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Abstract

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
(v) over bar (e) interactions up to a visible energy of 8.5MeV, using
the events at higher energies to build a cosmogenic background model
considering fast-neutrons interactions and Li-9 decays. The
backgroundmodel-independent determination of the mixing angle yields
sin(2)(2 theta(13)) = 0.094 +/- 0.017, 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.