{ u}_{e}$ without gadolinium loading. Spectral distortions from the $overline{ u}_{e}$ reactor flux predictions previously reported with the neutron capture on gadolinium events are confirmed in the independent data sample presented here. A value of $sin^{2}2theta_{13} = 0.095^{+0.038}_{-0.039}$(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 $sin^{2}2theta_{13} = 0.088pm0.033$(stat+syst).
The Reactor Experiment for Neutrino Oscillation (RENO) experiment has been taking data using two identical liquid scintillator detectors of 44.5 tons since August 2011. The experiment has observed the disappearance of reactor neutrinos in their interactions with free protons, followed by neutron capture on hydrogen. Based on 1500 live days of data taken with 16.8 GW$_{th}$ reactors at the Hanbit Nuclear Power Plant in Korea, the near (far) detector observes 567690 (90747) electron antineutrino candidate events with a delayed neutron capture on hydrogen. This provides an independent measurement of $theta_{13}$ and a consistency check on the validity of the result from n-Gd data. Furthermore, it provides an important cross-check on the systematic uncertainties of the n-Gd measurement. Based on a rate-only analysis, we obtain sin$^{2}$2$theta _{13}$= 0.087 $pm$ 0.008 (stat.) $pm$ 0.014 (syst.).
Using the Double Chooz detector, designed to measure the neutrino mixing angle $theta_{13}$, the products of $mu^-$ capture on $^{12}$C, $^{13}$C, $^{14}$N and $^{16}$O have been measured. Over a period of 489.5 days, $2.3times10^6$ stopping cosmic $mu^-$ have been collected, of which $1.8times10^5$ captured on carbon, nitrogen, or oxygen nuclei in the inner detector scintillator or acrylic vessels. The resulting isotopes were tagged using prompt neutron emission (when applicable), the subsequent beta decays, and, in some cases, $beta$-delayed neutrons. The most precise measurement of the rate of $^{12}mathrm C(mu^-, u)^{12}mathrm B$ to date is reported: $6.57^{+0.11}_{-0.21}times10^{3},mathrm s^{-1}$, or $(17.35^{+0.35}_{-0.59})%$ of nuclear captures. By tagging excited states emitting gammas, the ground state transition rate to $^{12}$B has been determined to be $5.68^{+0.14}_{-0.23}times10^3,mathrm s^{-1}$. The heretofore unobserved reactions $^{12}mathrm C(mu^-, ualpha)^{8}mathrm{Li}$, $^{13}mathrm C(mu^-, umathrm nalpha)^{8}mathrm{Li}$, and $^{13}mathrm C(mu^-, umathrm n)^{12}mathrm B$ are measured. Further, a population of $beta$n decays following stopping muons is identified with $5.5sigma$ significance. Statistics limit our ability to identify these decays definitively. Assuming negligible production of $^{8}$He, the reaction $^{13}mathrm C(mu^-, ualpha)^{9}mathrm{Li}$ is found to be present at the $2.7sigma$ level. Limits are set on a variety of other processes.
A new measurement of the $theta_{13}$ mixing angle has been obtained at the Daya Bay Reactor Neutrino Experiment via the detection of inverse beta decays tagged by neutron capture on hydrogen. The antineutrino events for hydrogen capture are distinct from those for gadolinium capture with largely different systematic uncertainties, allowing a determination independent of the gadolinium-capture result and an improvement on the precision of $theta_{13}$ measurement. With a 217-day antineutrino data set obtained with six antineutrino detectors and from six 2.9 GW$_{th}$ reactors, the rate deficit observed at the far hall is interpreted as $sin^22theta_{13}=0.083pm0.018$ in the three-flavor oscillation model. When combined with the gadolinium-capture result from Daya Bay, we obtain $sin^22theta_{13}=0.089pm0.008$ as the final result for the six-antineutrino-detector configuration of the Daya Bay experiment.
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