Study on reactor antineutrino spectrum at RENO

Abstract

Reactor antineutrino has played important role in the development of neutrino physics from the discovery of the neutrino, the discovery of the neutrino oscillation, and the measurement of the smallest mixing angle $\theta_{13}$. Reactor antineutrino will be a useful source for future experiments, such as determining the neutrino mass ordering, measuring the CP violation phase in the weak sector, and the search for sterile neutrino. However, currently, it has been found out that there is a significant difference in the theoretical prediction and the measured flux and spectrum by the RENO and the other experiments. Thus, it is the paramount task to measure the precise flux and spectrum by the experiment for future experiments and building a more robust reactor antineutrino model. The IBD yield is measured as (5.852$\,\pm\,$0.094$) \times 10^{-43}$\,cm$^2$/fission from the 2\,900 days of RENO live time. This corresponds to 0.941\,$\pm$ 0.015 to the prediction by Huber and Meuller. By unfolding the measured prompt spectrum to the true neutrino spectrum, it becomes possible to combine two data from two different detector responses. Using 2\,900 days of RENO and 180 days of NEOS spectra, a sterile neutrino search is conducted. We obtained improved exclude region with 95\% C. L. in the region $0.1<|\Delta m_{41}^2|<7$\,eV$^2$, and the best fit value at $|\Delta m_{41}^2|=2.41\,\pm\,0.03\,$\,eV$^2$ and $\sin^2 2\theta_{14}$=0.08$\,\pm\,$0.03 with 8\% p-value. This is the first result that the best fit is consistent with the allowed region that can explain the Reactor Antineutrino Anomaly\,(RAA).