Measurement of $\theta_{13}$ in Double Chooz using neutron captures on hydrogen with novel background rejection techniques

TL;DR

This paper reports a precise measurement of the neutrino mixing angle θ13 using neutron captures on hydrogen in the Double Chooz experiment, employing novel background suppression techniques over 462 days of data.

Contribution

The study introduces new background rejection methods enabling accurate θ13 measurement without gadolinium, using hydrogen captures and a larger dataset than previous analyses.

Findings

Measured sin^2 2θ13 = 0.095^{+0.038}_{-0.039} from event rate analysis.

Confirmed spectral distortions consistent with reactor flux predictions.

Achieved over an order of magnitude reduction in accidental background.

Abstract

The Double Chooz collaboration presents a measurement of the neutrino mixing angle $\theta_{13}$ using reactor $\overline{\nu}_{e}$ observed via the inverse beta decay reaction in which the neutron is captured on hydrogen. This measurement is based on 462.72 live days data, approximately twice as much data as in the previous such analysis, collected with a detector positioned at an average distance of 1050m from two reactor cores. Several novel techniques have been developed to achieve significant reductions of the backgrounds and systematic uncertainties. Accidental coincidences, the dominant background in this analysis, are suppressed by more than an order of magnitude with respect to our previous publication by a multi-variate analysis. These improvements demonstrate the capability of precise measurement of reactor $\overline{\nu}_{e}$ without gadolinium loading. Spectral distortions from the $\overline{\nu}_{e}$ reactor flux predictions previously reported with the neutron capture on gadolinium events are confirmed in the independent data sample presented here. A value of $\sin^{2}2\theta_{13} = 0.095^{+0.038}_{-0.039}$(stat+syst) is obtained from a fit to the observed event rate as a function of the reactor power, a method insensitive to the energy spectrum shape. A simultaneous fit of the hydrogen capture events and of the gadolinium capture events yields a measurement of $\sin^{2}2\theta_{13} = 0.088\pm0.033$(stat+syst).