Search for Active-Sterile Antineutrino Mixing Using Neutral-Current Interactions with the NOvA Experiment

This Letter reports results from the first long-baseline search for sterile antineutrinos mixing in an accelerator-based antineutrino-dominated beam. The rate of neutral-current interactions in the two NOvA detectors, at distances of 1 and 810 km from the beam source, is analyzed using an exposure of $12.51\times {10}^{20}$ protons-on-target from the NuMI beam at Fermilab running in antineutrino mode. A total of 121 of neutral-current candidates are observed at the far detector, compared to a prediction of $122\pm 11(\mathrm{stat}.)\pm 15(\mathrm{syst}.)$ assuming mixing only between three active flavors. No evidence for ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_s$ oscillation is observed. Interpreting this result within a $3+1$ model, constraints are placed on the mixing angles ${\theta }_{24}<25°$ and ${\theta }_{34}<32°$ at the 90% C.L. for $0.05{\mathrm{eV}}^2\le \mathrm{\Delta }{m}_{41}^2\le 0.5{\mathrm{eV}}^2$, the range of mass splittings that produces no significant oscillations at the near detector. These are the first $3+1$ confidence limits set using long-baseline accelerator antineutrinos.

Figure 1

The CNN likelihood output for neutral-current events used to distinguish between various interaction topologies applied to ND simulation and data. The signal and the dominant predicted background from CC beam interactions are shown. The red arrow indicates the selected events.

Figure 2

Spectrum of ND events passing the NC selection. The shaded band shows the total systematic uncertainty. The solid-fill component histogram displays the background contribution from beam CC interactions.

Figure 3

The reconstructed energy deposited in NC-candidate events in the FD. The observed data spectrum is shown along with the total prediction assuming standard three-flavor oscillations. The contribution from the main backgrounds is also shown.

Figure 4

Allowed regions in parameter space defined by the mixing elements describing neutrino oscillations in a $3+1$ model. Top: The 68% and 90% allowed values of the mixing angles ${\theta }_{24}$ and ${\theta }_{34}$. Bottom: The matrix element space, with limits from comparable measurements from Super-Kamiokande [25] and IceCube DeepCore [27]. The limits from Super-Kamiokande and IceCube are set using atmospheric neutrinos, a sample which is majority neutrinos but with a non-negligible antineutrino component. The NOvA limits are the first set from studying an antineutrino-dominated sample.