Reactor Rate Modulation oscillation analysis with two detectors in Double Chooz

TL;DR

This paper presents a background-model-independent analysis of reactor neutrino oscillations at Double Chooz, measuring b8
3 in a way that minimizes assumptions about background contributions, using extensive reactor-off data.

Contribution

It introduces a novel background-model-independent method for b8
3 measurement using reactor-off data, improving precision and reducing reliance on background assumptions.

Findings

Measured b8
3 as sin^2(2b8
3)=0.094b1017

Achieved a background rate estimate consistent with cosmogenic models

Reduced uncertainty on b8
3 to 0.015

Abstract

A $\theta_{13}$ oscillation analysis based on the observed antineutrino rates at the Double Chooz far and near detectors for different reactor power conditions is presented. This approach provides a so far unique simultaneous determination of $\theta_{13}$ and the total background rates without relying on any assumptions on the specific background contributions. The analysis comprises 865 days of data collected in both detectors with at least one reactor in operation. The oscillation results are enhanced by the use of 24.06 days (12.74 days) of reactor-off data in the far (near) detector. The analysis considers the \nue interactions up to a visible energy of 8.5 MeV, using the events at higher energies to build a cosmogenic background model considering fast-neutrons interactions and $^{9}$Li decays. The background-model-independent determination of the mixing angle yields sin$^2(2\theta_{13})=0.094\pm0.017$, being the best-fit total background rates fully consistent with the cosmogenic background model. A second oscillation analysis is also performed constraining the total background rates to the cosmogenic background estimates. While the central value is not significantly modified due to the consistency between the reactor-off data and the background estimates, the addition of the background model reduces the uncertainty on $\theta_{13}$ to 0.015. Along with the oscillation results, the normalization of the anti-neutrino rate is measured with a precision of 0.86\%, reducing the 1.43\% uncertainty associated to the expectation.