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High Energy Physics - Experiment, hep-ex, Physics, Instrumentation and Detectors, physics.ins-det
Abstract:
The Double Chooz collaboration presents a measurement of the neutrino mixing
angle $\theta_{13}$ using reactor $\overline{\nu}_{e}$ 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
$\overline{\nu}_{e}$ without gadolinium loading. Spectral distortions from the
$\overline{\nu}_{e}$ reactor flux predictions previously reported with the
neutron capture on gadolinium events are confirmed in the independent data
sample presented here. A value of $\sin^{2}2\theta_{13} =
0.095^{+0.038}_{-0.039}$(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 $\sin^{2}2\theta_{13} =
0.088\pm0.033$(stat+syst).
