Background-independent measurement of θ[subscript 13] in Double Chooz

Abstract

The oscillation results published by the Double Chooz Collaboration in 2011 and 2012 rely on background models substantiated by reactor-on data. In this analysis, we present a background-model-independent measurement of the mixing angle θ[subscript 13] by including 7.53 days of reactor-off data. A global fit of the observed antineutrino rates for different reactor power conditions is performed, yielding a measurement of bothθ[subscript 13] and the total background rate. The results on the mixing angle are improved significantly by including the reactor-off data in the fit, as it provides a direct measurement of the total background rate. This reactor rate modulation analysis considers antineutrino candidates with neutron captures on both Gd and H, whose combination yields sin[superscript 2](2θ[subscript 13]) = 0.102 ± 0.028(stat.) ± 0.033(syst.). The results presented in this study are fully consistent with the ones already published by Double Chooz, achieving a competitive precision. They provide, for the first time, a determination of θ[subscript 13] that does not depend on a background model.