Sterile Neutrino Search at the NEOS Experiment

An experiment to search for light sterile neutrinos is conducted at a reactor with a thermal power of 2.8 GW located at the Hanbit nuclear power complex. The search is done with a detector consisting of a ton of Gd-loaded liquid scintillator in a tendon gallery approximately 24 m from the reactor core. The measured antineutrino event rate is 1976 per day with a signal to background ratio of about 22. The shape of the antineutrino energy spectrum obtained from the eight-month data-taking period is compared with a hypothesis of oscillations due to active-sterile antineutrino mixing. No strong evidence of $3+1$ neutrino oscillation is found. An excess around the 5 MeV prompt energy range is observed as seen in existing longer-baseline experiments. The mixing parameter ${\sin }^{2}2{\theta }_{14}$ is limited up to less than 0.1 for $\mathrm{\Delta }{m}_{41}^2$ ranging from 0.2 to $2.3{\mathrm{eV}}^2$ with a 90% confidence level.

Figure 1

A simplified cross-sectional view of the NEOS detector.

Figure 2

Detector responses to $\gamma$ and $\beta$ sources: (a) ratios of full peak charges to the true $\gamma$ energies, (b) $\beta$-decay spectra for ${}^{214}\mathrm{Bi}$, and (c) ${}^{12}\mathrm{B}$.

Figure 3

(a) The IBD prompt energy spectrum. The last bin is integrated up to 10 MeV. The orange shaded histogram is the background spectrum measured during the reactor-off period. The detector response matrix in the inset shows the relation between the neutrino energy and the prompt energy. (b) The ratio of the observed prompt energy spectrum to the HM flux prediction weighted by the IBD cross section with the $3-\nu$ hypothesis. The predicted spectrum is scaled to match the area of the data excluding the 5 MeV excess region (3.4–6.3 MeV). (c) The ratio of the data to the expected spectrum based on the Daya Bay result with the $3\nu$ hypothesis, scaled to match the whole data area. The solid green line is the expected oscillation patterns for the best fit of the data to the $3+1$ $\nu$ hypothesis and the corresponding oscillation parameters $({\sin }^{2}2{\theta }_{14},\mathrm{\Delta }{m}_{41}^2)$ is (0.05, $1.73{\mathrm{eV}}^2$). The dashed red line is the expected oscillation pattern for the RAA best fit parameters (0.142, $2.32{\mathrm{eV}}^2$). The gray error bands in (b) and (c) are estimated total systematic uncertainties, corresponding to the square roots of diagonal elements of the covariance matrices.

Figure 4

Exclusion curves for $3+1$ neutrino oscillations in the ${\sin }^{2}2{\theta }_{14}-\mathrm{\Delta }{m}_{41}^2$ parameter space. The solid blue curve is 90% C.L. exclusion contours based on the comparison with the Daya Bay spectrum, and the dashed gray curve is the Bugey-3 90% C.L. result [10]. The dotted curve shows the Daya Bay 90% C.L.${}_{\mathrm{s}}$ result [37]. The shaded area is the allowed region from the reactor antineutrino anomaly fit, and the star is its optimum point [12].