Double Chooz: results towards the near detector phase

Abstract

Since the first indication in 2011 of a non-vanishing value of $\theta_{13}$ using reactor neutrinos by Double Chooz, this collaboration has developed new analyses leading to an increased precision on $\theta_{13}$ and different cross-checks of the oscillation results. Beyond the update of the analysis based on neutron captures on Gd in 2012, Double Chooz has also released a consistent measurement of $\theta_{13}$ by means of neutron captures on H. The combination of the n-Gd and n-H data samples, as well as 7.23 days of reactor off data, in a global rate+shape analysis yields $\sin^22\theta_{13} = 0.109\pm 0.035$. This measurement has been validated with a new background-model-independent approach, which compares the observed and expected neutrino rates as a function of reactor power. This Reactor Rate Modulation analysis yields $\sin^22\theta_{13} = 0.102\pm 0.043$ when combining the n-Gd and n-H samples. The Double Chooz collaboration is currently developing a new analysis increasing the signal-to-background ratio while reducing the background and detection systematics. This new approach is being prepared as a first step of the new phase of the experiment that will begin in summer 2014 with the operation of a near detector.