Direct measurement of backgrounds using reactor-off data in Double Chooz

Double Chooz is unique among modern reactor-based neutrino experiments studying ${\overline{\nu }}_e$ 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 $\beta \mathrm{\text{−}}n$-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.

Figure 1

${\overline{\nu }}_e$ candidates in the reactor-off data sample, with breakdown by components. Top and bottom figures show DCI and DCII selection results, respectively. Black points: data; histogram: background $+{\overline{\nu }}_e$ expectation.

Figure 2

Scaling of DC fast-$n$ background rates and comparison with quoted values. Empty (full) markers indicate quoted results using a selection without (with) an external muon veto; lines and shaded bands represent our scaling of the DC measurements with their uncertainty [blue, forward hatched (with OV veto); red, backward hatched (without OV veto)]. Values were scaled by number of H atoms and normalized to muon flux at DC far site.

Figure 3

Scaling of DC $\beta \mathrm{\text{−}}n$-decay rates and comparison with quoted values. Results were scaled by a number of carbon atoms and normalized to muon flux at the DC far site. Solid lines correspond to rate uncertainties in the reactor-off analysis: DCI (red solid line) and open data points compare the total $\beta \mathrm{\text{−}}n$ rate, while DCII (blue solid line) and filled data points correspond to analyses with an extended veto following showering muons. Shaded regions indicate uncertainties (red, back-hatched DCI; blue, forward-hatched DCII).