Evidence of Antineutrinos from Distant Reactors Using Pure Water at $\mathrm{SNO}+$

The $\mathrm{SNO}+$ Collaboration reports the first evidence of reactor antineutrinos in a Cherenkov detector. The nearest nuclear reactors are located 240 km away in Ontario, Canada. This analysis uses events with energies lower than in any previous analysis with a large water Cherenkov detector. Two analytical methods are used to distinguish reactor antineutrinos from background events in 190 days of data and yield consistent evidence for antineutrinos with a combined significance of $3.5\sigma$.

Figure 1

The top row shows the LR method, and the bottom row shows the BDT method. Left: The LR for internal events (a) and the delayed BDT for all events (d) that pass the criteria in Table 1. The signal (red) is normalized to the expectation. For both methods, there is good agreement between the data in the $\mathrm{\Delta }t$ sideband and the accidental estimation. Center: The $\mathrm{\Delta }t$ distribution for the observed events, including the region used as a sideband between 500 and $1000\mathrm{\mu }\mathrm{s}$. The fits include exponential and flat components to model the neutron captures and accidental backgrounds, respectively. In the LR method (b), the accidental background is not expected to be flat, because $\mathrm{\Delta }t$ is included in the LR, but making this simplifying assumption for the fit has no significant impact [28]. Right: The prompt energy distributions for the selected events compared to the expectation for the signal and backgrounds. The background components are normalized to their expected counts, while the IBD signal is normalized to the total number of observed events minus the total number of expected background events.