Search for sterile neutrino oscillations using RENO and NEOS data

We present a nearly reactor model independent search for sterile neutrino oscillation using 2 509 days of RENO near detector data and 180 days of NEOS data. The reactor related systematic uncertainties are significantly suppressed as both detectors are located at the same reactor complex of Hanbit Nuclear Power Plant. The search is performed by electron antineutrino (${\overline{\nu }}_e$) disappearance between six reactors and two detectors with flux-weighted baselines of 419 m (RENO) and 24 m (NEOS). A spectral comparison of the NEOS prompt-energy spectrum with a no-oscillation prediction from the RENO measurement can explore reactor ${\overline{\nu }}_e$ oscillations to sterile neutrino. Based on the comparison, we obtain a 95% C.L. excluded region of $0.1<|\mathrm{\Delta }{m}_{41}^2|<7{\mathrm{eV}}^2$. We also obtain a 68% C.L. allowed region with the best fit of $|\mathrm{\Delta }{m}_{41}^2|=2.41{\mathrm{eV}}^2$ and ${\sin }^{2}2{\theta }_{14}=0.08$ having a p-value of 8.2%. Comparisons of obtained reactor antineutrino spectra at reactor sources are made among RENO, NEOS, and Daya Bay to find a possible spectral variation.

Figure 1

Reactor antineutrino spectra obtained from the measured IBD prompt spectra of RENO and NEOS after unfolding their detector effects. The error bars are obtained as square root of the diagonal elements of the corresponding covariance matrices. Each spectrum is normalized by the total integral of the spectrum.

Figure 2

Fractional uncertainties of unfolded RENO and NEOS ${\overline{\nu }}_e$ spectra.

Figure 3

Comparison of prompt-energy spectra at NEOS and RENO. Upper: Ratio of the NEOS observed prompt spectrum relative to the $3\nu$ best-fit prediction at NEOS from the RENO measurement. The error bars represent the statistical uncertainty only. The error bands represent the systematic and prediction uncertainties. The areas of two spectra are normalized for a shape comparison. The gray band indicates the systematic uncertainty. Lower: Ratio of the NEOS $3\nu$ best-fit prediction at RENO relative to the RENO observed prompt spectrum. The red and magenta curves represent the best fits to the data. The blue curves represent spectral ratios expected with one of sterile neutrino oscillation parameters that are excluded by this analysis.

Figure 4

Comparison of the exclusion limits on sterile neutrino oscillations and an allowed region. The right side of each contour indicates an excluded region. The black curve (cyan filled region) represents a 95% (68%) C.L. exclusion contour (allowed region) obtained from the RENO and NEOS combined search using the Feldman and Cousins method (FC) [25]. The orange (blue) curve represents a 90% C.L. exclusion contour obtained from the RENO and NEOS combined search using the raster scan (RS) method where the spectral comparison is made at NEOS (RENO) detector. The best-fit parameter (red point) is found at $|\mathrm{\Delta }{m}_{41}^2|=2.41{\mathrm{eV}}^2$ and ${\sin }^{2}2{\theta }_{14}=0.08$. The second best-fit is found at $|\mathrm{\Delta }{m}_{41}^2|=1.75{\mathrm{eV}}^2$ and ${\sin }^{2}2{\theta }_{14}=0.05$. For the comparison, shown are the $\mathrm{NEOS}+\mathrm{Daya}$ Bay [10] 90% C.L. (gray shaded) and RENO far/near [8] 95% C.L. (blue dotted) limits on the disappearance. Also shown is a 95% C.L. allowed region of RAA [24] (black dotted) with the best fit [26] (star) at $|\mathrm{\Delta }{m}_{41}^2|=2.4{\mathrm{eV}}^2$ and ${\sin }^{2}2{\theta }_{14}=0.14$. Note that part of the best-fit area is excluded by STEREO and PROSPECT within 95% C.L. [27, 28].

Figure 5

(Upper) Comparisons of ${\overline{\nu }}_e$ spectral shapes of RENO, NEOS, and Daya Bay [29] relative to the HM model. The extracted ${\overline{\nu }}_e$ spectra are extrapolated to the production point at the reactor using the $3\nu$ oscillation model and the parameter values in Ref. [12]. The differences of fission fractions are corrected. Each spectrum is normalized by the area between 1.8–8 MeV. (Lower) Comparisons of ${\overline{\nu }}_e$ spectral shapes with an arbitrary normalization. The Daya Bay spectrum reported in 2017 [11] is recently updated [29].