  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
$\bar\nu_{e}$ signal has increased. The value of $\theta_{13}$ is measured to
be $\sin^{2}2\theta_{13} = 0.090 ^{+0.032}_{-0.029}$ from a fit to the observed
energy spectrum. Deviations from the reactor $\bar\nu_{e}$ 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.
