Direct Measurement of Backgrounds using Reactor-Off Data in Double Chooz

TL;DR

This paper reports on direct background measurements during reactor-off periods in Double Chooz, validating background models and providing data crucial for neutrino oscillation analyses.

Contribution

It presents the first detailed background measurement during reactor-off periods in Double Chooz, confirming the accuracy of background models used in oscillation studies.

Findings

Measured background rate of 1.0±0.4 events/day during reactor-off

Validated background models for various sources within uncertainties

Provided kinematic distributions of dominant cosmic-ray-induced backgrounds

Abstract

Double Chooz is unique among modern reactor-based neutrino experiments studying $\bar \nu_e$ disappearance in that data can be collected with all reactors off. In this paper, we present data from 7.53 days of reactor-off running. Applying the same selection criteria as used in the Double Chooz reactor-on oscillation analysis, a measured background rate of 1.0$\pm$0.4 events/day is obtained. The background model for accidentals, cosmogenic $\beta$-$n$-emitting isotopes, fast neutrons from cosmic muons, and stopped-$\mu$ decays used in the oscillation analysis is demonstrated to be correct within the uncertainties. Kinematic distributions of the events, which are dominantly cosmic-ray-produced correlated-background events, are provided. The background rates are scaled to the shielding depths of two other reactor-based oscillation experiments, Daya Bay and RENO.