Double Chooz: First Background-independent Measurement of θ13

Abstract

While the oscillation results published by the Double Chooz collaboration in 2011 and 2012 rely on a background model derived from reactor-on data, in this analysis we present a background model independent measurement of the mixing angle θ13. A global fit of the observed neutrino rates for different reactor power conditions is performed, yielding a measurement of both θ13 and the total background rate. The results on the mixing angle are improved significantly by including in the fit 7.53 days of reactor-off data, as they provides a direct measurement of the total background rate. In this reactor rate modulation analysis we consider neutrino candidates with neutron captures on both Gd and H nuclei, thereby boosting the statistical sample. The combination of the n-Gd and n-H candidates yields sin2(2θ13) = 0.102 ± 0.028(stat.) ± 0.033(syst.). The results presented in this study are consistent with the ones already published by Double Chooz, achieving a competitive precision. For the first time, they provide a determination of θ13 which does not dependon background models.