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 $12.51×{10}^{20}$ 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 $122±11\left(\mathrm{stat}.\right)±15\left(\mathrm{syst}.\right)$ 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 $0.05\text{}\text{}{\mathrm{eV}}^{2}\le \Delta {m}_{41}^{2}\le 0.5\text{}\text{}{\mathrm{eV}}^{2}$, 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.